The detector at the Gran Sasso end of the OPERA experiment. Credit: OPERA

The faster-than-light neutrinos seen by the OPERA particle physics experiment last year may have just been explained. By a loose cable. I wish I was joking.

To back up a little, the OPERA collaboration based at the Gran Sasso laboratory underneath the mountain of the same name in Italy published a paper to pre-print server arxiv.org last September saying that they had seen neutrinos, a type of sub-atomic particle, travel faster than the speed of light. They recorded neutrinos, which had travelled from CERN, Geneva, through the Earth to Gran Sasso, Italy, arriving at the laboratory 60 nanoseconds faster than they would had they travelled at the speed of light.

Since then, scientists around the world have been collectively scratching their heads and publishing papers that tended to fall into one of two categories: suggesting an error with the experiment (such as the clocks at the two laboratories not being synchronised properly), or suggesting an addition to the current theory of particle interactions that could explain the strange result (for example, a new dimension that the neutrinos could have skipped through to make their journey shorter – so they would have never actually travelled faster than light at any point).

But I don’t think anyone expected it to be something as simple as this.

Today, Science is reporting that a fibre optic cable connecting a GPS receiver and an electronic card in a computer was loose. They go on:

After tightening the connection and then measuring the time it takes data to travel the length of the fibre, researchers found that the data arrive 60 nanoseconds earlier than assumed

This news (though still unconfirmed) rather casts a shadow over another recent explanation, involving something slightly less ridiculous.

In a paper published in journal Astronomy & Astrophysics, Claudio Germana of the Astronomical Observatory of Padova, Italy, suggests that there was a problem with the synchronisation of clocks at the two ends of the experiment. His calculations suggest that if the experiment had been run at a different time of year, the neutrinos would in fact have arrived 50 nanoseconds later than light.

I spoke to Carlo Contaldi, a physicist at Imperial College London, who last year published a paper on arxiv.org pointing out a possible problem with clock synchronisation, about the new paper. Though he thought the calculations and the large effect the calculations seemed to show were “interesting”, he had some reservations:

[Germana] does not seem to mention the latest measurements that were carried out by OPERA in November 2012. Those showed a consistent value for the neutrino’s time of flight as the previous results and it would be interesting to see how that time frame fits in with these corrections.

It’s an interesting hypothesis though – and one that is easily testable by running the experiment at a different time of year.

This paper is just the latest in a long string of attempts to explain the faster-than-light neutrinos. For more of the explanations that have been offered over the last few months, have a look at a timeline I made that follows the story right from the beginning until now.

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All of these papers could have been for nothing, of course, if the new report of a loose cable is true. It would be a little disappointing if this turns out to be the case. I’m going to reserve judgement for now, at least until the “sources familiar with the experiment” become something a little more concrete.

I’ll be updating the above timeline as the story unfolds.

UPDATE: The Nature News Blog has an official statement from OPERA, that says they have “identified two issues that could significantly affect the reported result” – you can read the full statement over there.

Aurora as seen from the International Space Station as it crossed over the southern Indian Ocean on September 17, 2011. Credit: NASA

These light shows are the visible sign that a geomagnetic storm is raging overhead. But there’s another phenomenon that happens alongside the northern lights that you won’t have noticed at all. Surrounding our planet, way up above the atmosphere, is a doughnut shaped ring of charged particles held in place by Earth’s magnetic field. In fact, there are two of them. They’re called the inner and outer Van Allen belts.

The Van Allen belts were found in 1958 and were the first major scientific discovery of the space age. During geomagnetic storms, electrons in the Van Allen belts have been known to vanish – only to return a few hours later. This strange phenomenon was first spotted in the 1960s, and has puzzled physicists ever since.

Surely, they thought, at the height of a geomagnetic storm in which many energetic particles from the sun hit Earth’s atmosphere, there would be more electrons in the Van Allen belts, not less?

A new paper published online at Nature Physics seems to have the answer: the electrons are swept away by particles from the sun.

Drew Turner, from the University of California, Los Angeles, and his colleagues (also at UCLA) used data from three different spacecraft for this research: THEMIS, GOES and POES spacecraft.

THEMIS stands for Time History of Events and Macroscale Interactions during Substorms (and is also the name of a Greek goddess, something that I’m guessing wasn’t entirely coincidental) and was a NASA mission that investigated what causes auroras to go from moving slowly across the sky to dancing rapidly within minutes. It consisted of five identical satellites that lined up over North America once every four days to witness auroras.

The original THEMIS mission ended in 2009. Now two of the satellites have been sent off to orbit the moon and only three remain close to Earth. Those three were teamed up with two GOES (Geostationary Operational Environment Satellite) and six POES (Polar Operational Environmental Satellite) spacecraft, both run by the National Oceanic and Atmospheric Administration (NOAA), with the POES also jointly run by European Organization for the Exploitation of Meteorological Satellites, to witness a small geomagnetic storm on 6th January last year.

THEMIS and GOES both orbit Earth near the equator, with POES taking on the polar regions at a lower altitude, and pass through the Van Allen belts several times a day.

A solar flare accompanied by a coronal mass ejection (CME) erupted from the sun on January 23rd 2012. Credit: NASA/SDO

There are several solar phenomena that can cause geomagnetic storms. Coronal mass ejections, or CMEs, are one that we hear about a lot, possibly because of the amazing images that NASA mission Solar Dynamics Observatory (SDO) has been talking of them lately. But what caused the storm on 6th January 2011 was something called a co-rotating interaction region (CIR). CIRs are created because there are two different streams of particles coming from the sun: fast and slow. The fast stream taking over the slow one causes turbulence at the boundary between the two and creates a CIR.

During the 6th January storm, several satellites in the outer Van Allen belt noticed a ‘dropout’ of electrons – they appeared to go missing, but reappeared again around six hours later.

Turner and his colleagues looked at the data from the THEMIS, GOES and POES satellites and found that, while some electrons at lower energies did appear to have been replaced by electrons coming in with the CIR that caused the storm, ones with higher energies were pushed out from the Van Allen belts and away from Earth.

Some physicists thought that the electrons might have fallen downwards out of the belts during geomagnetic storms, but this new research is clear evidence that they seem to be pushed up and away instead. It might seem like a small distinction, but information on how the Van Allen belts work is important if we are to properly protect satellites flying around in them.

An upcoming NASA mission, Radiation Belt Storm Probes (RBSP) should be able to help give a fuller answer to what happens to the Van Allen belts during these storms. It’s due to launch this August – just in time to witness the many solar storms that will come our way in the run up to the next solar maximum.

Reference
Turner, D., Shprits, Y., Hartinger, M., & Angelopoulos, V. (2012). Explaining sudden losses of outer radiation belt electrons during geomagnetic storms Nature Physics DOI: 10.1038/nphys2185

Eros as seen on 14th February 2001 by the NEAR spacecraft. Credit: NASA

Fed up of simply reading about space and want to do some real science? Well, here’s your chance: astronomers are asking anyone with a pair of binoculars or telescope to train them on a new object visible in the night sky.

The object is an asteroid called 433 Eros. At 20 miles wide it’s one of the largest near-Earth asteroids, but it only really gets close to use once every 1.76 years because of it’s highly elliptical orbit. Its about to get the closest to Earth that its been in over thirty years – but don’t worry, at 16.6 million miles away it won’t pose any threat.

In fact, it could prove useful. From now until this Friday, the Eros Parallax Project is asking anyone with the right equipment to snap photos of Eros at specific times depending on their location. If you’re quick, you might be able to jump on board and help. There’s more information about the project here.

Depending on where you are on Earth, you will see Eros in a slightly different place in the sky relative to the background stars. This phenomenon is known as parallax. You can see it if you hold a finger up at arms length, look at where it is relative to the background with one eye closed, then switch eyes and watch it shift in relation to whatever is behind it.

Astronomers will use all the data submitted to find the distance to Eros. They will then use this to get a better estimate of the size of the solar system.

Aurora borealis above Bear Lake in Alaska. Credit: U.S. Air Force photo by Senior Airman Joshua Strang

The Sun is hotting up, and we can see the results right here on Earth. Across the northern hemisphere, fantastic light displays have been visible of late, and the frequency of these events is set only to increase as the Sun heads toward a peak in its magnetic activity.

In light of this (no pun intended), I decided a post about what is going on during an aurora was in order.

What exactly is happening with the Sun at the moment?

A coronal mass ejection that occurred on 1st August 2010 and caused a spell of aurorae that summer. Credit: NASA/STEREO

The Sun goes through cycles, each lasting around 11 years. During this cycle, its magnetic field increases and then decreases again. The magnetic field of the Sun is the source of its ‘activity’ – a term which describes solar phenomena like sunspots, faculae and prominences. Activity can also come in the form of coronal mass ejections (CMEs). These are huge bubbles of material with diameters a few times that of the Sun that explode into space, releasing billions of tons of charged particles, or plasma.

A few of years ago the Sun’s activity was at an exceptionally low and long-lasting minimum, but since then it’s been increasing and we’re heading for a maximum early in 2013. This means lots more activity is on the horizon: near a solar minimum we get around one CME a week, near a maximum this increases to two or three per day.

What has this got to do with the northern lights?

The northern lights (aka aurora borealis) are an amazing display of green and sometimes red light seen near to the magnetic north pole, and they’re caused by CMEs. Their southern equivalent occurs near the south pole, and is known as aurora australis.

After a CME erupts from the Sun, it can interact with the solar wind and cause huge interplanetary shock waves that go on to reach the Earth. When particles from the solar wind get to Earth, they are channelled down our planet’s magnetic field lines and end up accelerating towards the magnetic north and south poles. These particles then interact with atoms and molecules in our atmosphere and excite them, causing them to release photons. It is these photons that make up the light we see in the sky during an aurora.

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This BBC News article has a good illustration showing the solar wind’s interaction with the Earth’s magnetic field.

This post is a slightly modified version of one that appeared at my old blog in August 2010.

A composite image taken by Cassini on a fly by in 2005. This is roughly what Titan would look like to the human eye. Credit: NASA/JPL/Space Science Institute

Underneath Titan’s dense atmosphere lies something rather unusual, by terrestrial standards. Some features of the Saturnian moon, at first glance, might look similar to some features we have on Earth — it is the only other body in the solar system with lakes, and appears to have an active weather system. But instead of water, it’s methane that rains from the skies to fill Titan’s vast lakes, before it evaporates to form clouds that cover the surface. Curiously similar to the water cycle here on Earth, but at the same time rather alien.

The Cassini spacecraft has been able to take a closer look at this alien weather system and has seen that the distribution of lakes and clouds is not even across the surface of the moon. The lakes tend to cluster around the poles, in particular in the northern hemisphere. Clouds, meanwhile, prefer the south – that is the hemisphere that until recently was experiencing summer. A year on Titan is the equivalent of 30 years on Earth, so summer lasts a long time. Clouds stick around for about 25 out of these 30 years, but vanish for the remainder.

Some scientists at Caltech have come up with an explanation for this uneven distribution of clouds and lakes. They published their findings in Nature recently, and I wrote about their new model for Imperial’s student newspaper this week – head over to Felix Online to read all about it, if you wish.

I spoke to Dr Ingo Mueller-Wodarg, a planetary scientist in the Physics department at Imperial about the paper, and he explained why the new model is better than previous ones.

“What this study does is reproduce reasonably well, better than others before, the observations in terms of lake and cloud distribution. The significance of this is that we have gained a first understanding of what controls these features, namely a complex interplay of global wind transport, microphysical processes such as condensation and evaporation, cloud formation and radiative heating.

As far as I can tell this model has advanced on two fronts, namely being 3D rather than 2D and fully including the coupling between the atmosphere and surface in terms of methane transport and including surface reservoirs of methane. Many [previous] models are 2D, since calculation times otherwise become prohibitive due to the number of years that the models need to be run to assess seasonal trends (given that a Titan year is equivalent to 30 Earth years!). As far as I know, many models have significantly simplified the surface-atmosphere methane transport processes and hence got differing results. Importantly, many studies previously didn’t fully account for surface reservoirs of Methane and how these change over a year in response to the atmosphere.”

The Caltech team’s model has enabled them to make predictions about what the weather will be like on Titan in the next few years – to see if their model is right, all we will have to do is stay tuned (and make sure Cassini is making the observations needed to check their predictions!).

Refererence
Schneider, T., Graves, S., Schaller, E., & Brown, M. (2012). Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle Nature, 481 (7379), 58-61 DOI: 10.1038/nature10666

Size of the Sun now compared to how big it will expand to as a red giant. Credit: Wikipedia User:Mysid, User:Mrsanitazier.

Astronomers have found that the core of a red giant, the type of star that our Sun will eventually become, spins ten times as fast as its surface. And it happens because of a phenomenon we can see here on Earth, too.

You have probably seen a figure skater perform a so-called ‘scratch spin’, where she starts out with arms and free leg extended, before pulling them in – and spinning faster as a result. This happens because of a property known as angular momentum, a measure of how much an object is spinning. More specifically, it happens because the angular momentum of an object – in this case the figure skater – must stay the same before and after the manoeuvre. But angular momentum is not a property confined to figure skaters, people in general, or even things on Earth. Every spinning object in the universe has angular momentum, and each must obey the same physical law as the figure skater. In fact right at this very moment, across the universe, stars are performing scratch spins of their own.

Stars like our Sun run on hydrogen. When a star runs out of hydrogen, it is forced to burn other fuels. This switch triggers a change in the star. The core of the star collapses as the outer region expands and cools, creating a type of star known as a red giant.

We know that the angular momentum of the star must be conserved, so we also know that the core of the star that collapses must be spinning faster than the surface of the red giant. So far, though, our understanding of exactly how a star’s angular momentum changes as the star evolves is not especially good.

This is partly because we cannot directly observe how fast the core of a star is spinning. Now, though, an international collaboration of astronomers led by Paul Beck at the Institute of Astronomy at Leuven University in Belgium have found a way to measure the rotation of the core by probing the star’s interior using techniques from astroseismology. Astroseismology is a bit like the normal seismology that we use to study earthquakes, but instead of looking at waves traveling through Earth it looks at waves traveling through stars — starquakes. Their research was published in the latest issue of Nature, but is also available on arXiv.

Beck and his colleagues looked at small, regular variations in the light coming from several red giants observed by the Kepler spacecraft. Kepler’s main job is searching for planets outside of our solar system, so it is well suited to detecting extremely small changes in the brightness of stars, as this is a major way to spot that a star has a planet orbiting it.

The variations in light are caused by different waves traveling to different depths inside the star. Once Beck and his colleages had collected nearly two years worth of data on these variations, they compared what they had with theoretical predictions, and found that the core of the stars must be rotating at least ten times as fast as the surface.

This study advances astronomers’ knowledge of how the angular momentum of parts of a star change as it evolves, but there are still many questions left unanswered. The next step will be to study a larger sample of red giants at different stages in their lifetimes to learn more about how these stars change as they grow old, and what fate is in store for our Sun.

Reference
Beck, P., Montalban, J., Kallinger, T., De Ridder, J., Aerts, C., García, R., Hekker, S., Dupret, M., Mosser, B., Eggenberger, P., Stello, D., Elsworth, Y., Frandsen, S., Carrier, F., Hillen, M., Gruberbauer, M., Christensen-Dalsgaard, J., Miglio, A., Valentini, M., Bedding, T., Kjeldsen, H., Girouard, F., Hall, J., & Ibrahim, K. (2011). Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes Nature, 481 (7379), 55-57 DOI: 10.1038/nature10612

A false color image of Cassiopeia A using observations from both the Hubble and Spitzer telescopes, and Chandra X-ray Observatory. Credit: NASA/JPL-Caltech

They may come from completely different fields of study, but brain scans and supernovae have more in common than you would think.

In a new TED talk, Michelle Borkin explains how software developed for use in a hospital was able to help astronomers study the structure of supernovae.

An astronomer colleague of Borkin’s at the Harvard-Smithsonian Center for Astrophysics had eight years worth of data from the supernova remnant Cassiopeia A. She wanted to use the data to understand the remnant’s structure so she could work out how the star exploded. But there was a problem: she had no good way to look at the data. Luckily, Borkin did, and suggested that the astronomer try using 3D slicer software, originally developed in a hospital in Boston for looking at brain scans. It worked beautifully.

It is not just data analysis in these two fields that uses the same tools. The way data is collected from brain scans and radio telescopes is similar too. Even images in the fields of medicine and astronomy are alike: a confocal microscopy image of a human cornea looks much like a radio telescope image of star forming region NGC1333, despite the difference in scale.

This collaboration between astronomy and medicine is not the only example of an interdisciplinary connection in science – a lot of interesting science is now happening at the interface between two or more fields of study. Scientists working in all areas are looking outside their own lab in search of new ideas and methods, and more could benefit from joining them.

Video credit: TED

More about the Astronomical Medicine Project.

In order to make sense of the finding, I collected together lots of the coverage and papers concerning the result and had a go with interactive timeline making tool dipity.com. Have a look at the timeline below. You can zoom in on particular weeks and days, to see the detail of who published what and when, or you can zoom out for a broader overview of how the story unfolded. This is very much a work in progress and I plan to add to it as and when new events occur. If there’s something I haven’t included that you think should be on there please let me know in the comments.

If you need a refresher on how Opera experiment found this result, have a watch of the video by Minute Physics below, which provides a nice and simple explanation.

All that remains for me to say is happy new year to those already in 2012, and I’ll see the rest of you on the other side.

Supernova 2011fe in the Pinwheel Galaxy. Credit: Thunderf00t

When a star becomes a white dwarf — an old, extremely dense star that would have once been similar to our own Sun — the eventful part of its life is over. It releases what heat and light it has left over billions of years, slowly cooling until it no longer shines.

Usually. Some white dwarfs, however, are not content with this ending.

If a white dwarf exists in a two star system with a companion it can avert its fate and go out with a bang, not a whimper. It does this by causing a particular type of stellar explosion called a type 1a supernova. A type 1a supernova starts when the white dwarf drags material from its companion onto itself. It grows and grows until it cannot get any bigger. At this point it implodes, then rebounds and explodes in a supernova bright enough to outshine whole galaxies.

The companion star from which the white dwarf steals matter is instrumental in this dramatic event. Its identity, however, has long been a mystery.

Despite their origins being somewhat muddy, type 1a supernovae have given us a lot over the past year. Saul Perlmutter, Brian Schmidt and Adam Riess, along with their respective research groups, used them to discover the accelerating expansion of the universe — caused by the mysterious force we call dark energy — that won them the 2011 Nobel prize in physics.

Now, a type 1a supernova spotted in August this year has allowed astronomers to narrow down the range of possible companions that give white dwarfs the mass boost they need to explode.

At one minute to four in the morning on 24th August 2011, an alert was sent out to astronomers working on the Palomar Transient Factory (PTF). Their telescope had spotted an extremely bright object in the Pinwheel galaxy that hadn’t been there before — a new supernova, now known as supernova 2011fe. At that time it was, relatively speaking, quite faint, but over time it brightened. You may have seen it yourself: ten days after it was first seen it became bright enough to see through a pair of good binoculars.

The Palomar Transient Factory had noticed the star just 11 hours after it exploded, winning them the record for the earliest ever detection of a type 1a supernova, and were quick enough to get a glimpse of the light coming from it just 16 hours after explosion using an instrument on the robotic Liverpool Telescope in the Canary Islands. Since then, other telescopes have looked over their observations of that night to see if they saw it too.

Three possible scenarios before a type 1a supernova explosion. Li's group rule out scenario (a) in which the second star is a red giant, but cannot rule out (b) or (c). Credit: Nature / (a) ESO; (b) STSCI, NASA; (c) NASA/T. Strohmayer (GSFC)/D. Berry (Chandra X-Ray Observ.)

Watching a supernova as soon as possible after it starts is key to discovering what happened to make it explode in the first place. Theoretical models say the companion star to an exploding white dwarf can be only one of three types: a red giant, a main sequence star like the Sun, or another white dwarf. Astronomers are keen to narrow this down further.

In Nature earlier this month a team from California published two papers analysing observations of supernova 2011fe in the hope of finding clues that will enable them to do this. One paper, led by Peter Nugent from the Lawrence Berkeley National Laboratory found that the companion star was probably a main sequence star. Nugent’s paper also confirms that the star that exploded was a white dwarf.

The other paper, led by Weidong Li of the University of California, Berkeley, rules out a red giant companion.

Li used observations from the Keck telescope on the summit of Mauna Kea in Hawaii to pinpoint the precise location of the supernova, then analysed images taken by the Hubble Space Telescope from before the supernova explosion to look for clues about the pair of stars from which it was born. In the space where supernova 2011fe was later detected, they saw nothing. This did not mean they had made a mistake — just that the system preceding the supernova was not bright enough to be detected. This information was enough for them to rule out a red giant as the companion because, at one hundred times as luminous as the Sun, it would have been bright enough to show up. They could not, however, rule out other types of stars.

Supernova 2011fe is the first type 1a supernova to be discovered for many years and, because instrumentation has moved on considerably in that time, will become the most studied supernovae in history. These two papers are just the beginning.

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More on supernovae

• Double checking our cosmic tape measure
• We’re cosmic dust but you’re everything to me

References

Li W, Bloom JS, Podsiadlowski P, Miller AA, Cenko SB, Jha SW, Sullivan M, Howell DA, Nugent PE, Butler NR, Ofek EO, Kasliwal MM, Richards JW, Stockton A, Shih HY, Bildsten L, Shara MM, Bibby J, Filippenko AV, Ganeshalingam M, Silverman JM, Kulkarni SR, Law NM, Poznanski D, Quimby RM, McCully C, Patel B, Maguire K, & Shen KJ (2011). Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe. Nature, 480 (7377), 348-50 PMID: 22170681

Nugent PE, Sullivan M, Cenko SB, Thomas RC, Kasen D, Howell DA, Bersier D, Bloom JS, Kulkarni SR, Kandrashoff MT, Filippenko AV, Silverman JM, Marcy GW, Howard AW, Isaacson HT, Maguire K, Suzuki N, Tarlton JE, Pan YC, Bildsten L, Fulton BJ, Parrent JT, Sand D, Podsiadlowski P, Bianco FB, Dilday B, Graham ML, Lyman J, James P, Kasliwal MM, Law NM, Quimby RM, Hook IM, Walker ES, Mazzali P, Pian E, Ofek EO, Gal-Yam A, & Poznanski D (2011). Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star. Nature, 480 (7377), 344-7 PMID: 22170680

Artist's impression of one possible scenario — the supernova model — for the creation of the Christmas gamma-ray burst. Credit: NASA/Swift/Aurore Simonnet, Sonoma State Univ.

How did the Christmas gamma-ray burst explode? No, it’s not a geeky Christmas cracker joke, it’s a real question scientists have been trying to answer since Christmas day last year, when a gamma-ray burst called GRB 101225A first lit up the sky. The Christmas burst, as its come to be known, exhibted some rather unusual characteristics.

Gamma-ray bursts are short-lived flashes of gamma rays, made up of light that is more energetic than x-rays. Most are thought to be the result of massive stellar explosions in distant galaxies. Bursts can be over in milliseconds or last for several minutes, but no longer than that. After they finish they leave behind a longer-lived afterglow that can survive into weeks and months. While they last, they are the brightest objects in the known universe.

The Christmas burst was unusual. It went on for at least half an hour, but its afterglow faded much faster than was expected and displayed some features that are incompatible with current models. When it was first observed, astronomers were unable to work out how far away it was from Earth.

Now, two groups of scientists have come up with explanations of why the Christmas burst exploded the way it did. Both put forward their explanations in the same issue of the journal Nature earlier this month. Each group’s hypothesis attempts to explain why the Christmas burst lasted for longer than most others and why its afterglow faded so fast. In doing so, they also provide an estimate of the burst’s distance — and each come up with different answers.

Sergio Campana, an astronomer at the Istituto Nazionale di Astrofisica in Italy, and his group think that the Christmas burst was created in an atypical way — by a comet falling onto an extremely dense star. On the other hand, Christina Thöne, from the Instituto de Astrofísica de Andalucía in Spain, and her group propose a more conventional creation, involving a supernovae born out of two stars.

Artist's impression of the scenario in which a small object like a comet falls onto a neutron star and triggers the burst. Credit: NASA/Swift/Aurore Simonnet, Sonoma State Univ.

In Campana’s hypothesis a “small” object — a comet or asteroid — is whizzing past a dense neutron star just a little too close. It gets pulled in by the gravity of the star, but before it reaches the star’s surface, the smaller object is overwhelmed by the force of the gravity and breaks up into fragments. This debris is thrown into orbit around the star and then falls back to form a disk. Eventually, the debris falls on to the star itself and explodes, releasing vast amounts of energy. If this version of events is true, the explosion must have happened in our own galaxy, no more than 10,000 light years away in the Perseus spiral arm.

Thöne’s explanation involves a burst that starts as as binary system — two stars, one an extremely dense neutron star and the other a supergiant helium star, orbiting each other. The neutron star sucks the mass off the helium star until both are surrounded by an envelope of gas. The two stars then merge into a black hole or magnetar, a dense star with a powerful magnetic field, creating a jet of intense energy that bursts through the gas envelope. In this scenario, the explosion happened over four and a half billion light years away.

Enrico Costa, independent of both groups, pointed out that though both groups’ suggestions are “plausible”, at least one must be wrong. Once the burst’s host has been found, one (or perhaps even both) of these models will be ruled out. For now, though, the jury is still out on how this unusual Christmas burst came to be, so perhaps we should keep an open mind about the origins of some of the brightest explosions in the universe.

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Links
Astronomy Journal Club (#astrojc on twitter) dicussed this paper in their final meeting of the year. This is the review (and you can find the full meeting here).

For some animations of the two possible scenarios, go here.

References
Thöne, C., de Ugarte Postigo, A., Fryer, C., Page, K., Gorosabel, J., Aloy, M., Perley, D., Kouveliotou, C., Janka, H., Mimica, P., Racusin, J., Krimm, H., Cummings, J., Oates, S., Holland, S., Siegel, M., De Pasquale, M., Sonbas, E., Im, M., Park, W., Kann, D., Guziy, S., García, L., Llorente, A., Bundy, K., Choi, C., Jeong, H., Korhonen, H., Kubànek, P., Lim, J., Moskvitin, A., Muñoz-Darias, T., Pak, S., & Parrish, I. (2011). The unusual γ-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33 Nature, 480 (7375), 72-74 DOI: 10.1038/nature10611

Campana, S., Lodato, G., D’Avanzo, P., Panagia, N., Rossi, E., Valle, M., Tagliaferri, G., Antonelli, L., Covino, S., Ghirlanda, G., Ghisellini, G., Melandri, A., Pian, E., Salvaterra, R., Cusumano, G., D’Elia, V., Fugazza, D., Palazzi, E., Sbarufatti, B., & D.Vergani, S. (2011). The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star Nature, 480 (7375), 69-71 DOI: 10.1038/nature10592

The Higgs is the only particle predicted by the Standard Model of particle physics, currently the best theory we have to describe how particles interact, that we have not yet observed. Cern wants to change that.

Scientists working on Cern’s Large Hadron Collider (LHC) has been looking for the Higgs for quite a while now. I’m not quite sure how to break it to them, but I think I might know where it is…

Ok, ok. This is actually a stop motion animation that uses skittles (the sweet kind) to explain how the LHC works. It was made by a team of Imperial College Science Communication masters students, including myself, and the Higgs makes a blink-and-you’ll-miss-it appearance towards the end.

So whatever the announcement is tomorrow, remember that we found it first.

(For a slightly more serious post about the Higgs, go here to read why the Higgs matters)

Video credit: Peter Larkin, Harriet Jarlett, Heather Cruickshank, Sam Cavenagh, Kelly Oakes, Georgia Bladon, Antonio Torrisi, and Dharshani Weerasekera.

The Cygnus X star forming region contains several clusters of young, massive stars. In this image bright spots are where star formation is happening and ridges of dense gas mark the boundaries of cavities formed by the young massive stars. Credit: NASA/IPAC/MSX

Cygnus X is a star forming region in the constellation Cygnus in the night sky. It looks rather pretty in visible light, as shown at the beginning of the video below. But in radio, infrared and gamma ray wavelengths, Cygnus X really comes to life.

Recent Fermi Large Area Telescope (LAT) observations have shown that cavities created by massive stars within star forming clouds in the region are filled with gamma rays, created when the clouds are struck by cosmic rays.

Cygnus X is located 4,500 light years away from us. It has enough matter to make two million stars like our Sun, but contains stars that are much more massive than that.

Within Cygnus X are star clusters mainly made up of the hottest and brightest types of star — O and B type stars. These clusters, known as OB associations, are just five million years old — young, in stellar terms. O and B type stars are the most massive, which is why they’re so rare compared to other types of star. They “sculpt” the gas clouds in which they reside, emitting radiation and massive stellar winds that create cavities that surround the stars. In doing this the stars clear the area surrounding them of gas, making it much harder for more stars to form close by. They effectively climb up the ladder of star formation then kick it back down once they’ve got to the top.

Cygnus X was discovered in the 1950s as a source of radio waves. Now, Fermi LAT observations show a huge 160 light year across “cocoon” of cosmic rays exists in the region. The Fermi team said in a paper published in Science on November 25th that the cosmic rays “flood” the cavities carved out by massive stars in young star clusters.

Cosmic rays are beams of subatomic particles, usually protons, that travel through space at close to the speed of light — much like particles in a particle accelerator like the Large Hadron Collider.

When cosmic rays travel through space they get pushed about by magnetic fields which alter their path, making it difficult to see where the cosmic ray originated. When they meet the gas the lives between stars, they produce gamma rays. Gamma rays, unlike cosmic rays, are able to travel through whatever is in their way, arriving at Earth having followed a straight line path for their whole journey — so we can see where they originated.

The source of cosmic rays is a long standing problem in astrophysics. By tracing gamma rays, Fermi’s LAT can play a part in helping astronomers work out exactly where cosmic rays come from.

The newly discovered “cocoons” of cosmic rays are held together by magnetic fields created by the outflows from massive stars in the region. Shockwaves from the stars knot up the magnetic field so the cosmic rays become trapped.

Gamma Cygni, the remnant of a dying star — or supernova — exists within the Cygnus X region, making it a candidate for the cosmic ray source. But multiple shockwaves created by massive stars in the OB association is also considered as a possible source by the Fermi team.

This paper has shown that the life of a cosmic ray could be a lot more eventful than astronomers previously thought.

Reference
Ackermann, M., & et al (2011). A Cocoon of Freshly Accelerated Cosmic Rays Detected by Fermi in the Cygnus Superbubble Science, 334 (6059), 1103-1107 DOI: 10.1126/science.1210311

For those who can't read what it says on the trophy (i.e. everyone): "Royal College of Science Union, Science Challenge 2011, Imperial College Physics Prize, Kelly Oakes". Basically, I won a thing.

Every year the Royal College of Science Union at Imperial College runs an essay competition called the Science Challenge. There are usually four questions to answer and a number of prizes for the essays that answer them best.

I’ve been shortlisted before, but this year I finally won something — the Physics prize. Check out the photo to the left for a glimpse of my shiny trophy…

The essay question I answered was “Why should the average person care whether we discover the Higgs boson?” I took it as an opportunity to go off on one about why particle physics is important…

*

From afar it may seem entirely disconnected from the real world, but the Higgs boson is much more integral to life, the universe and, well, everything than you may think.

Have you ever contemplated why you weigh what you do? I am not alluding to the second doughnut you had the other morning, or the ill-advised chips on the way home from the pub, but rather the fundamental reason why the atoms that make up your body, and everything else in the world, have a certain mass. If you haven’t, you are not alone — until recently, scientists hadn’t thought much about it either.

Before the standard model of particle physics came along, the origin of mass was not even considered a problem; that an object had mass was simply assumed. But when scientists began probing objects at smaller and smaller scales, they discovered that it was not quite as simple as that: according to the standard model, fundamental particles should weigh nothing at all.

The standard model describes the behaviour and interactions of all of the most fundamental particles we have seen — and one other particularly elusive one that, physicists hope, we will see in the near future. The model was developed throughout the 20th century and finalised when the existence of quarks, the particles that make up protons and neutrons, was confirmed in the 1970s. At the time many of the particles predicted by the standard model were yet to be seen. Over the years since then, physicists have ticked these particles off, one by one, like items on a shopping list. Now they are left with just one remaining unfound particle — the Higgs boson.

Peter Higgs, a theoretical physicist at the University of Edinburgh, came up with the idea of the Higgs field and its associated particle — the Higgs boson — in 1964. The field he proposed extends throughout the universe, and interacts with matter particles in such a way as to give them mass. After an interaction the field leaves behind a telltale sign — the Higgs boson. Finding a Higgs boson would prove that the Higgs field exists.

Two experiments that are part of the Large Hadron Collider (LHC) at CERN are searching for the Higgs boson. Thousands of people from all around the world — including physicists, engineers and even anthropologists — work at CERN. If a Higgs boson is discovered there, there will be more than a few celebratory glasses of champagne — and an inevitable Nobel prize for Peter Higgs.

Elegant though the mathematics is that describes the Higgs mechanism, there is a chance that it does not actually describe nature. In this case, we have to look to slightly less elegant sounding ‘Higgsless’ models to discover the origin of mass. Some Higgsless models use extra dimensions to fix problems that would remain without the Higgs, while others use different mathematical tools. In fact, some physicists are more excited about the prospect of not discovering the Higgs, as this would leave the door open for other solutions that go beyond the standard model, and solve more problems than just the origin of mass.

So there are a few people at least for whom the discovery — or not — of the Higgs would be a momentous occasion.

But what about the rest of us? Well, there are many practical reasons to care about the search for the Higgs — if not that actual discovery. From conception through to the first collisions and beyond, particle accelerators spark many technological advancements that can be applied to fields as wide ranging as medicine, sustainable energy development and security. These advances would never have been made if we were not searching for as yet undiscovered particles like the Higgs.

However, one suspects that spin-off technologies and their economic benefits are not what the physicists at the LHC have in mind while running experiments and trawling data for signs of the Higgs boson. Peter Higgs told the Guardian why he was drawn to theoretical physics in the first place: “It’s about understanding! Understanding the world!” His enthusiasm is not abnormal in the physics community, even if it can sometimes be dampened by long hours spent staring at a computer screen analysing data. As humans we have a natural curiosity about the world around us, and we should not suppress that curiosity simply because the practical benefits of following it are not clear at the outset. Without such a curiosity the modern world as we known it would not exist.

Many people, including Peter Higgs himself, subscribe to the view that science for the sake of understanding the world around us is inherently valuable. If however, you need a more concrete reason to care about the Higgs, allow me to borrow some words from Carl Sagan: everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives on the pale blue dot we know as Earth — and none of it would have ever existed without the Higgs boson.

The GOODS field with 18 of the newly discovered colourful dwarf galaxies highlighted. Click for a bigger image. Credit: NASA, ESA and CANDELS

Hubble has uncovered a goldmine of young dwarf galaxies that are undergoing intense bursts of star formation.

Dwarf galaxies are the most common in the universe but until now astronomers had seen few examples of distant dwarf galaxies because they are small and not very bright. Observing distant dwarf galaxies used to require training telescopes on very small patches of sky — but the CANDELS survey, which started in 2010, is able to observe much larger patches of sky in the same detail.

A CANDELS team led by Arjen van der Wel from found sixty-nine distant dwarf galaxies in near-infrared images taken with Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys. The team stumbled upon the young galaxies while looking in two regions of the sky called the Great Observatories Origins Deep Survey (GOODS) and the UKIDSS Ultra Deep Survey (part of the UKIRT Infrared Deep Sky Survey). They weren’t looking for these galaxies specifically, but, once they’d spotted the galaxies’ unusual colours, could not resist taking a closer look. Their results were published in the December 1st issue of Astrophysical Journal.

This video zooms in on the GOODS South Deep field, right up to some of these newly discovered dwarf galaxies.

The galaxies are much smaller than the Milky Way but are creating stars at a massive pace, accumulating stars with a total mass of a hundred million times that of our Sun in just 30 million years. This intense activity is what produces their brilliant colours.

Now, these dwarf galaxies are nine billion light years away, which means we are seeing them as they existed nine billion years ago in the early universe. In astronomy, saying something is distant is the same as saying something existed when the universe was young. We see distant objects as they were when the light we use to see them left. We see something 9 billion light years away as it was nine billion years ago. That’s why light years are handy distance units to use when talking about astronomy.

Astronomers do think that galaxies in the early universe must have created stars at a much higher rate than they do today, but these dwarf galaxies are going fast even when this is taken into account.

One solution to this apparent problem is to say that star formation in small galaxies is not continuous. Simulations have shown that stars could form in short bursts. In between these bursts the galaxies would take a break from creating stars.

Stars form when clouds of gas collapse. The news stars could then reheat the gas and blow it away from the galaxy. In time, the gas would cool and collapse again, starting the next burst of star formation. But these dwarf galaxies appear to be going through bursts much more intense than simulations have produced.

This stop-start method of star formation does not fit with what astronomers have observed in near by dwarf galaxies. There are several dwarf galaxies orbiting the Milky Way as satellite galaxies, and astronomers have been able to look in detail at the rates of start formation in these ones before. Those surveys seemed to suggest that star formation takes place over billions of years.

Arjen van der Wel and colleagues conclude that these extremely intense bursts of star formation in the early universe may provide the majority of stars we see today in more mature dwarf galaxies.

While creating some problems of its own, this conclusion may help to solve another — one relating to that elusive substance known as dark matter. Simulations have shown that dark matter should be clustered towards the middle of galaxies, but observations show that in reality it is distributed much more evenly. This discrepancy could be explained if, during bursts of star formation, dark matter from the centre of a galaxy was pushed out towards the edges. We already know that normal matter does get pushed around in this way.

The James Webb Space Telescope, which was recently saved from the scrap heap, will be able to observe dwarf galaxies at a greater distance — meaning seeing them at an even earlier time — and will hopefully be able to offer more details surrounding their evolution.

Video credit: NASA, ESA, and G. Bacon (STScI)

Reference
van der Wel, A., Straughn, A., Rix, H., Finkelstein, S., Koekemoer, A., Weiner, B., Wuyts, S., Bell, E., Faber, S., Trump, J., Koo, D., Ferguson, H., Scarlata, C., Hathi, N., Dunlop, J., Newman, J., Dickinson, M., Jahnke, K., Salmon, B., de Mello, D., Kocevski, D., Lai, K., Grogin, N., Rodney, S., Guo, Y., McGrath, E., Lee, K., Barro, G., Huang, K., Riess, A., Ashby, M., & Willner, S. (2011). EXTREME EMISSION-LINE GALAXIES IN CANDELS: BROADBAND-SELECTED, STARBURSTING DWARF GALAXIES AT z > 1 The Astrophysical Journal, 742 (2) DOI: 10.1088/0004-637X/742/2/111

Galaxy cluster MACS 1206 as seen by the Hubble Space Telescope (click for a bigger version). Credit: NASA, ESA, M. Postman (STScI), and the CLASH Team

Dark matter makes up 23% of the universe but we know very little about it. A multi-wavelength survey called the Cluster Lensing And Supernova survey with Hubble (words which together make the obviously completely unintentional acronym CLASH) hopes to change that, by observing 25 clusters of galaxies in greater detail than ever before. The CLASH team have already completed observations of six clusters and plan to finish another five before the year is out.

MACS 1206 is a galaxy cluster that was recently surveyed by CLASH. It lies 4.5 billion light-years from Earth in the constellation Virgo in our night sky. The image of MACS 1206, above, was taken between April and July 2011 with Hubble’s Advanced Camera for Surveys and the Wide Field Camera 3. The observations, along with observations of other galaxy clusters to be surveyed by CLASH, will help astronomers construct detailed maps of dark matter in galaxy clusters.

Galaxy clusters are the perfect test site for dark matter’s gravitational effects because they are the biggest gravitationally bound objects in the universe. There are three defining features of a cluster: they contain hundreds of galaxies (at least — sometimes thousands); between the galaxies there are huge clouds of hot gas (we’re talking a hundred million degrees); finally, they contain dark matter, and lots of it.

We can’t see this dark matter — that’s where the ‘dark’ bit of its name comes from. We can, however, work out how much of it a galaxy cluster harbours using an effect known as gravitational lensing. Because of their huge masses, galaxy clusters act like giant ‘gravitational lenses’, bending and magnifying light that passes through them. This includes light from galaxies that lie beyond the cluster in our line of sight.

Gravitational lensing can cause the same galaxy to show up twice or more in different parts of an image, and can change how that galaxy looks in the image too.

The amount a galaxy is distorted depends on the total amount of mass in the cluster. The total mass includes that from the galaxies, the gas between the galaxies, and the dark matter. Astronomers are able to use the distortion of galaxies that lie behind galaxy clusters in our line of sight to measure the mass of the intervening galaxy cluster.

They can measure the amount of ‘normal’ matter in a galaxy cluster, so they know how much mass a cluster should contain if there were no dark matter.

Then, by using the amount of distortion to measure the total mass of the galaxy cluster, astronomers can work out how much dark matter exists within a particular cluster and how that dark matter is distributed. The distribution of dark matter in a cluster can give them some clues about how and when that galaxy cluster formed.

When Hubble surveyed MACS 1206 it found 47 multiple images of 12 galaxies that lie behind the cluster. A lot of the galaxies in the Hubble image of MACS 1206 look like they have been smeared slightly. If you look closely, you’ll notice that they are mostly smeared so that their longest edge is facing the centre of the image, making it look like they are in some kind of whirlpool. They are facing the centre of the galaxy cluster. By measuring how much the galaxies are smeared out, and in what way, astronomers hope to gain one more piece in the puzzle that is dark matter.

The Tarantula Nebula as seen by the TRAPPIST national telescope at La Silla. Credit: TRAPPIST/E. Jehin/ESO

The Tarantula nebula is an extremely bright emission nebula in the nearby Large Magellanic Cloud — a dwarf galaxy that orbits our own Milky Way. The Tarantula nebula measures a gigantic 1000 light years across, making it the largest star forming region in our Local Group of galaxies. Were it at the same distance away from us as the Orion nebula, it would take up a large chunk of the sky and cast shadows. Luckily for northern arachnophobes like myself, this nebula lies in the southern constellation Dorado.

The Ghost Head Nebula, as seen by Hubble. Credit: ESA, NASA, & Mohammad Heydari-Malayeri (Observatoire de Paris, France)

The Ghost Head nebula is another star forming region located in the constellation Dorado and also belongs to the Large Magellanic Cloud. This one only spans fifty light years, though. The two bright regions — the “eyes” of the ghost — are actually glowing lumps of hydrogen and oxygen made by intense radiation and fast stellar winds from massive, young stars at the heart of the nebula. The image was made using representative colours: red and blue regions show hydrogen gas that’s been heated by nearby stars, and green shows oxygen gas.

The Little Ghost Nebula, taken by Hubble. The white spot at the centre of the nebula is a white dwarf. Credit: NASA and The Hubble Heritage Team (STScI/AURA)

The Little Ghost nebula is a planetary nebula in the Ophiuchus constellation. It looks more like an all-seeing eye than a ghost, in my opinion, but either works for Halloween. Planetary nebulae aren’t anything to do with planets at all, but got the name because they looked similar to gas giants through the small, optical telescopes of the 18th century, when they were discovered. They form at the end of a Sun-like star’s life, when it throws off its outer layers while its core shrinks into a white dwarf. Our own Sun will go through something similar in five billion years… Scary stuff.

Composite image of the Cat's Eye Nebula, using images from Hubble and X-ray data from the Chandra X-ray Observatory. Credit: J.P. Harrington and K.J. Borkowski (University of Maryland), and NASA

The Cat’s Eye Nebula, three thousand light years away, was one of the first planetary nebulae to be discovered but still holds many mysteries. Concentric rings surrounding the inner nebula are actually bubbles of ejected mass and appear to have been thrown off in 1500 year intervals. There are several possible explanations for this, but none have yet been confirmed. The nebula is so complex that astronomers think the bright central object may not be just one star, but two — in a binary system. If that doesn’t scare you, have a look at some pictures of the Eye of Sauron from the film adaptation of Tolkien’s Lord of the Rings — see the resemblance?

The Snake Nebula. Credit: Wikipedia/en:user:Friendlystar

The Snake nebula is made up of a series of dark absorption clouds of molecular gas and interstellar dust, and can be seen in the constellation Ophiuchus — but only in areas with very little light pollution. Dust grains made mainly of carbon absorb visible light and reradiate it as infrared light, which is invisible to humans, blocking out stars that lie behind the Snake nebula in the sky.

The entire Orion Nebula as seen by the Hubble Space Telescope in visible light. Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

Being a student of Imperial College has a few perks. Our campus is on the same road as three of the biggest museums in London: the Natural History Museum, the Victoria and Albert, and the Science Museum. Not that you get much time to visit them when you have days full of lectures, seminars, tutorials and lab work…. but that’s another story. The lovely people at the Science Museum also let Imperial students into their IMAX cinema for free (provided it’s not the school holidays — but, trust me on this one, you do not want to be around South Kensington in half term anyway).

At the start of the summer, in that weird time between finishing exams and getting my degree result, I had a bit of time on my hands, so decided to take advantage of my soon-to-expire ID card and head to the Science Museum to see a film called Hubble 3D.

Hubble 3D (warning: elaborate website intro) is a breathtaking film and watching it is probably the closest most of us will ever get to being in space. The film focuses on the efforts of seven astronauts aboard Space Shuttle Atlantis who are tasked with repairing the Hubble Space Telescope. The crew on the servicing mission did on-site repairs for two of Hubble’s instruments: the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS) and, best of all for us lot stuck on the ground, an IMAX 3D camera was there to capture it.

But my favourite part of the film was made using images taken by Hubble itself. Narrator Leonardo DiCaprio takes viewers on a journey from Sirius, one of the closest stars to Earth and the brightest in the night sky, to inside the Orion Nebula. The Orion Nebula is located just underneath Orion’s belt in the constellation of the same name, and is visible with the naked eye. It’s a huge cloud of gas and dust, 24 light years across, in which stars are forming. Luckily for us, super fast wind from the young stars has blown a cavity in the side of the nebula facing Earth — so Hubble can peer right inside and see the stellar nursery in action. Hubble has even seen protoplanetary disks forming within the Orion Nebula — these disks are believed to be the starting point in the formation of planetary systems like our own solar system. One hundred and fifty protoplanetary disks have been found, suggesting that planetary systems are more common in the universe than we previously thought.

Without further ado, here’s that part of the film. It’s 2D (obviously) but still captures some of the magic of the 3D IMAX version, especially if you choose the highest resolution your computer can handle and watch it in full screen.

There’s also a video explaining how Frank Summers and hiscolleagues at the Space Telescope Science Institute took black and white 2D images taken by Hubble and turned them into 3D colour objects that the viewer could travel through. It’s interesting stuff, and doesn’t spoil the first video at all — I promise!

Open star cluster NGC 188 in the constellation Cepheus. Credit: Roberto Mura

When astronomers observe stars from Earth (or from orbit) there are two properties they can measure: the colour of the star and its magnitude. Magnitude is a measure of how bright a star looks from here so is not intrinsic to the star. If we know the star’s distance as well as its magnitude, though, we can work out its luminosity — how bright it really is. Astronomers can also take the colour of the star and work out its temperature. Anyone who has ever touched something that was “red hot” will be aware that colour and temperature are related, and so it is with stars. The hottest ones are blue and the coolest are red, but the spectrum of colours runs the full gamut between these — stars can be blue, white, yellow, red or anywhere in between.

Astronomers use the luminosity and temperature of a star to plot it on a Hertzsprung-Russell diagram (like the lovely hand drawn picture below). In any cluster of stars, most will sit along the middle stretch of the H-R diagram on the main sequence. The main sequence is where stars spend most of their life; our Sun is on the main sequence at the moment, has been there for just less than 5 billion years and has the same amount left to go. Once a star has exhausted all of its hydrogen, it will move off the main sequence — it does this at the ‘turn off’ — and evolve into a red giant.

A Hertzsprung-Russell diagram showing main sequence stars, red giants and blue stragglers. Temperature is shown in ºC (or Kelvin, whichever you prefer) and luminosity is shown is multiples of the Sun's luminosity. Click for a bigger version. Credit: Me (in case you hadn't guessed)

All different colours of stars lie on the main sequence. The bluest, and hottest, ones still on the main sequence mark the point of the ‘turn off’ — past that, we’re in straggler territory.

Blue stragglers are much younger than the other stars in their cluster and have masses larger than those at the main sequence ‘turn off’. Their formation cannot be explained using our usual ideas of star formation.

As early as the 1960s, astronomers had begun thinking of explanations for these unusual stars. In 1964, astronomer William McCrea suggested that blue stragglers were created in close binary star systems where one star has already evolved into a giant and the other is still on the main sequence.

In binary systems, both stars orbit a common centre of mass. The distance between the stars varies and depends on where they are in their orbits. In close binary systems, the stars are near enough to each other that each star can influence the other’s outer atmosphere.

In McCrea’s paper, he suggested that a blue straggler is created when material at the surface of the giant gets pulled towards its smaller companion. The companion then grows as it steals more and more material from the giant star, eventually leaving the giant star with no outer layers. The giant collapses into a white dwarf, and the companion star is now larger, and brighter and hotter, than stars at the main sequence “turn off” — it is a blue straggler.

However, the strongest theory now seems to be that blue stragglers form when stars collide.

Both these hypotheses can be tested by looking for white dwarfs as binary partners to blue stragglers. Formations by collision would likely result in larger companions, but those arising from the transfer of mass from a giant star would almost certainly result in a white dwarf. If you try to look for white dwarfs next to blue stragglers, however, you’ll quickly run into a problem: they are so small and faint that they cannot be observed directly. Astronomers had to find some way to indirectly search for white dwarfs.

Aaron Geller and Robert Mathieu, from the University of Wisconsin-Madison, did just that.

Geller and Mathieu looked at blue stragglers in the star cluster NGC 188. Twelve of the 16 blue stragglers in the cluster have extremely long orbital periods — taking over three years to orbit their binary companions. All but two take over 100 days, so these long period blue stragglers are the ones that Geller and Mathieu focussed on.

By looked at how the light from these blue stragglers changed over time, Geller and Mathieu were able to confirm that they did indeed have binary companions. The next, and trickier, step was to find out the mass of these companions. By assuming that the companion masses follow a particular distribution, they were able to compare the changes in light from the blue stragglers, caused by orbiting their companions, and come up with a mass distribution for the companions.

Their results show that the average blue straggler companion has a mass about half that of the Sun. Overall, the companion masses do not vary much from this average. This suggests only one thing: all of the blue stragglers in the NGC 188 cluster formed by mass transfer, not by collisions.

To test their observations against current theories, Geller and Mathieu set into motion a simulation of the cluster NGC 188, using current knowledge of stellar evolution (including that of stars in binary systems) and basing as many parameters as they could on real observations. After speeding through 7 billion (7,000,000,000) years of evolution, the simulation matched NGC 188 almost perfectly.

When they looked a little closer at the simulated blue stragglers, however, they found that about half of them had formed by mass transfer and half through collisions between stars. But this didn’t match their observations of the real blue stragglers in NGC 188. In NGC 188, all of the blue stragglers formed through mass transfer.

Geller and Mathieu also couldn’t quite rule out the hypothesis that some of the blue stragglers in NGC 188 formed by mergers between stars in triple systems. However, they think that this is quite unlikely and hope to rule it out once they have a chance to observe NGC 188 using the Hubble Space Telescope. They hope the Hubble observations will allow them to directly detect the light coming from the white dwarfs, too.

So why the difference between simulation and reality? Geller’s and Mathieu’s results suggest that our theories of stellar evolution, at least when the stars are close together, could do with some tweaking. As usual, the answer seems to be: it’s a little more complicated than we originally thought.

Reference
Geller AM, & Mathieu RD (2011). A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions. Nature, 478 (7369), 356-9 PMID: 22012393

NGC 5253, the dwarf starburst galaxy studied by Jordan Zastrow and her colleagues. Credit: ESO

If you’re reading this at night, look outside. Even in a city you’ll be able to see a few stars, if it’s not too cloudy and your eyes are up to it. If you’re lucky, the view from your window or garden will include a whole host of stars. Either way, you will be looking out on the vast blackness of space and seeing pinpricks of light that have traveled for millions of years before reaching your eyes.

This is only possible because of something that happened in the early universe. Shortly after the Big Bang, the universe quickly filled with a glowing plasma of mainly hydrogen and helium. As it expanded and cooled, atomic nuclei and electrons combined to make a dense fog of neutral gas. At this point, photons — particles of light — were freed and became the Cosmic Microwave Background Radiation that astronomers are studying today. But we’re not interested in that. It’s the opaque fog the photons left behind that would get in the way of seeing stars. Of course, it doesn’t, because it no longer exists — so where did it go?

Reionisation is the name of the process that took the universe out of the cosmological dark ages and into the light. In all, the dark ages lasted about half a billion years. They were ended by the formation of the first galaxies.

Astronomers believe that star-forming galaxies are the most likely suspects in the search for the source of radiation that was needed to clear away the cosmic cobwebs — or, in more technical terms, turn the neutral intergalactic medium, composed mainly of hydrogen, into the charged plasma that it is today. Until now, though, astronomers weren’t sure how the radiation could have got out of the star-formaing galaxies in which it was created.

The galaxies that astronomers think are the source of the radiation needed to reionise the universe are known as starburst galaxies. Unsurprisingly, this is because they go through intense bursts of star formation. Starburst galaxies are rare today, but astronomers think they were much more common in the early universe. One starburst galaxy in particular, NGC 5253 — a dwarf starburst not too far away, cosmologically speaking — has been helping astronomers with their inquiries into exactly how the radiation required to clear the early universe made its escape.

NGC 5253 has just undergone an episode of star formation that resulted in huge clusters of massive, young stars. Massive stars can create a wind that expands out through the galaxy. As it goes, the wind removes any obstacles in the way of radiation created by the stars, allowing it to pour out of the galaxy along the same path.

It’s ultraviolet light which is the radiation under scrutiny, so Jordan Zastrow, a graduate student at the University of Michigan, and her colleagues looked for signs of ultraviolet light coming from NGC 5253. They didn’t look for the light itself, but its interaction with other gas close to the galaxy.

In a paper published in Astrophysical Journal Letters last week Zastrow and her colleagues describe results that show ultraviolet light is emanating from the galaxy in a narrow cone, and making gas in the interstellar medium evaporate as it goes. Outpourings like this one could be what started reionisation.

Zastrow and her colleagues cannot completely rule out an active galactic nucleus as the source of the radiation. Astronomers think active galactic nuclei are caused by the accretion of mass on to supermassive black holes at the centre of a galaxies. AGN sometimes eject radiation, which could look a lot like what Zastrow and her colleagues see around NGC 5253. But Zastrow thinks massive stars are much more likely to be causing the outpouring in this case.

That the radiation escapes the starburst galaxy in a narrow cone is important, because it means that, unless it is oriented in the right way, we won’t be able to tell whether or not a starburst galaxy is emitting ultraviolet light. As well as explaining why we have not seen this phenomenon before, this could mean that NGC 5253 may be the only nearby starburst galaxy that can teach us something about how reionisation began.

Reference
Jordan Zastrow, M. S. Oey, Sylvain Veilleux, Michael McDonald, & Crystal L. Martin (2011). An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253 Asrophysical Journal Letters arXiv: 1109.6360v1

Fermilab's two accelerator rings, as seen from the air. Credit: Fermilab, Reidar Hahn

Tomorrow, the Tevatron particle accelerator at Fermilab will shut down. The end will be no song and dance: the accelerator operators will simply stop putting new protons and antiprotons into the machine. The last few particles will whiz around the accelerator until the number of collisions per second drops below a useful level, after which time the remaining particles will be diverted to a metal target. The process is the same as that used for annual maintenance shutdowns, but this time there will be no going back. By the end of December, the thousands of superconducting magnets that surround the accelerator tunnel will have been warmed up from their close-to-absolute-zero temperature before being removed, and the rest of the accelerator will have been cleared of gases and fluids, and dismantled. Its a rather understated end to 26 years of smashing particles together in the name of science.

From the placing of the first magnet to those final few collisions, the Tevatron has been a major engineering achievement and a source of plenty of new science for particle physicists to sink their teeth into. Even after the shutdown, scientists working on Fermilab’s CDF and DZero experiments will carry on analysing Tevatron data and in all likelihood will end up publishing at an even higher rate than they did when the accelerator was running, for a couple of years at least.

In honour of the shutdown, the nice people Fermilab have produced an interactive timeline of the Tevatron’s history. You can see the timeline here. And for a bit more detail on one of the most important discoveries made there, read on…

March 2, 1995

Just before the 10th anniversary of the first proton-antiproton collisions witnessed by the CDF collider detector, both the DZero and CDF experiments submitted papers to Physical Review Letters detailing their discovery of a new particle — the top quark. The discovery was announced in seminars at Fermilab and a press release sent out on the same day.

Thinking that quarks should come in pairs, particle physicists had been hunting for the top quark since the discovery of its smaller partner, the bottom quark, at Fermilab in 1977. The CDF experiment saw hints of the top quark in 1994, but there wasn’t quite enough data to say for certain that it existed. Once more data had been collected and analysed, and the DZero experiment had conducted its own, independent investigation, a clear signal of the tops existence was found, allowing both groups to say for certain that they’d seen it.

The top quark is the heaviest fundamental particle we have ever observed, by a long way. It is 100,00 times heavier than the lightest quark — the up quark — and has about the same mass as a gold atom, though it is much smaller. Its mass is why it was the last quark to be discovered: the more massive a particle, the more energy is required to create it in a particle accelerator. Before the Large Hadron Collider, the Tevatron was the only accelerator in the world capable of reaching energies high enough to make the top quark.

To create a top quark, a proton and antiproton collide at nearly the speed of light. Sometimes, this results in the production of a top quark, which always comes with an antiquark. The pair don’t hang around for long, and within a fraction of second they both decay into lighter particles. A chain of decays often results in a burst of particles that physicists call jets. Finding the signature of a top quark in the vast amounts of data would be near impossible, given its short lifetime, so instead physicists look for its decay products. A top-antitop pair decay into two W bosons (force particles that mediate the weak nuclear force) and two bottom quarks (the top’s much lighter parter). In turn, one of the W bosons turns into a muon and neutrino pair and the other turns into an up and down quark pair. The up and down quarks then decay into jets, shortly followed by the bottom quarks that go the same way. So, the signature to look for when searching for top quarks is: one muon, one neutrino and four jets.

The CDF and DZero experiments had each seen approximately ten top-antitop quark pairs, which was enough for them to be sure that the events were not just something that looked like top quarks, but the real thing.

In the years after the discovery, scientists at Tevatron have been able to determine the top quark mass to high precision. This has not only helped to reinforce our knowledge of the standard model of particle physics, but also refine our ideas about the Higgs boson — the only particle in the standard model that has yet to be found. Now that the Tevatron is shutting down, it will be up to the LHC to continue the hunt for the Higgs.

References

CDF Collaboration. (1995). Observation of Top Quark Production in p¯p Collisions with the Collider Detector at Fermilab Physical Review Letters, 74 (14), 2626-2631 DOI: 10.1103/PhysRevLett.74.2626

DZero Collaboration., Abachi, S., & et al. (1995). Search for High Mass Top Quark Production in pp¯ Collisions at s= 1.8 TeV Physical Review Letters, 74 (13), 2422-2426 DOI: 10.1103/PhysRevLett.74.2422

It may look all serene from this vantage point, but underneath the Gran Sasso mountain is a hive of neutrino-detecting activity. Credit: Wikipedia/w:nl:Gebruiker:Idéfix

Unless you’ve been hiding under a rock for the past 24 hours, you’ve probably heard about the neutrinos that turned up at the Gran Sasso Laboratory in Italy a few nanoseconds earlier than they were supposed to, in a feat that would have required them to travel faster than the speed of light.

The story has been covered by many news outlets already, and, while some headlines may have raised a few eyebrows, most of the coverage has been suitably cautious. Heck, even the Daily Mail has generously thrown a few “ifs” and “mights” into their take on the findings.

The results were first announced, unceremoniously, in a tweet by Reuters Science last night. The tweet linked to a story on reuters.com that describes how three years worth of measurements show that some neutrinos must have travelled faster than the speed of light.

The neutrinos in question undertook a journey from CERN in Geneva, through the Earth, and finally ended up at the Gran Sasso laboratory deep underneath the mountain of the same name in Italy. The neutrinos were produced by the Super Proton Synchrotron at CERN, along with a lot of other sub-atomic particles. The trick particle physicists use to get a beam of only neutrinos, and the reason the detector for the experiment is located so far away from the source of the beam, is to send the beam of particles off to travel underground for several miles. Neutrinos are the only particles that survive the journey, because they pass through matter completely unscathed whereas the others do not. The now pure neutrino beam takes less than 3 milliseconds to travel the 730km between CERN and Gran Sasso, and the neutrinos are detected by apparatus belonging to the OPERA experiment, consisting of around 150,000 bricks of photographic film interleaved with lead plates.

The neutrinos, says the paper released by the OPERA experiment after the news was first broken, reached the detector 60 nanoseconds before they would have done had they been travelling at the speed of light. The result amounts to a statistical significance of 6-sigma. “Sigma” is shorthand for standard deviation, a statistical tool that can be used to give an estimate of the certainty of a result. Generally, the higher the number of sigma, the more trustworthy the result. A minimum of 5-sigma, equivalent to a one in 1,744,278 chance that the result is a fluke, is normally required to claim a discovery. 6-sigma, equivalent to a one in 506,797,346 chance is even more convincing.

If any other particle physics result had been shown to have a statistical significance of 6-sigma, champagne corks would be popping in laboratories all over the world. But this one is different.

The universal, unwavering, cosmic speed limit — the speed of light — is the most fundamental of constants. Without it, we can wave goodbye to relativity as we know it. Causality — the fundamental relationship between cause and effect — would have to go. As Subir Sakar, head of particle theory at Oxford University put it to the Guardian yesterday: “Cause cannot come after effect and that is absolutely fundamental to our construction of the physical universe. If we do not have causality, we are buggered.”

Aside from being inconsistent with both relativity and causality, the result doesn’t appear consistent with the past behaviour of neutrinos either. I’ve written before about supernova 1987a, whose arrival was heralded by a burst of neutrinos. In this case, the neutrinos, travelling at the speed of light, reached Earth 3 hours before the light did. The light from the supernova was delayed because it had to get through the remnants of the stellar explosion, not because of any sneaky faster-than-light travel on behalf of the neutrinos. Dr Ben Still, a particle physicist working on the T2K neutrino experiment, blogged earlier today about the Opera result and calculated that if the neutrinos from supernova 1987a had exhibited the same odd behaviour as those that are under the spotlight today, they could have arrived 4.14 years before the light did. The neutrinos and light from supernova 1987a are well documented and this does not appear to have been the case.

Of course, there could be some explanation that is able to bring all of the so far seen problems together and solve them in one fell swoop. There could also be mistakes in the paper that have been missed by physicists working for the OPERA experiment. Time will tell.

For what it’s worth, I’d be prepared to put money on Einstein winning this one, but that doesn’t mean that the excitement and interest these results have generated was for nothing. While, arguably, the announcement of the result could have done with a little more careful planning, the OPERA experiment has now got what it needed: lots of pairs of eyes looking over the results, and checking for something everyone else so far might have missed. Looked at in the right way, this episode is the perfect insight into how science really works. Scientists test hypotheses and current theories (yes, even ones as seemingly solid as relativity) and pipe up when they see something out of the ordinary, to allow others in the field to double and triple check their analysis. This is what is happening now.

While scepticism is necessary in situations like this — I’m sure we’re all aware of the famous Carl Sagan quote, “extraordinary claims require extraordinary evidence” — progress is not made by shouting down anything that does not fit within the current status quo. You never know, perhaps this result will be the one that topples relativity. (They probably didn’t, but there’s a chance, however slim, that those neutrinos did travel faster than light — and that’s a very interesting prospect indeed).

In science, when something out of the ordinary appears, the next step is often to repeat the experiment and try to recreate the results. So, if someone could just lend me a particle accelerator, a mountain and a deep underground mine, I’ll get to it…

*

Now you’ve had my two pence, there’s plenty more speedy neutrino goodness to be had:

My SciAm blogs colleague Caleb Scharf has a post up with lots of links to more coverage on this story. For a great overview of the science I’d particularly recommend Ethan Siegel’s blog post at Starts with a Bang.

At Discover blogs, the Bad Astronomer has his say and Sean Carroll at Cosmic Variance also has his take on the paper.

And finally,Randal Munroe at XKCD was extremely quick of the mark and had a neutrino-themed comic up and ready earlier today.

Reference

The OPERA Collaboraton (2011). Measurement of the neutrino velocity with the OPERA detector in the CNGS
beam . arXiv: 1109.4897v1

The "impossible" star, along with a pie chart showing its composition; it is made of mostly hydrogen and helium, with trace amounts of heavier elements. Credit: ESO/Digitized Sky Survey 2

In the beginning, the only elements that existed were hydrogen, helium and very small amounts of lithium. All of the other elements in the period table came later and, rather than forming out of the primordial soup of sub-atomic particles that existed shortly after the big bang, the elements from lithium up to and including iron, were made in the nuclear furnaces at the centres of stars. But all the stars we’ve ever seen have contained some of these heavier elements, a fact which raises the question: were there ever any stars made only of those three first elements?

Astronomers call any element heavier than helium a “metal”. That’s an awful lot of elements — in fact, 116 of the 118 elements we have discovered so far fall into this category. Astronomer’s also talk about the “metallicity” of a star — the fraction of elements in the star, by mass, that are metals. Metallicity is an indicator of a star’s age. Or, more accurately, an indicator of how many previous generations of stars have lived, died and been recycled to make the new star.

Star populations were named in the order the were discovered: I, II and III. Population I stars were discovered first, but created last, and are the youngest stars. Because of this, they have the highest metal content. The Sun is a population I star. Population II stars have lower metal content than population I stars. Population III stars hypothetically have no metal content at all, but none have been observed. Yet.

This poses a problem.

It has been suggested that low-mass stars should not be able to form in the early universe, because the primitive interstellar medium — the gas and dust made of hydrogen, helium and trace amounts of lithium that filled the early universe and still exists between stars today — did not contain enough metals.

To solve both the problem of where the first metals came from and why we haven’t seen any stars without them, astrophysicists have suggested that perhaps only high mass zero-metallicity stars formed. These stars would have had masses around a hundred times that of the sun and their lives would have been over in a (relative) flash, thanks to an inverse relationship between stellar mass and lifetime. When these stars neared the end of the lives, they will have been able to fuse the first 26 elements in the periodic table — up to iron — in their cores. Once they had exploded in supernovae, the newly formed elements would have been spread far and wide throughout the universe, and added to the mix when new stars formed.

These stars would have pushed up the metal content of the interstellar medium and allowed stars with lower masses and higher metallicities to form. This second generation of stars would be population II.

But (isn’t there always a “but”?) a group of astrophysicists, lead by Elisabetta Caffau from Heidleberg University in Germany, recently announced that they have found a low-mass star that also has a low metallicity.

The star resides in the Galactic halo and has a metallicity lower than any other star ever observed. Put simply: this star, which goes by the name “SDSS J102915+172927”, shouldn’t exist. Caffau and her colleagues published their findings earlier this month in Nature.

The star gets the first part of its name because it was catalogued in the Sloan Digital Sky Survey (SDSS), and the second part because of where it is in the sky. Simone Zaggia, a co-author on the paper, made a nice animation showing the path of the star in the Milky Way.

Caffau’s team analysed the composition of the star using the spectrographs X-Shooter and UVES of ESO’s Very Large Telescope (VLT) in Chile. Spectrographs are instruments that split the light from a star into colour components to find out how much of each element the star contains. When they did this, they found that the star contains no carbon, oxygen or nitrogen.

Low-mass, low-metal stars have been seen before, but these tend to be rich in carbon, nitrogen and oxygen. Carbon and oxygen are thought to be key to the formation of low-mass stars, because they cool the gas and dust in the interstellar medium during star formation. The existence of a low mass star without carbon or oxygen seems to indicate that what we think are the necessary levels of these two elements for low-mass star formation are, in fact, not necessary at all. If this were the case, it would mean a dramatic increase in the diversity of stars in the early universe.

Caffau and her team don’t think that SDSS J102915+172927 is an anomaly. They expect that between 5 and 50 stars from those that can be analysed by the VLT, and even more in the whole SDSS sample, will have similar or lower metallicities to the new most primitive star.

This newly discovered star may indicate that, if we dig a little deeper, we could discover that, in terms of diversity, the oldest stars in the universe are not so different to the youngest ones after all.

Reference
Caffau E, Bonifacio P, François P, Sbordone L, Monaco L, Spite M, Spite F, Ludwig HG, Cayrel R, Zaggia S, Hammer F, Randich S, Molaro P, & Hill V (2011). An extremely primitive star in the Galactic halo. Nature, 477 (7362), 67-9 PMID: 21886158

Face-on and edge-on views of the visible light (left) and gas density (right) of Eris, a simulated galaxy similar to the Milky Way. Credit: Guedes et al (2011)

Judging by its starlight and gas content (as seen in the image above), Eris looks to be a near match for our own Milky Way galaxy — except that it exists only as a simulation inside a supercomputer. Until recently, realistic simulations of the Milky Way were not forthcoming. Now astrophysicists at The University of California, Santa Cruz, and the Institute for Theoretical Physics at the University of Zurich have simulated our galaxy better than ever before, all by adjusting how the stars in it form. Their results are due to be published in the Astrophysical Journal, but in the meantime are available on arXiv.

Simulations that have previously tried to recreate a galaxy like the Milky Way have failed to come up with something that matches reality closely enough. The Milky Way is 100,000 light years in diameter and 1,000 light years thick, with sweeping spiral arms connected by a bar leading to a central bulge. The bulge contains mainly older stars, whereas the spiral arms are populated with the hottest young stars. Our solar system lies about two thirds of the way out from the centre of the galaxy in one of the spiral arms known as the Orion-Cygnus Arm. A halo containing globular clusters of old stars surrounds the galaxy in a roughly spherical shape.

An artist's impression of the Milky Way, showing its spiral arms and central bar and bulge. Credit: NASA/JPL-Caltech/R. Hurt

Previous simulations tended to create galaxies with bulges that were too large and disks that were too small. Javiera Guedes, a graduate student at UC Santa Cruz at the time of the research, and her supervisor Piero Madau, along with two colleagues from the Institute of Theoretical Physics in Zurich, came up with a simulation in which the bulge and disk were much more like they are in the Milky Way. It is the first simulation that can create galaxies with features that are all consistent with our observations of the Milky Way. They named it Eris.

Key to Eris’ success was a more realistic approach to star formation. Star formation in real galaxies occurs in a more clustered way than previous simulations had been able to recreate. Eris has a higher resolution than these previous simulations so was able to model this more complex star formation. It did so by setting the threshold for star formation higher, enabling differences in density to form across the simulated interstellar medium, which is made up of gas and dust. This meant that stars formed only in dense regions of the interstellar medium, creating the clusters that appear when stars form in reality.

Stellar explosions typically blow gas away from their surroundings. Because stars in this simulation were forming in dense regions, when the shorter-lived massive stars exploded, there was more gas to blow away. The gas that was ejected during these explosions never reached the central bulge of the galaxy, meaning that the bulge did not grow as large as it could have — making the simulated galaxy more like the Milky Way, with its relatively small central bulge.

The simulation involved 60 million particles, including all the gas, dark matter and stars in the simulated galaxy. Because of this, the simulation required lots of computer power and time — 1.4 million processor hours on NASA’s Pleiades supercomputer, as well as time on supercomputers at the researchers’ institutions.

Guedes and her colleagues worked out it was the high resolution of the simulation, and its effects on star formation, that made Eris closer to the Milky Way by running a twin simulation with a lower resolution. Eris’ lower resolution twin came up with a galaxy less like the Milky Way and more similar to those in previous simulations.

The success of Eris supports the ΛCDM model of the universe, the simplest model we have that is in agreement with observations. ‘Λ’ (pronounced ‘lambda’) refers to the dark energy component of the universe, as this was the symbol Einstein originally used for his cosmological constant — a mathematical appendage he added to his field equations to make them describe a static universe. When astronomers realised the universe wasn’t static, the cosmological constant was taken away, but eventually came back as a useful way to describe the dark energy that is forcing the universe into a period of accelerated expansion. The ‘CDM’ part of the name stands for cold dark matter. ‘Cold’ because it doesn’t move very fast, and ‘dark’ because it doesn’t interact with light, making it nearly impossible to detect. The ΛCDM model combines both dark matter and dark energy to explain the universe, and it works rather well.

Milky Way simulations may not be “just right” yet, but they’re certainly getting there.

Reference
Javiera Guedes, Simone Callegari, Piero Madau, & Lucio Mayer (2011). Forming Realistic Late-Type Spirals in a LCDM Universe: The Eris
Simulation Astrophysical Journal arXiv: 1103.6030v2

A composite infrared and x-ray image of the remnant of a type 1a supernova. Credit: NASA/MPIA/Calar Alto Observatory, Oliver Krause et al.

In the late 90s there was a race going on between two astronomy collaborations. Both were on the verge of making a discovery that would change the field of cosmology forever, though they may not have realised it at the time. The High-z Supernova Search Team and the Supernova Cosmology Project were both studying a peculiar sort of exploding star, known as a type 1a supernova, and trying to figure out the ultimate fate of the universe — would it expand forever, or eventually slow to a stop and reverse in on itself in a “big crunch”? The answer: neither. Both teams reported, in separate papers, one published in 1998 and the other in 1999, that the expansion of the universe was actually accelerating. That is, it is expanding faster today than it was yesterday, and tomorrow it will be moving apart even faster than it is today. This was a rather unexpected result.

Astronomers think that the type of supernova used to make this discovery, type 1a, occurs when a white dwarf — an old, dense star that was once similar to our own Sun — with a binary companion begins to drag material from its companion star on to itself, growing bigger and bigger until, eventually, it can no longer sustain itself. At this point, it implodes, then explodes in a bright supernova. The upper mass limit of a white dwarf, above which it will turn into a supernova, is called the Chandrasekhar mass, after the Indian astrophysicist who predicted its existence in 1930. It is caused when a phenomenon known as electron degeneracy breaks down. Electron degeneracy is a result of the Pauli exclusion principle, which states that two particles cannot occupy the same quantum state at the same time. In effect, the electrons in the white dwarf refuse to be pushed closer together, creating a repulsive force that can, for a while, balance the extra gravity caused by the accreting material. If enough material is accreted on to the white dwarf, however, the gravitational force will outweigh that from electron degeneracy and the white dwarf will implode. The Chandrasekhar limit is the point at which these two opposing forces exactly balance. Once a white dwarf passes the Chandrasekhar limit, and after it goes through the supernova stage, it becomes a neutron star or, sometimes, a black hole.

The European Southern Observatory (ESO) has a nice video showing how the white dwarf sucks material from its companion:

There are two different scenarios in which a type 1a supernova may be created. In the first, only one star in the binary system is a white dwarf, the other is a main sequence star like the Sun, or a more evolved star such as a red giant. In the second scenario, both stars are white dwarfs.

In a situation with only one white dwarf, there should be matter left over after the supernova. This material, which was not accreted on to the white dwarf before it imploded, is known as circumstellar matter because it surrounds the stars. Its detection near a type 1a supernova would indicate that the binary system from which the supernova was born contained only one white dwarf.

Knowledge of a supernova’s predecessors could prove invaluable. Type 1a supernovae can be used to measure distances on a cosmic scale. Predictable patterns in the way type 1a supernova brighten and fade reveal their true luminosity, which can be compared with how bright they look to us to find their distance. Measuring distances using these supernova was how the two teams of astronomers made their discovery of the accelerated expansion of the universe. We say that this expansion is caused by dark energy, but the phrase “dark energy” could, at the moment, mean a number of different things. A better understanding of one of the main pieces of evidence for dark energy may help us to figure out what it actually is. Which brings us back to those type 1a supernovae…

Astronomers have looked for circumstellar material around certain type 1a supernovae before and found evidence to suggest that it is there. Dr Assaf Sternberg, of the Benoziyo Center for Astrophysics at the Weizmann Institute of Science in Israel, and his colleagues wanted to go one step further and find out whether all type 1a supernova have this material close to them.

By looking at spectra from type 1a supernovae, Sternberg and colleagues hoped to test their hypothesis. A spectrum contains the signature of everything that the light from a star had to pass through on its way to Earth. Spectra from supernovae with circumstellar material close by should provide a telltale sign of this in their spectra. The sign Sternberg and his colleagues looked for was absorption of light by sodium atoms, as this would indicate that there was cool, neutral gas present — the circumstellar material. They expected the lines in the spectra made by sodium absorption to be blueshifted, too. When something is “blueshifted” it is moving towards the observer — us — and away from its source, in this case the supernovae.

Sternberg and his colleagues looked at the spectra from 35 type 1a supernovae and 11 of another type, known as “core collapse” supernovae, that they got from the High Resolution Echelle Spectrometer (HIRES) at the Keck Observatory in Hawaii and Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan telescopes which are part of the Carnegie Institution for Science. They also studied 6 previously published type 1as supernova spectra and 7 core collapse ones.

Sternberg and his colleagues first found out how far away each supernova was and what kind of galaxy it was from, by looking in astronomical databases and using existing images taken in the Sloan Digital Sky Survey. They then looked carefully at the spectrum from each supernova, keeping an eye out for features that would give away the existence of circumstellar matter.

What the team found was that, more often than not, the spectra contained blueshifted sodium absorption features. They checked to see whether these features could have been produced by clouds of gas in the host galaxy of the supernova or wind blown by the host galaxy, as well as checking whether interstellar matter was the cause, but found that the most likely explanation was an inherent feature of the supernovae themselves — the circumstellar matter they were looking for. Their results suggest that type 1a supernovae, or at least some type 1a supernovae in nearby spiral galaxies, are born of binary star systems with only one white dwarf.

Hopefully, this will be one of the first steps on a path that will lead to a better understanding of the origin of these supernovae, and we will no longer be left with the lingering question of whether they really are a reliable tape measure for cosmic distances.

Reference
Sternberg A, Gal-Yam A, Simon JD, Leonard DC, Quimby RM, Phillips MM, Morrell N, Thompson IB, Ivans I, Marshall JL, Filippenko AV, Marcy GW, Bloom JS, Patat F, Foley RJ, Yong D, Penprase BE, Beeler DJ, Allende Prieto C, & Stringfellow GS (2011). Circumstellar material in type Ia supernovae via sodium absorption features. Science (New York, N.Y.), 333 (6044), 856-9 PMID: 21836010

Video credit: ESO/M. Kornmesser

Composite image of the central part of the Orion Nebula, near where molecular oxygen was found. Credit: ESO/M.McCaughrean et al.

The densest parts of the ISM are molecular clouds, which are dense enough that molecules can form within them. Molecular hydrogen, H₂, is the most common inhabitant of molecular clouds, but a recent discovery in one particular cloud — part of the Orion molecular cloud complex — has revealed another constituent, as well as the answer to a longstanding question in astronomy: molecular oxygen does exist in space.

Oxygen is the third most abundant element in the universe. Astronomers have known that atomic oxygen exists in space for a while now, although the amount of atomic oxygen is less than expected. Searches for molecular oxygen — two atoms of oxygen stuck together, O₂ — had been, so far, unsuccessful.

Now, molecular oxygen has been found near the Orion Nebula by Paul Goldsmith, from the Jet Propulsion Lab at Caltech, and an international team of collaborators. Their paper, which will be published in the Astrophysical Journal, explains how Goldsmith and his colleagues used the Herschel telescope to detect signals that gave away the existence of molecular oxygen.

They knew they had detected molecular oxygen when they saw three specific spectral lines using the HIFI far-infrared instrument on Herschel. The relative intensities of these lines enabled them to estimate the temperature of the source of the O₂ to be between 65 and 120 Kelvin (-208 to -153ºC).

A series of reactions that culminate in molecular oxygen formation are triggered when a cosmic ray ionises molecular hydrogen, H₂, turning it into H₂⁺. The H₂⁺ then reacts with more H₂, resulting in H₃⁺, which then reacts with atomic oxygen, O, to make OH⁺. When the OH⁺ bumps into hydrogen molecule, the reactions leads to H₂O⁺ and H₃O⁺, which in turn combine with electrons and make water (H₂O) and OH. The OH can react with atomic oxygen to make O₂. (Phew!)

Each step in this long chain of reactions has a reaction rate, many of which have been estimated using measurements in the lab. The rate of the final step, which produces the O₂, has been a topic of debate, but even the lowest estimates should produce lots of molecular oxygen in all but the coldest clouds of gas and dust. So why wasn’t any O₂ spotted before?

Two missions that have looked for molecular oxygen and failed are Sweden’s Odin mission, and NASA’s Sub-millimetre Wave Astronomy Satellite (SWAS). When each of these missions released results showing that molecular oxygen abundance must be lower than expected, astronomers tried to explain why this might be the case.

One hypothesis put forward was that oxygen could be freezing onto dust grains in areas with low temperatures. On the surface of the dust grain it would be converted into water ice, making it invisible to missions looking for oxygen.

Astronomers realised that when the dust grains in a molecular cloud are warm enough, the dust grain’s surface should release the water ice in which the oxygen is locked up. This meant that warm areas should contain more oxygen gas, with lots of it in the form of molecules.

Believing that warmth would be key to finding oxygen, Goldsmith and his colleagues pointed the telescope towards a region of Orion near which stars are forming. They hoped the star formation would heat up the surrounding gas, allowing oxygen molecules to exist on their own, not locked up on the surfaces of dust grains. And it must have, because Goldsmith and his colleagues found what they were looking for. By their estimation, in the region they looked at there was one oxygen molecule for every million molecules of hydrogen.

Goldsmith and his colleagues are sure that they have found molecular oxygen, but admit that there are still questions to be answered about the source of the emission. For starters, they didn’t see a lot of oxygen, and they still do not fully understand why some regions have more oxygen than others. But it’s a start.

Reference
Paul F. Goldsmith, Rene Liseau, Tom A. Bell, John H. Black, Jo-Hsin Chen, David Hollenbach, Michael J. Kaufman, Di Li, Dariusz C. Lis, Gary Melnick, David Neufeld, Laurent Pagani, Ronald Snell, Arnold O. Benz, Edwin Bergin, Simon Bruderer, Paola Caselli, Emmanuel Caux, Pierre Encrenaz, Edith Falgarone, Maryvonne Gerin, Javier R. Goicoechea, Ake Hjalmarson, Bengt Larsson, Jacques Le Bourlot, Franck Le Petit Massimo De Luca, Zsofia Nagy, Evelyne Roueff, Aage Sandqvist, Floris van der Tak, Ewine F. van Dishoeck, Charlotte Vastel, Serena Viti, & Umut Yildiz (2011). Herschel Measurements of Molecular Oxygen in Orion Astrophysical Journal arXiv: 1108.0441v1