Category Archives: Physics

How the Sun lost its spots

It may look like a static yellow ball from here, but in reality the Sun is alive with activity. Right now it is becoming more active each day as we get closer to the next solar maximum, which is expected to peak in July 2013. However, a couple of years ago it was quieter than it had been for nearly a century. It had very few sunspots and radiated very little energy. This variation is normal — the Sun goes through regular cycles where its activity and number of sunspots go up and then down again. What was unusual was the depth of this solar minimum.

Dibyendu Nandy, from the Indian Institute of Science Education and Research in West Bengal, and colleagues Andres Munoz-Jaramillo and Petrus Martens, from Montana State University, think they might have found the reason for this almost unprecedented solar calm.

An image of the Sun taken in September 2008 — not a single sunspot in sight. Credit: SOHO/ESA/NASA

Each solar cycle lasts roughly 11 years. After this time, its magnetic field flips over. After two cycles the magnetic field has flipped twice and it ends up back where it started. During these cycles the amount of solar activity goes up and down too.

Sunspots are a good measure of the amount of activity going on in the Sun at any point, and the number of sunspots on the Sun follow the 11 year solar cycles; there are more sunspots at a solar maximum and less at a minimum. A sunspot’s magnetic field is very strong and stops the transfer of heat from the interior of the Sun to the surface. Sunspots look dark because this loss of heat makes them cooler than their surroundings. In fact the surrounding area is brighter than it would be without the sunspot. This means that, counterintuitively, the more sunspots there are on the Sun, the more energy radiates out of it — even though it looks darker than usual.

Spotless days in red, number of sunspots in blue. Only cycle 14 had a deeper minimum than the last one (cycle 23). Credit: Nandy et al, Nature, 3rd March 2011

The last solar minimum was unusual because there were a very high number of days — about 800 — without any sunspots at all. Nandy and colleagues created a computer model to try to work out why this happened.

They found that great loops of electrical current, which flow in the plasma that makes up the Sun, were interfering with the formation of new sunspots. In a plasma, the electrons have been stripped away from their atoms, leaving them free to move about and conduct such currents. The currents flow around the surface of the Sun, going down into the interior at the poles and resurfacing at the equator. Dying sunspots get dragged underneath the surface, where their magnetic field is given a boost. They are then sent back up to the top to form a new sunspot.

Close up picture of a sunspot taken in ultraviolet light by NASA's TRACE spacecraft. Credit: NASA

During a deep solar minimum, however, it doesn’t quite happen like this. In the first half of the solar cycle the plasma flows quickly, but in the second half it slows down. This fast movement at the start stops strong magnetic fields forming inside the Sun, so that it eventually runs out of steam and stops making sunspots during that cycle. The slow plasma flow afterwards means that the formation of the next lot of sunspots takes a bit longer to get going that usual.

This all adds up to long stretches of time without a single spot on the surface of the Sun.

The team’s simulation, which modelled this physics, reproduced what we saw during the last solar minimum, showing that very deep solar minima are generally linked to the Sun’s weakened magnetic field.

Being able to predict when solar minima like this are going to occur is a very useful thing. When the Sun’s magnetic field is weakened, so is the solar wind. The solar wind is a stream of charged particles that are ejected from the Sun’s atmosphere and into space, and is responsible for aurorae, geomagnetic storms and the tails of comets, amongst other things. It also stops lots of cosmic rays getting into the solar system. When the Sun’s magnetic field is weakened, the solar wind lets more cosmic rays through, making space a more dangerous place. This new model will hopefully mean we can predict hazardous changes in space weather and plan missions accordingly.

Nandy D, Muñoz-Jaramillo A, & Martens PC (2011). The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations. Nature, 471 (7336), 80-2 PMID: 21368827


3D Sun iPhone app for photos and videos of the latest solar activity — very cool (hat tip to @Psycasm for telling me about this)

Sunspot plotter — find the number of sunspots on any day back to Jan 1st 1755

NASA animations of plasma flows and the sunspots they create

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Cut down to size: supermassive black holes turn out not to be so “super” after all

This post was chosen as an Editor's Selection for

You might not be able to tell from wherever you are reading this, but black holes in the distant universe just shrunk down to as little as a tenth of their previous size. This is not some cosmic disappearing act; a new analysis of supermassive black holes at the centres of active galactic nuclei has revealed that their masses were previously overestimated by up to a factor of ten. The paper was published in Nature last week.

A composite image of active galaxy M82, using Hubble, Chandra and Spitzer data. Credit: NASA, ESA, CXC, and JPL-Caltech

Active galactic nuclei, or AGN, are among the most luminous objects in the universe and are powered by massive black holes millions of times the mass of the Sun. Gas clouds, known as “broad line regions” for reasons that will become clear later, surround the black holes. These gas clouds range from a few light days to hundreds of light days across; they are much wider than our solar system. Astronomers have been studying these clouds for over thirty years, but had not worked out the why some of them were flatter than others — until now.

Wolfram Kollatschny and Matthias Zetzl from the Institute for Astrophysics, at the University of Göttingen in Germany, looked into the relationship between the shape and width of spectral lines observed in the emission spectra of AGN. An emission spectrum is produced when an object, a gas cloud for example, blocks a light source, such as a star. The light coming the source gets absorbed into the gas cloud, and is eventually re-emitted. Astronomers can measure the intensity and wavelengths of the light that gets re-emitted. Spectral lines are spikes in the emission spectrum, and represent a lot of light at a certain wavelength. They can tell astronomers what elements are present in the gas cloud.

However, for the gas clouds surrounding black holes, it is not quite that simple. The regions are spinning very fast around the central black hole, and the light emitted from them is subject to the Doppler effect. The Doppler shift is seen, or rather heard, in more everyday situations too — when the pitch of an ambulance siren seems to rise as it speeds towards you in the street and fall as it gets further away. That’s the Doppler shift affecting the sound waves. It happens because the ambulance is moving as it emits the sound waves, so the frequency of the waves _appears_ to change.* When gas rotates around a black hole, the same thing is happening to the light rays because some of the gas is moving away from the observer quickly and some is moving towards the observer quickly. This makes the spectral line astronomers eventually observe broader — an effect known as Doppler broadening. This is the reason the gas clouds are called “broad line regions”.

Kollatschny and Zetzl looked at 37 active galactic nuclei. They worked out that fast rotating AGN created broader spectral lines, and slower ones made more narrow lines. From their observations, they saw that faster rotating AGN had flatter gas clouds surrounding them, and slower ones had more rounded gas clouds. As they now knew how fast AGN were spinning, they were also able to come up with new, more accurate estimates of the masses of their central black holes. Previous estimates used just the spectral lines to estimate masses. This is a problem, particularly for very distant AGN, as astronomers can usually only see one spectral line from these — so there’s nothing else to check any estimated against.

The new black holes masses came out between two and ten times smaller than the previous estimates. While this isn’t going to cause any major problems for the black holes themselves — they’re still the most massive objects in the universe — it may pose a problem for astronomers studying the formation of black holes.


*Wikipedia has a good animation illustrating the Doppler Effect.

Kollatschny, W., & Zetzl, M. (2011). Broad-line active galactic nuclei rotate faster than narrow-line ones Nature, 470 (7334), 366-368 DOI: 10.1038/nature09761


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Caught in the act: sneak preview of galaxy cluster that’s still forming

Galaxy clusters are some of the largest structures in the universe. Astronomers have found these clusters, which are large groups of galaxies bound together by gravity, as far back as only 4 billion years after the Big Bang (less than a third of the age of the universe). They know they contain stars that formed even earlier than that. But nobody had caught a cluster while it was still forming — until now.

Region around starburst galaxy COSMOS AzTEC-3. The green circle is 13 million light years across. Credit: Capak et al. / Nature

Astronomers have found a “protocluster” that was around only 1 billion years after the Big Bang (that’s a redshift of 5.3 for anyone that’s counting). It sits in a region that is 40 million light years across and is rich in young stars.

The protocluster was found in data from the Cosmological Evolution Survey, COSMOS. COSMOS uses the Hubble, Spitzer and Chandra space telescopes with the ground based Keck Observatory and Japan’s Subaru Telescope to get an good look at the universe. COSMOS looks at a tiny region of space — about 0.005% of the whole sky, or two square degrees — in all wavelengths of light, from radio to gamma waves.

Peter Capak, the lead author on the paper published in Nature last week, and colleagues knew that extremely bright objects such as starburst galaxies (galaxies with an unusually high amount of star formation) and quasars (the bit at the centre of a massive galaxy that surrounds the supermassive black hole) should exist in very young galaxy clusters, so they first looked for objects giving off a lot of radiation. They found objects emitting a lot of visible light by measuring optical and near-infrared radiation, starburst galaxies by taking radio wave measurements and quasars using X-rays. Once they had located these extreme objects, they looked in the areas surrounding them for unusually large numbers of galaxies given the size of the area — something they called “overdensities”. They then used Hubble and Subaru to measure how far away these extreme objects were, and the Keck II telescope in Hawaii to confirm the observations.

Capak and colleagues were particularly interested in an “overdensity” near a starburst galaxy known as COSMOS AzTEC-3. The area in question contained over 50 billion times the mass of the Sun in gas (and ten times more dark matter), and was brighter than 10 trillion Suns. Stars in the region are forming at a rate of over 1500 a year, more than a hundred times the average value.

Six of the 11 very bright objects in the protocluster. COSMOS AzTEC-3 is labelled "Starburst". Credit: Capak et al./Nature

Around COSMOS AzTEC-3 there were 11 bright galaxies — 10 more than would normally be expected. This led to the conclusion that what they were seeing was the beginnings of a galaxy cluster, known as a protocluster.

Chandra X-ray observations helped to pin down a quasar very close to the protocluster. It can be difficult to find quasars that are this far away because, although they are the most luminous objects in the universe, they’re not usually bright enough to be seen by the telescope. But this one was. The astronomers worked out that the quasar’s black hole must have a mass of between ten and a hundred million times the mass of the Sun.

Putting all of this information together, astronomers worked out the total mass of the protocluster. It weighs in at at least ten billion time the mass of the Sun, but could be up to a hundred billion Suns. This confirmed what the astronomers suspected: they were looking at one of the biggest and brightest objects at this distance.

Astronomers also measured the amount of gas in the protocluster. This is what will fuel the protocluster’s growth. They found more than enough to point to a very bright, and massive, future for the baby cluster. It will eventually evolve (or rather, confusingly, already has evolved — depending on which way you look at it) into a massive galaxy cluster. Massive clusters of galaxies have been found from around 4 billion years after the Big Bang, giving this one at least 3 billion years to grow up.

Capak PL, Riechers D, Scoville NZ, Carilli C, Cox P, Neri R, Robertson B, Salvato M, Schinnerer E, Yan L, Wilson GW, Yun M, Civano F, Elvis M, Karim A, Mobasher B, & Staguhn JG (2011). A massive protocluster of galaxies at a redshift of z ≈ 5.3. Nature, 470 (7333), 233-5 PMID: 21228776

It’s also on arXiv.


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A galaxy far, far away… (take two)

Last November I wrote about the most distant galaxy ever seen. Since then, one even further away has been found. This post is (slightly) adapted from an article I wrote for Felix about this new galaxy.

The newly discovered galaxy is circled in the top left hand corner. Credit: NASA/ESA

There haven’t always been stars and galaxies in the universe, and the time when they began to form — known as the reionisation epoch — is the subject of much interest in astrophysics. A paper published in Nature details a discovery that could tell us more about this mysterious time.

In the paper, Rychard Bouwens and colleagues say they have detected the most distant galaxy ever seen; the light from the galaxy was emitted only 500 million years after the Big Bang. This age puts it well within the epoch of reionisation.

The galaxy has the highest redshift ever observed; it was from this that the team were able to calculate the galaxy’s age.

Cosmological redshift is a measure of how fast an object is moving away from the Earth and is a consequence of the expanding Universe. Objects with higher redshift are moving away from the Earth faster than those with lower redshifts, which means they are further away from Earth and also further back in time. To put it simply, the higher the redshift the older the galaxy.

Bouwens and colleagues used something known as the Lyman-break technique to identify the galaxy. This technique relies on a sharp drop in the spectrum of a galaxy that is due to the absorption of energetic photons by neutral gas that surrounds galaxy forming regions. The discovery can then be confirmed by looking at optical images.

For a long time the observations required to study the reionisation epoch were out of reach, but recent images from Hubble are making the detection and study of far away galaxies possible for the first time. The new galaxy was discovered in images taken by Hubble’s Wide Field Camera 3.

Bouwens and colleagues also looked into the rate of star formation at the time just after the newly discovered galaxy. They discovered that in just 200 million years the rate of star formation increased tenfold. This confirms that the newly discovered galaxy is right in the heart of the reionisation epoch, and sheds new light on how the stars and galaxies we see today formed.

It seems that galaxies around at the same time as this new discovery may not have been able to fully reionise the universe. This leaves the means by which the universe went from being a neutral gas to an ionised one, with electrons and protons stripped away from each other, a mystery. However, the existence of galaxies at this time does point to the first stars forming 100 million years beforehand, or around 400 million years after the Big Bang.

“We’re seeing huge changes in the rate of star birth that tell us that if we go a little further back in time we’re going to see even more dramatic changes,” said Garth Illingworth, a co-author of the paper from the University of California at Santa Cruz. “We’re moving into a regime where there are big changes afoot. Another couple of hundred million years back towards the Big Bang, and that will be the time when the first galaxies really are starting to build up.”

Bouwens, R., Illingworth, G., Labbe, I., Oesch, P., Trenti, M., Carollo, C., van Dokkum, P., Franx, M., Stiavelli, M., González, V., Magee, D., & Bradley, L. (2011). A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 Myr Nature, 469 (7331), 504-507 DOI: 10.1038/nature09717

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First Planck results: from the coldest to the largest objects in the universe

Last week astronomers working on the European Space Agency’s Planck experiment convened in Paris to talk about their first results, and they weren’t short of things to say. No less than 25 papers were announced on Tuesday 11th January — and this is before work has even started on the mission’s main aim of putting together a detailed picture of the Cosmic Microwave Background, or CMB.

The CMB is a uniform glow of microwave radiation, with only tiny fluctuations, that gives us a snapshot of the universe around 380,000 years after the Big Bang. We’ve seen it before, courtesy of WMAP in 2003 and COBE in 1992. But Planck has the power to look at this faint glow in never-before-seen detail, revealing more about the universe than every before.

Video showing locations of the different compact objects found by Planck.

Before they can get to work on this new view of the CMB, however, astronomers must study the foreground noise of the picture in detail. This “noise” is made up of structures formed after the CMB: galaxies, galaxy clusters, and matter within the Milky Way, such as gas and dust.

Planck astronomers studied this “noise” in order to better understand how they can remove it from the picture, and just see the CMB. It’s called noise because it gets in the way when we try to look at the CMB, but actually it’s very interesting in its own right. These structures can tell us a lot about the formation of stars, galaxy clusters and even the universe itself — and that’s what some of the 25 new papers announced last week are about.

One of the superclusters detected by Planck and confirmed by XMM-Newton. Credit: ESA/Planck Collaboration

First up, we have galaxy clusters. They’re the largest structures in the universe, and the Planck mission has just completed the first all-sky survey of them using something called the Sunyaev-Zel’dovich effect (SZE). Galaxy clusters don’t just contain galaxies; they also hold hot gas and a large amount of dark matter. The SZE arrises when high energy particles in the hot gas interact with the CMB and distort it. Astronomers can see this distortion in the CMB and use it to detect galaxy clusters.

In total, 189 galaxy clusters have been detected by Planck using the SZE. This includes 169 that had already been detected using other methods, and 20 brand new ones. The really interesting thing about these galaxy clusters is the huge range of masses they encompass — between one and fifteen hundred trillion times the mass of the Sun. Galaxy clusters are extremely sensitive to the underlying framework that describes our universe, and so can shed light on the evolution and structure of the universe.

By working together with another experiment at the ESA, the XMM Newton X-ray observatory, 11 of the newly discovered galaxy clusters have already been confirmed. XMM-Newton has been able to get a closer look at some and reveal that two of the new clusters are in fact superclusters. That is, they are clusters of galaxy clusters, rather than simply clusters of galaxies.

Map showing cold, dense clumps of dust in the Milky Way. Credit: ESA/Planck Collaboration

Not content with studying the largest structures in the universe, Planck has also taken a look at the coldest.

Thanks to Planck, we can now detect material at lower temperatures than ever before. And we can do it more accurately than ever before, too. Astronomers working on the Planck mission have just finished looking at the results of the first all-sky survey of compact cold dust clumps in the Milky Way, and cool dust in other galaxies. These clumps are some of the coldest objects in the universe, and are key to understanding some of the hottest — cold, dusty clumps, like the ones seen by Planck, are believed to be sites of star formation.

These clumps have temperatures of only 7 to 16 degrees above absolute zero. Most of the clumps Planck found were only a few light years away from Earth, but some were up to eight thousand light years away*.

Though Planck was only able to look in detail at this dust in our own galaxy, the results are vital to understanding the behaviour of similar dust in other galaxies. When we look at galaxies that are further away, we also see them as they were further back in time. As we learn more about star formation in our own galaxy from these cold clumps, we will begin to have a better understanding of star formation in galaxies that are further away — and further back in time.

However, Planck has limitations, and these mean that it cannot look into the heart of these cold objects. This is where the ESA’s Herschel space observatory comes in. It has a much higher resolution than Planck and has no problem seeing the detailed structure of the clumps. Between them, Herschel and Planck can form a complete picture of the clumps at both small and large scales. With their help, we can effectively reel in far away galaxies for a closer look and learn about star formation throughout the history of the universe.

Cold dust clumps and galaxy clusters are just two of the interesting discoveries Planck has made in the two full sky surveys it’s completed since it launched in May 2009. It will continue to survey the sky until at least the end of 2011, but full results, including that new detailed picture of the CMB minus the noise, will not be published before early 2013. That might seem like a long way away, but these first results should keep astronomers busy for a little while yet. Then they can get on with the job of studying the CMB, which will no doubt keep them busy for an even longer time to come.

*A light year is roughly three thousand billion miles. For comparison, the Sun is just less than a hundred billion miles, or eight light minutes, from Earth

More on Planck
Watch the press conference
More about Planck’s findings at the BBC
The 25 papers published by the Planck team


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Black holes are not fed by colliding galaxies after all

This post was chosen as an Editor's Selection for

It’s not a question you’re likely to have ever considered, but the source of “food” for some of the most active black holes has been a longstanding line of inquiry for the astrophysics community. Many thought they had the answer when several studies seemed to show a link between collisions of similarly sized galaxies and the formation of a very active black hole at the centre of the merged galaxy. But a new survey of 1400 galaxies has answered the question once and for all and it turns out that, in most cases, this link doesn’t actually exist.

Jet powered by a black hole at the centre of the galaxy M87, 50 million light years from Earth. Image: NASA

It’s long been known that at the centre of most galaxies lies a black hole. Some are relatively quiet, like the one in our own galaxy, but others manage to take in some of the matter that surrounds them and then spit it out in the form of huge amounts of energy. There’s a name for what’s created by the most lively ones: Active Galactic Nuclei, or AGN for short. Scientists don’t quite understand why some black holes in galaxies are more active than others, but research done last year by NASA’s Chandra X-ray Observatory did shed some light on how black holes grow in galaxies, by looking at how many of them are active at any one time and the size of the galaxies they inhabit. A new study, published in the Astrophysical Journal today, goes one step further and tests whether galaxy mergers trigger the creation of energetic AGN.

AGN give themselves away by emitting radiation. To do this, they take in gas and dust from their surroundings and heat it up before pouring it out again in the form of X-rays, radio, UV or other radiation. What we don’t know is how they take in this matter in the first place. Contrary to popular belief, black holes don’t actually suck in everything that surrounds them. In fact, unless some matter is heading in a straight line towards a black hole, it is much more likely to end up in orbit around it than falling into the black hole itself. This may sound counterintuitive, but it happens for the same reason that Earth and all the other planets in the solar system orbit the Sun — as long as an object has some sideways motion, or angular momentum, it will orbit rather than fall into the more massive object. Scientists have some idea of how matter could overcome this angular momentum on smaller and larger scales, but not in this situation.

Active and inactive galaxies showing varying levels of distortion. A large amount of distortion would give them them away as having undergone a major merger with another galaxy. Image credit: NASA/ESA and M. Cisternas (MPIA)

This new paper uses galaxies imaged by the COSMOS survey. What makes this study different from all the others is that the researchers included a control sample. As well as images of 140 galaxies with active black holes at the centre, they used 1264 images of galaxies they knew to be inactive. The pictures were given to ten galaxy experts at eight different institutions who were asked to classify them as “distorted” or “not distorted”. A galaxy with an active galactic nucleus will have certain tell tale signs giving away its AGN, so these giveaways were removed to make the trial “blind” — a technique always used in medical trials and other branches of science, but not often required in physics where the work is usually done by computers. Blinding the study makes sure that the people judging whether the galaxies are distorted are not, consciously or subconsciously, biased by any belief that active galaxies are more likely to be distorted than inactive ones.

The researchers found that, in the majority of the cases, there was no evidence that galaxy merger trigger the creation of active galactic nuclei. This means that someone will need to some up with another, more peaceful suggestion of how AGN get fed. A few ideas exist, such as “galactic harassment” — the fly by of another galaxy that gets close enough to disturb but not to do any damage or merge with the original galaxy — or the collisions of clouds of gas within the galaxy. But more research is needed to establish which, if any, of these ideas are the right one.

However, only galaxies around in the last eight billion years were included in the study, so the questions still remains of whether AGN created in the more distant past were triggered by mergers. In fact, this is the next problem on the groups to-do list.


Cisternas, M., Jahnke, K., Inskip, K., Kartaltepe, J., Koekemoer, A., Lisker, T., Robaina, A., Scodeggio, M., Sheth, K., Trump, J., Andrae, R., Miyaji, T., Lusso, E., Brusa, M., Capak, P., Cappelluti, N., Civano, F., Ilbert, O., Impey, C., Leauthaud, A., Lilly, S., Salvato, M., Scoville, N., & Taniguchi, Y. (2011). THE BULK OF THE BLACK HOLE GROWTH SINCE z ~ 1 OCCURS IN A SECULAR UNIVERSE: NO MAJOR MERGER-AGN CONNECTION*
The Astrophysical Journal, 726 (2) DOI: 10.1088/0004-637X/726/2/57


Click here to read the paper on arXiv.


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Scientific American Guest Blog post

I have a post up on the Scientific American Guest Blog today. It’s all about strange exoplanet discoveries, and what they can tell us about our own solar system. I really enjoyed researching and writing it, so I hope you’ll enjoy reading it too…

“Habitable and not-so-habitable exoplanets: how the latter can tell us more about our origins than the former”

Beta Pictoris and its planet

The image above is of one of the planets I talk about, and its star. It’s called Beta Pictoris b and is only around 60 light years from Earth. You’ll have to go read my post to find out what’s unusual about it, though.

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