Wednesday, 29 July 2015

The nearest site of star formation, the Taurus Molecular Cloud

Following my recent post about how stars form I wanted to take some time to talk about a few of the most prominent star forming regions that astronomers have studied and what they have taught us about the star formation process. The first of these regions I want to talk about is the Taurus Molecular Cloud.

The Taurus Molecular Cloud is, unsurprisingly, in the constellation of Taurus. It is the nearest star forming region to our Sun and so it is the region that astronomers have been able to study in the most detail. The proximity of the Taurus Molecular Cloud means that it spans a relatively large area on the sky, almost 10 degrees across, but the lack of a bright emission nebula means that all that can be seen even with a good backyard telescope is the obscuration of the background stars due to the gas and dust in the cloud.

The image below shows where the Taurus Molecular Cloud can be found in the night sky, approximately halfway between Elnath (the second brightest star in the constellation of Taurus, to the upper left) and the Pleiades open star cluster.

Location of the Taurus Molecular Cloud within the constellation of Taurus, and near the Pleiades star cluster. North is up in this image, East to the left (Image adapted from one by David Malin)

The Taurus Molecular Cloud was discovered in 1852 by J.R. Hind, which he noted as a faint nebulous object on the sky. Astronomers were soon able to take a spectrum of the light from the nebula and were able to confirm that it was indeed a giant cloud of gas. However, its importance as a site of nearby star formation wasn't immediately recognised.

The archetypal young star, T Tauri, visible in the centre
of this image and surrounded by a small dusty cloud
At the beginning of the 20th Century many astronomers became interested in a number of variable stars identified in the vicinity of the Taurus Molecular Cloud, the most prominent of which was named T Tauri after the constellation in which it was found.

Searches for other variable stars in the vicinity of dark nebulae produced many candidates in the 1940s and 1950s. These stars were often found in the vicinity of young OB stars (massive stars with particularly short lives, hence they must be young), leading many astronomers to believe that they were particularly young stars themselves. This discovery led to the realisation that the dark nebulae that these stars were found near was likely where these stars had formed. This was how the modern theory of star formation began!

The star T Tauri is now firmly recognised as the prototypical young star, and its name has been given to the class of young stars that share its properties, T-Tauri stars. It is thought to be less than a few million years old, already formed but still accreting material and growing in mass. The star itself is actually in a binary system with a fainter star, and is thought to be surrounded by a disk of material that is in the process of accreting onto the star, which partly explains the variability of the star that initially brought it attention.

An infrared view of the Taurus Molecular Cloud (Credit: FCRAO)
With the invention of infrared detectors in the second half of the 20th century astronomers were able to start peering into these dark nebulous clouds to study the star formation process within them, and as the nearest such cloud Taurus was a major target for early infrared astronomy.

The infrared image shown here penetrates the dusty molecular cloud and allows astronomers to see the giant gas clouds that are in the process of collapsing into stars. This image shows the cloud in immense detail, with filamentary tendrils of gas and dense cores where stars will one day form.

Because the Taurus Molecular Cloud is not large enough to be forming any really massive stars the region is spared the destructive powers that these stars can inflict on their surroundings. This means that there is still considerable molecular material in the cloud, including many molecular ices, despite the fact that many stars have already formed. This means that star formation is still ongoing and may be able to continue for a while, all thanks to the tranquil nature of the stars formed here.

The rise of infrared astronomy has also led to an increase in the number of young stars discovered in the cloud. Prior to this only the bright and optically visible young stars, such as T Tauri itself, had been identified. But infrared observations allowed astronomers to peer into the dark and obscuring clouds and identify many more young stars, and several hundred are now known.

The distribution of young stars (red stars and triangles) in the Taurus Molecular Cloud, show against a map of the molecular hydrogen in the cloud. Yellow diamonds, blue squares and green circles show young stars with known outflows. (Credit: Narayanan et al. 2012

This large sample of young stars has been vital in helping astronomers learn about star formation. For example, the distribution of these stars, as can be seen in the image above, coincides strongly with the distribution of the dense molecular gas, suggesting that stars form in regions of particularly dense gas. Furthermore with so many young stars all roughly of a similar age, astronomers have been able to produce models for how young stars of a given age would appear as they finished forming, and were then able to compare these models with the stars discovered in Taurus.

All in all the Taurus Molecular Cloud has been vital for how astronomers have learnt about the star formation process. It has provided a rich, nearby laboratory to study the dark nebulous clouds in which stars form and also to observe the final stages of the star formation process itself. Next time you look up and see the constellation of Taurus, see if you can spot the dark clouds of the Taurus Molecular Cloud and think about how important this region has been for astronomy!

Thursday, 23 July 2015

How do stars form?

Stars, like our Sun, are forming all the time across out Galaxy and in other distant galaxies. Understanding how stars form and what causes different types of star to form is one of the most important areas of research in astrophysics. Today I'd like to discuss the star formation process, what we know about it, and what we are still trying to understand.

Stars form out of dense clouds of gas (mostly made of hydrogen and helium) known as molecular clouds, so-called because many of the atoms in them have cooled and formed molecules. These molecular clouds are huge and are mostly found in the spiral arms of galaxies such as our own. The clouds are very cold, with temperatures of only 10 to 20 Kelvin (about -253 Celsius) and made of molecular gases such as H2 and CO.

The Whirlpool galaxy imaged in visible light (left) showing young stars and star-forming regions delineating the spiral
arms and a radio image (right) showing emission from the CO molecule tracing the molecular clouds in which stars form (Credit: NASA / PAWS)

These molecular clouds are thought to be held in balance between the inward force of gravity (which tries to make them collapse) and the outward pressures of magnetic fields and the motions of the molecules in the cloud (which are trying to make the cloud expand and disperse).

Eventually though something has to give and some part of the molecular cloud will begin to collapse. As it does so it will also cool as the molecules in the cloud release energy through as process known as radiative cooling, which helps the cloud collapse further. If the molecules weren't able to cool down while the molecular cloud contracted then the increase in density would cause them to heat up and the molecular cloud would expand and disperse, so this cooling is critical for star formation.

The dark cloud Barnard 68 (Credit: Marco Lombardi)
As this happens the molecular cloud will begin to fragment into smaller and smaller clumps of gas, each becoming denser and denser as they contract in towards their centres. In fact the density can reach so high that no light can penetrate to the centres of these clumps, making them so dark that they even block the light from background stars. We call these objects dark clouds, because they appear as dark patches on the night sky!

Once these dark clouds are dense enough that they can block out starlight then they cool even faster because they are no longer being heated by the light from nearby stars. Once these clouds have cooled even further then they can even block infrared radiation and become so cool as to not even emit infrared radiation. Only the coldest objects in the Universe are so cold as to not emit infrared radiation!

Once the centre of the clump has collapsed considerably a dense, gravitationally stable core forms in the centre, known as a protostar, which begins to heat up as it continues to contract. The protostar continues to grow in size by accreting more material from the surrounding molecular cloud, its core getting denser and hotter as it does so, and after a while the protostar begins to radiate energy into the surrounding molecular cloud.

A forming protostar surrounded by a disk of material accreting onto it
(Credit: ESO)
At this point the protostar is massive enough that it attracts considerably more material from the surrounding molecular cloud, which falls towards the star. Due to the conservation of angular momentum this material spirals in towards the star and forms a disk of material that orbits the star, slowly accreting onto the star in bright bursts that illuminate the surrounding cloud. With each burst of accretion the star becomes hotter and more massive.

Eventually the core of the protostar becomes so dense and hot that the temperature is high enough for nuclear fusion to take place. At first the star can only burn deuterium, but as it gets hotter it will eventually burn hydrogen just like our own Sun. The star is now beginning to shine quite brightly and the radiation from the star prevents further material accreting onto the star and may even begin to disperse the remaining material in the disk that still surrounds the star.

Once the star has started fusing hydrogen into helium we say that it has fully formed. Hydrogen fusion is the process by which the vast majority of stars create their energy, and the star can usually maintain this for billions of years before it runs out of hydrogen in its core.

This is the rough process by which we think stars form, and there is a lot of evidence to support this picture, including observations of forming stars and computer simulations that try to model the entire process. There are however a number of outstanding questions that scientists are still trying to answer, such as: How are stars clustered when they form (for example in clusters and OB associations) and what causes this? What causes stars to form with different masses? And what brings the star formation process within a molecular cloud to a halt? These are questions that astronomers such as myself are actively trying to answer!

Tuesday, 14 July 2015

Big data from new telescopes

I read an article recently on the Guardian's science website called Big universe, big data, astronomical opportunityThe article discussed the rise of large datasets in science, particularly in astronomy, and the need for astronomers to adapt to the challenges faced by this by acquiring new skills for analysing such data.
The Large Synoptic Survey Telescope
(Credit: LSST)

The article made many good points about the rise of big data. We are, after all, on the verge of a number of large projects that will produce more data than we've ever seen before. These include the Gaia satellite (which I've talked about before), new sub-mm and radio telescopes such as ALMA and the Square Kilometre Array, and the Large Synoptic Survey Telescope. The latter is particularly noteworthy because once it enters operation in 2022 it will produce 30TB of data each night, more than most telescopes produce in a year!

Astronomers will need to learn not just to manage these datasets, but to effectively sort through and process them. New algorithms will need to be designed, utilising artificial intelligence and advanced machine learning methodologies, as well as new approaches to visualise and understand the results. Astronomers will have to embrace this because this is the way science is going, and the telescopes that will produce this data are already being built, or in some cases are coming online as we speak.

Despite making these good points the article then appears to suggest that these new telescopes and satellites, designed to answer the most far-reaching questions humanity has ever asked, aren't what we need. "Have we leaned all that we can from the data that we have?", the author asks, suggesting that we should analyse all the data we have before collecting more. The article even suggests that the data we have could "hold answers to some of the fundamental questions of the Universe we are seeking".

I strongly disagree with the author on this point and find this attitude rather short-sighted. The data that we have may provide answers to some questions, but certainly not the big questions we want to answer, and that's where we should be investing our efforts. We won't find answers in our existing data to questions such as how the Sun and the Earth formed, how the Universe began, or whether there is life out there, it's just not possible.

Astronomical research is the continuous pursuit of answers to the greatest questions we can ask. We usually can't answer these questions in one go so instead we answer them step-by-step, performing experiment after experiment, refining the question as we go. For each experiment we design the equipment needed to answer the question, and then when that question has been answered we take the research further. If the same equipment or telescope can be used to answer the next question then we use it, but often the next question involves looking further, deeper and at different wavelengths, requiring new telescopes and equipment.

Just because a lot of data already exists doesn't mean that the answers to our questions can be found there. The great explorers of the past didn't discover new continents by searching around at home, they built great ships and went in search of discovery. Astronomers shouldn't be searching around in old data, but building great telescopes and searching the skies for their discoveries!