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!

Big data from new telescopes

I read an article recently on the Guardian's science website called Big universe, big data, astronomical opportunity. The 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!

X-rays from stars

Tomorrow I'm going to a small conference at the University of Warwick to talk about X-ray emission from stars and our Sun. It may surprise many of you to learn that our Sun is a relatively bright source of X-rays, and it certainly surprised most astronomers when this was discovered in the 1940s!

Why is this such a surprise? Well, the surface temperature of the Sun is a relatively modest 6000 C, which may seem like a high temperature to you and I, but its certainly not hot enough to emit X-rays, which require temperatures of millions of degrees. And yet when astronomers launched the first rockets above our atmosphere in the 1940s they found that the Sun was a bright X-ray source!

X-ray image of our Sun taken from the Solar Dynamics Observatory
(Credit: NASA/SDO)

As it turns out these X-rays don't actually come from the surface of the Sun but from a region above the surface called the corona. You can see this in the X-ray image above, which shows the corona reaching above the surface of the Sun. The corona is a hot, low density plasma of gas, which reaches temperatures of millions of degrees, making it very bright in X-rays but almost completely invisible in the optical part of the spectrum.

So why is the corona so hot if the surface of the Sun is much cooler? Well this is actually an open question that many astronomers are still trying to answer, and its a question I've tried to answer by studying other stars. Our current best guess is that the corona is heated by the release of magnetic energy generated deep within the Sun in a type of dynamo. Unfortunately we can't see within the Sun to work out how and why this is happening, which is why this is still an unanswered question. Hopefully tomorrow's meeting will shed some light on this topic!

Tycho Brahe's gin

Last year I attended a conference in Copenhagen and while I was there I wanted to visit Tycho Brahe's observatory. For those who don't know, Tycho Brahe is considered to be one of the greatest astronomical observers in history, particularly in the pre-telescope era in which he lived, and he made many notable contributions to astronomy, including reporting on the 1572 supernova that now bears his name.

Hven gin, made on the island that was once home to
Tycho Brahe's observatory.

In 1576 he constructed an observatory called Uraniborg, which was apparently very impressive. It was built on the island of Hven in the Baltic Sea between Sweden and Denmark, and I was interested in visiting the observatory to see what remained. Unfortunately, as I found out,  the observatory doesn't exist any more, which is a great shame. Luckily for me though I was at least able to bring some part of Hven home with me when I found out that the island is now home to a gin distillery, and since my wife enjoys gin I was able to find a bottle in a Copenhagen liquor store and bring her a bottle home. I'm told it's very good, Tycho would have been proud!

Which parties will improve science in the UK at this year's general election?

While this year's general election in the UK may not be focussed on science and engineering, these are areas that are very important to the economy of our country. Despite this, science and research have suffered over the last decade, particularly under the last government. As a fraction of GDP the UK's research spending is currently the lowest amongst the G8 and the world's major economies, and in 2012 research spending dropped below 0.5% of GDP for the first time ever.

Science funding as a percentage of GDP (Credit: The Guardian)

The UK is clearly dropping behind these other economies in terms of research spending and productivity, which will seriously affect our future productivity and economy. Since this is an important issue it can be useful to know where the major parties stand on science education and funding, and what they plan to do about these issues if they were to get into power.

To answer this question I've trawled through the various parties' manifestos, as well as their letters to the Campaign for Science and Engineering, to try to find out what the different parties plan to do about science after the election. This meant a lot of sorting through vague statements of support for science and education to find actual plans and promises that might mean something over the next 5 years. Here's what I found.

The two main parties, the Conservatives and Labour, aren't really making any strong promises. The Conservatives are promising to continue the current science budget for another year, though since that is a fixed budget it would likely equate to a drop in actual spending power due to inflation. Labour are promising to ensure that all young people study Maths until age 18, which would probably benefit scientific literacy levels, but only as a by-product.

The leaders of the four major UK parties, but who is best to
improve science in the UK? (Credit: BBC) 

The Liberal Democrats go a little further, partly by promising to continue the ring-fencing of the science budget, but also by ensuring that by 2020 both research capital and revenue spending increase in line with inflation. This is an improvement over the Conservative's fixed science budget, though it doesn't go as far as other parties are promising to go, as we'll see later. The Liberal Democrats also have a more promising track record on these issues, by attempting to get more scientifically-literate MPs into parliament and also by introducing a 5p plastic bag charge into supermarkets.

The UK Independence Party are promising to abolish tuition fees, though only for students of STEM subjects who stay and work in the UK for 5 years following their graduation. Offsetting this strong move though UKIP are also promising to leave the European Union, which they claim will reduce the regulations which hamper science and technology (I can't speak for any of these regulations myself), but this will also lose UK science of up to 1 billion Euros of funding that we currently receive from the EU every year. Furthermore UKIP are promising to repeal the Climate Change act of 2008, which includes carbon budgets and targets for this half century, and this, I think, is a dangerous move given the way the world's climate is going at the moment.

The Green Party
(Credit: Wikipedia)

The Green party have, perhaps unsurprisingly, made some of the most impressive promises, including doubling public spending on research over the next ten years to reach 1% of GDP, the sort of levels already close to being reached by major research powerhouses across the world such as the USA and South Korea. In addition to this they want to end undergraduate tuition fees and reintroduce student grants, both initiatives that should encourage more people to go to University, study the sciences, and therefore improve the scientific literacy of our workforce.

And finally, while I myself can't vote for either the Scottish National Party (SNP) or Plaid Cymru, since I do not live in either Scotland or Wales, it is worth noting that both parties promise to establish free education for all, including abolishing University tuition fees.

To the majority of people in the UK, there's a stark choice between the commitments the major political parties have made to science. The two largest parties, Labour and the Conservatives, clearly think that science is important, but not important enough to make clear and ambitious promises for how to improve science and engineering in the UK. The Liberal Democrats are offering a little more, as do the SNP and Plaid Cymru (if you are able to vote for them), but none of the parties go as far as the Green Party in making commitments to science for the next parliament.

Hopefully this has been useful for deciding who to vote for in the next election. Whoever you vote for, and whoever gets into power, I hope that science and engineering do well under the next government, for the benefit of our own country and everyone in it.