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!