How do you make a star?

If you live somewhere with a very dark night sky you can see about 6000 thousand individual stars, assuming your eyesight is not to bad either. That’s out of a couple of hundred billion stars in our galaxy, the Milky Way. So clearly there is a lot we can’t see, some because they are simply too far away and not enough of their light reaches our eyes to register seeing them or others because there’s too much stuff in the way. As it turns out the stuff that’s in the way is actually very important for ensuring there are stars to see. Often when looking at stars I think about how did they appear? Did they just suddenly fire up and start fusing? The good news is we don’t have to speculate too much and we don’t have to dream up weird explanations, as our understanding of the Universe has allowed us to know how stars are formed. I’ve been doing a fair bit of reading on this lately so here is the Milky-Way.Kiwi version of star formation.

We start out on this journey in a cold patch of the Milky Way. It has to be cold because star formation doesn’t work very well where it is hot. This patch of space needs to be cold, freezing cold, ‘very’ extremely cold, so cold that it’s not far off absolute zero. It’s only about 10-30 degrees above absolute zero (which is -273 degrees Celsius), which we measure on the temperature scale in Kelvins (K). This super cold patch of space, fortunately, is not empty, it has some gas, not much, but some. The gas is mainly hydrogen but it’s not the hydrogen that’s whizzing around in those hot hydrogen clouds, this is molecular hydrogen. That’s hydrogen atoms that are joined together as a molecule, there’s two of them in the molecule, hence molecular hydrogen. If you grabbed a cubic centimetre of this space, then counted all of the hydrogen molecules you could find – you would find about 300. This might not seem much to kick off a star, a cold patch of space with hardly anything in it, how could that produce a blazing hot inferno that weighs 2 X 10^30 kilograms like our Sun? Well, as it happens, there’s quite a few of these cubic centimetres with 300 or so molecules in each and all together they can attract each other under the force of gravity.

Now if our cold, low density cloud of molecular hydrogen weighs about as much as about 90 Suns then we’ve got enough stuff to make a star. To get it to start clumping together is a bit complex and requires a bit of turbulence, not too much of a magnetic field and maybe a wee bit of a rotation just starting to be perceptible. When this happens the cloud starts to compress and this compression squishes the hydrogen molecules together and they start bouncing off each other and this increases the temperature of the cloud which increases the pressure that is working against gravity. When this local compression is well over 1000 times the average density of the cloud then gravity can overcome the thermal pressure that would normally stop the cloud squishing. Gravity is trying to squish the cloud more and if the heat has no where to go then it will hold up gravity and the star won’t form. Luckily the photons that are created when the molecules hit each other can get out of the compressing cloud so the energy is able to escape allowing the cloud to squish even more. This process keeps on happening as gravity crushes the cloud more and more and the temperature increases, as long as the heat can escape then all is good and the little hot patch of cloud will get denser and denser as gravity keeps winning the battle against thermal pressure.

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Little protostars captured in X-Ray (Credit: ESA/XMM/Hamaguchi)

The process eventually gets to a point, though, where the little clump of dense hot spinning gas cannot radiate the heat away so easily as the molecules are so close together. When this happens the protostar is born, it temperature and pressure rises dramatically and it retains almost all of its thermal energy which is then used to balance against gravity. The little baby star is now at about 3000K on its surface and energy is circulating through a process of convection. This situation can’t last forever as the protostar would eventually run out of energy, over quite a few million years. It needs to grow. Around the little protostar is a growing disk of gas that is being heated up and losing some of its angular velocity and it starts in falling onto the surface of the little protostar. This adds mass and the protostar grows, gravity from the new mass crushes the star more and the temperature within increases. Then a big change happens within the protostar. Deep inside the core most of the electrons have been stripped from the hydrogen atoms so photons that are being created in the core aren’t bouncing around and being absorbed by electrons, they are able to more freely exit the protostar and the main way transporting energy goes from being convective to being radiative. This greatly increases the amount of energy getting to the surface so the temperature goes up even more.

As the radiation can get out a bit more easily, gravity continues to win and crush the protostar even more until the temperature within the core hits about 10,000,000 K. At this point the hydrogen atoms, stripped of their electrons and are now just essentially protons, and they are moving so fast that they no longer easily repel each other through the electromagnetic force as they can get close enough that the strong nuclear force allows some of them to bind together. This is when fusion kicks off and the little protostar is all grown up into a proper star. It might grow a little more from the disk of material surrounding the star but as fusion stabilises and balances the inwards crush of gravity the star’s solar wind will blow the material away and clear out a nice clean space around the star. If it’s a star like out Sun then it has about 10 billion years worth of hydrogen to fuse in its core. And that is the short version of how to make a star.