Twinkle Twinkle Little Star

A young protostar and its signature outflow peeks out through a shroud of dust in this infrared image from NASA's Spitzer Space Telescope. Spitzer/Caltech

I thought I’d take a look at what stars are, how they form, how they work and what happens to them when they get old and there are no pension schemes for stars!

Out there, in the vastness of space, are huge, diffuse clouds of gas  and dust, the remnants of earlier generations of stars that have lived and died and whose remains now drift through space. They can continue  to do so indefinitely, the movement of the molecules creating pressure that holds the cloud up. Unless something happens. It could be that the gas cools sufficiently for gravity to take over, or the process can get a helping hand from a nearby supernova, sending a shock-wave through space which hits the cloud and causes ripples in it, waves of slightly higher density. These ripples start a chain reaction. Where the cloud is slightly denser, gravity will be a tiny bit stronger. This pulls in more gas which makes the local gravity stronger still in an ever-increasing spiral.

inducedsfArizona University

There are some factors which can limit this and disturb the formation of the new star. If there is too little gas then it will never become a star, as simple as that. But if there is too much gas accumulating too quickly then the heat generated by  the collapsing gas will act against gravity and either slow it down or blow it apart entirely.

Okay, so we’ve got all this gas collapsing in on itself due to its own gravity. What next? Well eventually the temperature and pressure in the centre reach the point where Hydrogen atoms begin getting crushed together to form Helium atoms thus giving off a little bit of energy. It isn’t actually very much energy each time it happens, but it happens is such vast quantities that it adds up to a lot. At this point the energy being released by this fusion reaction is enough to stop the protostar collapsing more and the star  is born in a burst of energy. After a while it settles down to a reasonably stable existence on the Main Sequence, which we’ll look at in more detail later. It’s a bit like a wild teenager becoming an adult and settling down with a secure job, a family and a sensible car.

What happens in fusion is that at extreme temperatures and pressures, the natural repulsion of 2 positively charged protons is overcome and they are forced together. It is actually a bit more complicated than that but the gist of it is as follows. In average stars like the Sun, the main process is called the p-p chain and what happens is that 2 protons, Hydrogen nuclei, fuse to give  a Helium2 nucleus, a positron, (an anti matter electron), a Gamma ray photon and a neutrino, ( an almost massless sub-atomic particle). This stage happens twice. Another proton then collides with each of the Helium2 nuclei to give Helium3 and a Gamma ray photon, (times 2!). Then these two Helium3 nuclei fuse to form Helium4, which is stable, and 2 protons which then whizz off to start the whole thing over again. You see, even stars recycle!


We can actually work out how much energy is produced. The mass of Hydrogen is 1.673533 x 10^-27 kg and the mass of Helium is 6.6464782 x 10^-27 kg and there are 4 Hydrogen nuclei per reaction which is 4 x 1.673533 x 10^-27= 6.6941303 x 10^-27 kg. The ” ^” means ‘to the power of’ and 10 to the power of -27 is just 10 point, and then 17 zeros like this 0.0000000000000000000000000006694…..etc. A very small number.

(6.6941303 x 10^-27)- (6.6464782 x 10^-27)= 0.047652 x 10^-27 kg of mass lost in the reaction. This lost mass is converted into energy as explained by good old Uncle Albert and what is probably the world’s  most famous equation




Using E=mc^2 we don’t have to be Einstein to plug in the numbers and get

E= (0.047652 x 10^-27) x (3.0 x 10^8)^2 = 4.2885 x 10^-12 J per reaction. ( The 0.047625×10^-27 is the mass of 4 Hydrogen  atoms minus the mass of 1 Helium atom).

If you think that doesn’t sound like very much then you are right, it isn’t. But then you have to multiply that by the millions of times it is happening every second and suddenly you have a very large amount of energy indeed. If we assume, for the sake of convenience, that the Sun consists only of Hydrogen, and that these reactions are going on throughout the Sun, then we can work out (very) roughly how much energy there is in the Sun.

We have worked out that each reaction produces about 4.3×10^-12 Joules of energy and each fusion reaction requires 4 Hydrogen atoms, so the total amount of energy available in the Sun is:

4.3×10^-12 x   the mass of the Sun divided by 4 times the mass of 1 Hydrogen atom or:

4.3×10^-12 x  (2×10^30)/ (4×1.67×10^-27) = 10^45 J.

That’s a lot of energy and enough to keep the Sun burning for billions of years. In fact we can even work out how many billion years this will last. By dividing the amount of energy stored in the Sun by its Luminosity, (its energy output) we get:

10^45 divided by 4×10^26 (multiplied by 3×10^7 to get the number of seconds in a year)= 8×10^10 years.

This is of course wildly inaccurate as these reactions only take place in the core of a star and in our Sun this about 40% of the Sun’s mass in about 10% of its volume. It also  assumes that the reaction is 100% efficient which it isn’t, nowhere near, but it does give an idea of the vast amounts of energy available through nuclear fusion and how stars can keep burning for billions of years.


The most massive stars can use Carbon to create Nitrogen and then Oxygen through collisions with Hydrogen but this is only possible in the most massive Main Sequence stars and the first generation of really big stars would have used the p-p reaction as there simply wasn’t enough Carbon in the early Universe, but here’s how the CNO cycle looks:


The “λ” sign is a Gamma ray photon here, the “e with a plus sign” is a positron and the “ν with a little e” is a Neutrino.

So that’s the basics of what a star is and how it works. Next time I’ll write about the Hertzsprung-Russell diagram and what it tells us about stars and then we’ll look at what happens when stars get old.


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