What elements are left behind when a star dies?

Question posed by Kunal

To answer this question it would be helpful to understand a little bit about the elements and how they are made. Below is an excellent periodic table hosted by the University of Arizona website. Please click to visit it, and see a full-sized (and easier to read) version:


All of the elements are made from the same building blocks: protons, neutrons and electrons. The properties of each element depend on how many of each of these particles any particular element is made up of. The atomic number, in the top left corner of each box on the periodic table above, shows how many protons make up that element and is the main factor in defining each individual element. For example, oxygen always has 8 protons, iron always has 27 protons, and molybdenum always has 42 protons.

Most of these building blocks are locked up in the atoms of different elements, with the most abundant being the lightest one, hydrogen.


So which elements are left behind by stars?
All of them.

But it depends mostly on how massive a particular star is as to which elements it leaves behind.

All stars begin with hydrogen as a fuel and, through a process called fusion which is sparked off by the gravitational collapse of the star, turn it into the next-lightest element, helium. For small stars up to around the mass of our Sun, this is pretty much the whole story, and all that is left behind after the star dies is mostly helium.

For stars up to around eight solar masses, a little bit more goes on in their lifetime. Things start off in the same way as for all stars: A mostly hydrogen fuel source is 'ignited' into fusion, and helium is produced. After a while, because of the extra gravity due to the greater mass, the helium can join in with the fusion process as well, and heavier elements like carbon and oxygen begin to be produced. The more massive the star, the further this process goes, working all the way up to the production of iron in the heaviest of stars.

Once we get to iron, though, things change. Forming each element so far results in an energy release, which is what keeps a star burning. Once you get past iron, the formation of the elements require extra energy to be put in to the equation, rather than energy being given out.


So how do we get heavier elements?
When a large star dies it does so spectacularly. A star, when it stops burning via the processes outlined above, will collapse in on itself. If the star is massive enough, this collapse will ignite one final, furious burn which causes the outer layers of the star to explode. This is called a supernova. Supernovae supply the extra energy required to form elements heavier than iron.


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