• Question: How does a star die and what happens to it

    Asked by dbryce to Andrew, Daniel, Hayley, Natalia, Peta on 22 Nov 2011.
    • Photo: Daniel Scully

      Daniel Scully answered on 21 Nov 2011:

      When a star is “alive” it burns the different elements in it. The easiest to burn is hydrogen, so it does that until all the hydrogen is gone. Then it burns the helium and so on until it gets to Iron. Iron can’t be burnt by a star, so at that point it runs out of fuel.

      When small stars run out of fuel, they expand out – because there’s no longer enough mass to hold it together. The Sun will do this and it will expand out beyond the Earth. After it has cooled down, it collapses back into a small cold object called a “brown dwarf”.

      When big stars run out of fuel, there’s still a lot of mass pulling it together, but there’s no longer enough heat to hold it up. The star’s material collapses back in on itself.

      This may crunch together to make a super-dense object called a “neutron star”. If the star is very big though, it keeps collapsing and eventually explodes as a “Super Nova”.

      Super Nova are some of the brightest events in the Universe, and they can be brighter than a million stars put together.

    • Photo: Hayley Smith

      Hayley Smith answered on 21 Nov 2011:

      Stars have really interesting lifecycles and these can end in many different ways…
      In the main part of a stars life, when it’s most stable, it is converting Hydrogen to Helium in a process called nuclear fusion (the two Hydrogens are pushed together).
      But when the Hydrogen runs out the star has to start some other “burning” or fusion cycles in order to continue… so it starts trying to fuse the Helium together.
      This takes more energy than fusing together Hydrogens.
      This process continues on and on and on burning all the different elements up until until the star tries to fuse to Iron with Iron and it finds it can’t do this anymore. Up until this point the energy obtained from burning things together is enough to sustain the reaction – but when it gets to Iron-Iron it would take more energy to push these together than it would get out.
      All this time the star is contracting and expanding and getting cooler.
      And by this point the star is quite heavy so it will collapse in on itself – this is called a “Type II Core Collapse Supernova” and it’s pretty impressive. As it collapses in on itself it’s radius shrinks and the centre heats up again – there is essentially a big explosion, the outer layers are blasted off, there are many high energy gamma rays released and the gravitational collapse is only halted eventually when the core is formed into a really really dense neutron star.

      There are many other types of supernova but I, personally, think that’s the most impressive….
      We summarised it neatly, and jokingly, in a University seminar by saying it’s “bigger small bigger smaller bigger smaller bigger smaller bigger smaller………..BOOM!”

    • Photo: Peta Foster

      Peta Foster answered on 22 Nov 2011:

      Yep Daniel and Hayley are right and i agree with them and i would only add one little thing…

      when the universe began there was only hydrogen and then fusion brought us helium and essentially all the heavier elements, of which we ourselves are made, were made through the process of fusion…. inside a star!

      So essentially we are all made of stardust! How awesome is that? 😀

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