7

Are there any stars that use any other elements as its fuel besides hydrogen? If not why not?

Fred
  • 2,169
  • 1
  • 10
  • 27
Kavin Ishwaran
  • 2,640
  • 1
  • 8
  • 38
  • 3
    Certainly. See https://en.wikipedia.org/wiki/Stellar_nucleosynthesis But all stars start out burning hydrogen (well, deuterium) because it has the lowest fusion temperature. And of course, hydrogen & helium are far more abundant than any other element. See https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements – PM 2Ring Aug 24 '21 at 06:00
  • 5
    You might be interested in this: https://astronomy.stackexchange.com/questions/28274/does-a-star-fuse-helium-to-beryllium-on-the-main-sequence – Nilay Ghosh Aug 24 '21 at 10:25
  • That's where all those other elements come from, isn't it? – mattdm Aug 25 '21 at 19:08

2 Answers2

26

For nuclear fusion to occur, there must be a certain temperature and pressure. Generally for heavier elements, higher temperatures and pressures are needed, because the nuclei have a greater electric charge, and so more energy is needed to get them close enough together to fuse.

In the centre of a main sequence star there is hydrogen and helium. The energy released by the fusion of hydrogen prevents further collapse and so keeps the star from reaching the temperatures and pressures needed for helium fusion.

In a star like the Sun, as the amount of helium builds up, an inert core of helium forms, until (and if) it reaches the required temperature when it will start burning -- as the core has become degenerate matter at this point, the burning will increase the temperature without causing the core to expand and lower the pressure, so the core explodes. There can then be a period with shells of helium burning and hydrogen burning. By this point the star has become an "asymptotic red giant branch" star.

In stars heavier than the sun, the process can continue further with carbon, oxygen and heavier elements fusing. However the extreme temperatures at which these processes occur mean that they don't last long, and the star will eventually become unstable and blow itself apart.

So, in main sequence stars it is not hot enough to fuse other elements. In evolved giant stars it is hot enough, but it is so hot that the process doesn't last long.

Basil Bourque
  • 111
  • 3
James K
  • 120,702
  • 5
  • 298
  • 423
  • 2
    seem to be missing a word in "without causing the core to and lower the pressure" (core to expand?) – llama Aug 24 '21 at 14:52
  • 1
    Note to OP: the third paragraph of this answer is describing the Sun, or any star around solar mass. The details of the exact processes will differ for much more massive stars (which become supergiants) and much less massive stars (red dwarves). – YiFan Aug 24 '21 at 23:24
  • "don't last long" — long at what scale: human or star? Does it take less than a few earthly years or even days? – Ruslan Aug 25 '21 at 12:41
  • 1
    There is an answer on that somewhere on here, the later stages in which there are shells of burning C, Ne, O, Si, Mg, Si can be very short: days . what-is-the-length-of-time-for-each-fusion-process-in-a-massive-star – James K Aug 25 '21 at 12:53
  • so the core explodes. There can then be a period with shells of helium burning and hydrogen burning. Wait, what? I must have missed the part where our star will at some point have the core explode but then continue the fusion process. – Michael Aug 25 '21 at 18:13
  • 1
    Yes that happens. The inert helium core will become electron degenerate, Until such time as when the temperature/pressure is sufficient for triple alpha process, and then much of the helium will burn to carbon at once. It is called the "Helium flash" Once the thermal pressure exceeds degeneracy pressure, the core can expand, and this thermal expansion will absorb nearly all the energy of the flash, so there is little effect on the star at the surface. – James K Aug 25 '21 at 21:31
2

Brown dwarf 'stars' are too small (13 to 80 Jupiter masses) to burn hydrogen, but burn (for awhile, there isn't much of the stuff available) deuterium. Larger (over 65 Jupiter masses) brown dwarfs can also burn lithium.

AlaskaRon
  • 186
  • 4