What drives a star from one stage of the cycle to the next?

Question

Could someone please explain to me why stars change as a result of the struggle between gravity and thermal pressure? What drives the stars from one stage to the next? I have trouble grasping this concept from reading the textbook, I've read and re read again but it just doesn't click, hopefully someone can explain it better? Thank you(:

Answer 1

The precise answer to your question is: matter density. The more dense the matter is in the core of a star, the more will be the fusion reactions that occur. All stars start out as rarefied nebulae, with usually ~90% Hydrogen, ~9-10% Helium, and less than 1% other elements. Only the Hydrogen fuses in the beginning, which suffices to keep the star on the main sequence for a long, stable period. However, the stability is only approximate, since as more and more Hydrogen gets converted to Helium, the matter density of the core slowly increases, which in turn leads to more rapid fusion of Hydrogen, making the star grow slowly hotter. This is the phase that our own sun is in at present, and is the phase that all main sequence stars are in. However, there comes a critical moment, i.e. a "tipping point", where the matter density gets so high that Helium begins to fuse into Carbon, and this suddenly adds more heat to the equation. The result is the star swells into a red giant for a time, and maybe even initially blows off a planetary nebula, until much of the Helium is converted to Carbon, at which point the star will usually slowly shrink into a white dwarf, and then finally, a black dwarf. Massive stars reach other more advanced tipping points as their mass density increases, ultimately resulting in supernovae. (A white dwarf can go supernova as well, if and only if it slowly gets extra mass from another star, until it reaches ~1.44 solar masses, at which point it explodes) I hope this answers your question better than the others.

P.S. The others are wrong when they say that a star "runs out of Hydrogen". All stars always have plenty of hydrogen, even at the end of their lifetimes, when supernova or white dwarf status is eminent. A substantial amount of Hydrogen to Helium fusion is continuing, though somewhat diminished, even down to the bitter end. But this is also offset by an increase in proton to Helium fusion via the Carbon-Nitrogen-Oxygen pathway, also down to the bitter end. Thus, when a supernova finally occurs and explodes, it blows mostly Hydrogen back into the interstellar milieu.

Answer 2

It's the fuel that a star burns to maintain thermal equilibrium that determines what stage of evolution it is in. The smaller a star is, by the way, the longer it lasts because thermal equilibrium is easier to maintain, costing less nuclear fuel.



All stars begin in the main-sequence stage in which hydrogen is fused in the core. This is the longest-lasting stage of any type of star, because hydrogen fusion releases the most energy per mass. When the hydrogen in the core is gone, another stage begins.



A red dwarf will not reach another stage, due to its lack of mass, but will become a degenerate white dwarf composed of helium, the byproduct of hydrogen. This may take a trillion years or more, however, longer than the age of the universe.



For a star like our sun, hydrogen fusion will continue outside the degenerate helium core. It will expand to a red giant in order to dissipate the additional heat. The next stage begins as the helium core then ignites, about a billion years later. This will last only about 100 million years until there is a degenerate core composed of carbon and oxygen, the byproduct of helium fusion.



In even heavier stars, carbon fusion continues when helium fusion is over. It's still in the red giant phase, however. When that's over, in even shorter time, a degenerate core of magnesium, neon, and oxygen remains.



In massive stars, the main sequence is a blue giant star. It will have a red supergiant phase instead of a red giant phase. In that it will fuse elements beyond carbon. It will continue with neon, oxygen, and then silicon, each the byproduct of the previous. But each will burn faster. The silicon burning phase will last about a week, building up an iron core. When fusion can no longer produce enough energy, the core will collapse not to electron degeneracy, but to neutron degeneracy: a neutron star. It is no longer iron, but a single atomic nucleus weighing more than the sun. Further nuclear fusion is not even possible in this state.

Answer 3

What drives the evolution of star is the amount of material the star has. Stars first fuse hydrogen into helium. When a star runs out of hydrogen it needs to now fuse helium into Carbon. However, the pressure and temperature required to fuse helium into carbon is much greater than that required to fuse hydrogen into helium. To do this, the core of the star will contract as the fusion of hydrogen slows down until pressure is reached that helium fusion can start. As soon as that happens, the core will rapidly expand due to the new thermal pressure from the re-started fusion of helium.



Now, pressure comes from the gravity of the star. The more massive the star, the higher the gravity, and hence the higher pressure it can obtain. For a star the size of the Sun, helium fusion is obtainable but further fusion of Carbon is not possible. Bigger star can fuse Carbon and even bigger stars can fuse further.