Nabeshin said:Quick answer is that there's simply not enough mass in the star to cause the kind of pressures needed at the core to begin fusing helium. What happens in lieu is a little more complicated and tied to more advanced stellar evolution.
vertices said:What happens after all the He is converted into C though? I mean nuclear reactions cease, so by the virial theorem the core must contract and heat up (if it radiates away energy). You're saying its low mass doesn't allow it to heat up enough to trigger He burning... so I presume electron degeneracy stops the core from contracting (and thus heating up) enough for He burning?
Orion1 said:
The physics of degeneracy yields a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation.
Although white dwarf material is initially plasma—a fluid composed of nuclei and electrons—it was theoretically predicted in the 1960s that at a late stage of cooling, it should crystallize, starting at the center of the star.
Nuclear burning is the process by which stars generate energy. It occurs when the high temperatures and pressures in a star's core cause hydrogen atoms to fuse together to form helium, releasing large amounts of energy in the process.
Low mass stars, also known as red dwarfs, have a mass less than 0.5 times that of the Sun. Due to their lower mass, they have a lower core temperature and pressure, which are necessary for nuclear fusion to occur. As a result, these stars do not experience sustained nuclear burning and have a longer lifespan compared to higher mass stars.
Low mass stars generate energy through a process called convection, where hot gas rises from the core to the surface, carrying energy with it. This convection process allows these stars to continue burning hydrogen through a slow and steady process, known as the proton-proton chain, rather than the explosive nuclear fusion seen in higher mass stars.
One implication is that these stars have a longer lifespan compared to higher mass stars, as they are able to continue burning hydrogen at a slower rate. This means that red dwarfs can potentially live for trillions of years, far longer than the estimated lifespan of the universe. Additionally, the lack of nuclear burning in low mass stars means they do not produce heavy elements, which are essential for the formation of planets and life.
It is highly unlikely that low mass stars will ever experience sustained nuclear burning in their cores. While they may undergo brief periods of nuclear fusion during certain phases of their lifespan, their lower mass and core temperature make it difficult for them to sustain the high temperatures and pressures necessary for sustained nuclear fusion. However, with advancements in technology, it is possible that scientists may one day find a way to artificially trigger nuclear fusion in these stars.