Black Holes: Difference between revisions

As the star ages, it exhausts its hydrogen supply by converting it into helium. However, the star will continue to live, as its core collapses inward on itself further, increasing the pressure and temperature at the core. Eventually, the core becomes hot enough that helium starts to fuse into heavier elements such as carbon and then oxygen. During the short period where the core is pure helium, the shell of the star, which continues to fuse hydrogen, gains enough kinetic energy to expand away from the star, becoming the outer layers of a red giant. Once the star contains a core of pure iron, however, the star's life is at its end. The cold math of hot fusion tells us that fusion releases energy only up to iron; beyond that it requires an input of energy, and a small star doesn't have the gravitational oomph necessary to provide that input. The outer layers are shed off as a planetary nebula after the core becomes solid iron. No longer supported by fusion, the exposed core of the star condenses into a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf[[hottip:*:<ref>[[Insistent Terminology|Not to be confused with "brown dwarfs", which are substellar bodies that were never massive enough to sustain fusion to begin with.]]</ref> only a fraction of a degree above absolute zero.