The Hayashi Track

When a star first forms, it is powered not by nuclear fusion but simply by gravity. It shrinks, which causes a release of gravitational energy. This tends to heat it, which slows its shrinking. But it releases energy in the form of light, which tends to cool it down. Then it can keep shrinking. So you need math to figure out exactly what happens.

In 1961, Chushiro Hayashi figured it out. Stars between one tenth and twice the mass of the Sun begin life by shrinking while staying at the s...