What Is the Thermal History of a Star?

All stars start off as cold clouds of gas, that's how they all start. If you have a big cloud of gas, if it starts out too hot, it can't form stars. Stars get hot from gravity, and that temperature allows fusion to happen,. And then the fusion sustains them. The reason that they're hot is that gravity takes the gas and squeezes it into a smaller space, and that heats up the gas.