Look up in the night sky and you’ll see lots of stars. But what is a star? In a scientific sense, a star is ball of hydrogen and helium with enough mass that it can sustain nuclear fusion at its core. Our Sun is a star, of course, but they can come in different sizes and colors. So let’s learn what a star is.
75% of the matter in the Universe is hydrogen and 23% is helium; these are the amounts left over from the Big Bang. These elements exist in large stable clouds of cold molecular gas. At some point a gravitational disturbance, like a supernova explosion or a galaxy collision will cause a cloud of gas to collapse, beginning the process of star formation.
As the gas collects together, it heats up. Conservation of momentum from the movement of all the particles in the cloud causes the whole cloud to begin spinning. Most of the mass collects in the center, but the rapid rotation of the cloud causes it to flatten out into a protoplanetary disk. It’s out of this disk that planets will eventually form, but that’s another story.
The protostar at the heart of the cloud heats up from the gravitational collapse of all the hydrogen and helium, and over the course of about 100,000 years, it gets hotter and hotter becoming a T Tauri star. Finally after about 100 million years of collapse, temperatures and pressures at its core become sufficient that nuclear fusion can ignite. From this point on, the object is a star.
Nuclear fusion is what defines a star, but they can vary in mass. And the different amounts of mass give a star its properties. The least massive star possible is about 75 times the mass of Jupiter. In other words, if you could find 74 more Jupiters and mash them together, you’d get a star. The most massive star possible is still an issue of scientific disagreement, but it’s thought to be about 150 times the mass of the Sun. More than that, and the star just can’t hold itself together.
The least massive stars are red dwarf stars, and will consume small amounts over tremendous periods of time. Astronomers have calculated that there are red dwarf stars that could live 10 trillion years. They put out a fraction of the energy released by the Sun. The largest supergiant stars, on the other hand, have very short lives. A star like Eta Carinae, with 150 times the mass of the Sun is emitting more than 1 million times as much energy as the Sun. It has probably only lasted a few million years and will soon detonate as a powerful supernova; destroying itself completely.
Most stars are in the main sequence phase of their lives, where they’re doing hydrogen fusion in their cores. Once this hydrogen runs out, and only helium is left in the core, the stars have to burn something else. The largest stars can continue fusing heavier and heavier elements until they can’t sustain fusion any more. The smallest stars eject their outer layers and become white dwarf stars, while the more massive stars have much more violent ends, become neutron stars and even black holes.