What is a Star?

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.