Photo: Joe Plocki (Flickr)
One of our readers wrote in with this question:
Recently, Moment of Science did a segment on how the solar system was originally a whirling cloud of dust and gas. Then gravity caused it to start clumping together in the middle, and eventually that clump got so big it became the sun. In the middle of this swirling cloud you’ve got a lump of matter getting bigger and bigger. Gravity is causing it to pull more and more stuff into itself. Here’s my question: Why would it ever turn into a star? Why not just a really big lump?
The matter in that lump is all pushing down on its center. The bigger it grows, the more pressure there is on the core. Eventually, the pressure gets to be so great that hydrogen atoms in the core are crushed together, and fuse into another element, helium. When they fuse, a lot of energy is released. That energy is what makes the star burn!
Planets do form the same way stars do — but although they seem huge to us, planets just aren’t big enough to crush hydrogen into helium in their cores. Still, if we could somehow add more and more matter to the earth, eventually it would become a star. But you’d have to add a lot of matter!