What causes a body (formation) to be a star or planet?

Question

What mechanisims in space determine whether or not the body will be a star or planet? In other words, why do stars form instead of planets sometimes? Or why do planets form instead of stars?

Thanks

Answer 1

They often form together. A star is a very massive body that consists mainly of hydrogen, with some helium and very small amounts of more massive elements. Gravity causes the star to collapse under its own weight. This compresses and heats the gas to the point that the hydrogen starts fusing into helium. That generates the energy that causes a star to produce light and heat.

A planet is much smaller than a star. There are planets with very large amounts of hydrogen, such as Jupiter, but not enough to stimulate fusion. When a gas cloud collapses, it starts spinning in order to conserve angular momentum (because the original cloud was already spinning, but very slowly). This causes it to flatten out and produce an "accretion disk." Heavier atoms end up in the disk and they coalesce into bodies called planetesimals, which begin orbiting the star. The orbits usually cross each other a lot at first, so the planetesimals then collide and stick together, forming planets.

So a star is the central body of such a system, and its presence is what keeps the other material in a region of space so that it can form planets.

Answer 2

A star is made when a massive gas cloud collapses in on itself. It's so dense that the friction between the particles causes ignition. A planet is formed from heavier elements, but their always formed together.