Question

Why is the sun bigger than the planets?

Answer 1

See explanation...

Explanation:

Normal stars and planets form by the clumping together of gases, rock, etc under the force of gravity.

First of all, beyond a certain size a rocky planet would have such strong gravity that it would tend to hang onto lighter gases like hydrogen and helium. It would then tend to become a gas giant or a star:

Beyond a certain mass - say about `12` times the mass of Jupiter and double the size - some fusion reactions will start and the gas giant become a brown dwarf - i.e. a star with low luminosity.

If it continues to accumulate gases from a surrounding cloud then it may become large enough to start proper hydrogen fusion and become a normal star like our sun.

There are stars which are much smaller, the remains of collapsed stars, namely white dwarfs and neutron stars. A white dwarf is incredibly dense, being typically `60%` the mass of our sun, but about the size of the earth. Neutron stars are even more dense.