Question

How is a star different from a planet?

Answer 1

Mass

Explanation:

It is strictly a matter of mass. A celestial body that has enough mass to have high enough gravity to start fusion at it's core is a star. That mass is debatable even but thought to be between 13 to 65 times the mass of Jupiter. This would only be the most basic of fusions and stars of this mass are not always referred to as stars. They are called Brown Dwarfs.

A more precise mass would be around 85 times the mass of Jupiter which is where gravity would be high enough to fuse hydrogen into helium. A star of this mass would be a Red Dwarf.

So definitively a body with less than 13 Jupiter masses would be a planet and a body with 85 Jupiter masses or more would be a star and everything in between would be debatable.

Keep in mind this is mass not volume.