What is bigger out of a planet or a star?

Answer 1

It depends upon what kind of planet and what kind of star.

Usually, stars are FAR larger than planets.

The sun is 10 times Jupiter's diameter and 108 times Earth's.

Even a tiny star such as Red Dwarf would be at least 2.5 times the diameter of Jupiter.

However,

If a star has reached the end of its life and collapsed, it can take 3 forms:

A white dwarf

A neutron star

A black hole

Once the star can no longer produce energy at its core, it cannot stand up against gravity. It collapses like a balloon with the air let out (only much more).

A white dwarf is a super dense remnant of a stellar core, weighing as much as 1/2 the Sun's mass but with a similar diameter of Earth. It is held stable at that size by electrical repulsion between atoms. Gravity at the surface is ferocious - at least 150,000 to 200,000 times that of Earth.

If the original star was somewhat larger, the core might be more massive, and it will collapse beyond the stage of a white dwarf, to a neutron star. Maybe 12 to 15 miles across. Electrons are jammed against the protons, resulting in much higher density. Only the "weak" nuclear force prevents it from collapsing further. Gravity on the surface of a Neutron star is insanely high - 200 Billion times that of Earth.

If the original star was larger yet, even nuclear forces cannot hold it up. Gravity is so strong the object compresses to infinite density, but with mass of 1.4 times the Sun's. No surface exists, but there is an invisible boundary called an "event horizon" where space is warped so completely that even light cannot escape. Gravity at the edge of the event horizon is trillions of times that of Earth. Event horizons size depends on the mass of the black hole. For a small one, that's about 3 miles across.

Answer 2

Some stars are only a few kilometres wide, but in general, stars are very much larger than any planet.