A star is a huge ball of mostly hydrogen that has compacted from gravity until it gets so hot that the hydrogen starts to fuse into helium.
You may have noticed that when you pump up the tire on a bicycle, the tire can get warm. This heat comes from the act of squeezing the air into the tire. When gravity starts to compress a cloud of hydrogen gas it gets hot in the same way. But there is a lot more hydrogen so it gets much hotter. Eventually it gets so hot that it becomes like a hydrogen bomb and starts making helium and releasing enormous amounts of energy. When it first fuses hydrogen into helium it becomes a star.
Even the biggest planets are much smaller than stars. Planets never fuse hydrogen. But big planets like Jupiter do get hot from gravity's compression of gas. Jupiter, Saturn, Uranus, and Neptune are all much hotter than they would be if they were warmed only by the Sun.
Stars are basically giant hydrogen bombs held together by their own gravity. The pressure of gravity that holds the hydrogen together is enough to ignite nuclear fusion reactions within the stars, which heats them up and causes them to shine.
Planets are objects the gravity of which is not sufficient to cause the pressures and temperatures needed to cause nuclear fusion of hydrogen. Consequently, planets do not shine; they only reflect light that lands upon them from objects that emit light (like stars).
The Earth is an example of a planet. The Sun is an example of a star.
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