What is the main difference between stars and planets?

Answer 1

Stars emit all kinds of radiation, planets don't. This is why:

Stars are made mainly of Hydrogen. Planets are made mainly of heavier elements.

Stars have lot of Hydrogen and big mass. Big mass produces strong gravity force (which raises pressure) and it raises temperature inside the star. High temperature and pressure ignite atomic reaction called Nuclear Fusion, combining Hydrogen atoms into Helium atoms. (Hydrogen bomb works the same way.) Fusion releases energy and stars emit their own radiation, including heat, visible light, UV rays, X-rays, Gamma rays... and atomic and sub-atomic particles. Released Fusion energy also speeds up the further Fusion.

Planets are smaller and their mass (and gravity and pressure) is lot lower. They have no atomic reactions and they don't emit their own radiation. We can see them when they reflect star light.

Answer 2

A star is a burning ball of fire and a planet is a part of the solar system.

Additional answer

A star is much bigger than a planet and emits serious amounts of heat and light from the reactions going on within it. A planet is much colder and smaller and orbits its sun.

Answer 3

Stars are luminous bodies made up of Hydrogen and some Helium which are so large the gravitational attraction has pushed the Hydrogen atoms so close together that they fuse into Helium converting a lot of mass into energy.

Planets are not as big, some are rocky some gaseous.

Answer 4

Stars are so massive that their internal temperature and pressure is high enough for nuclear fusion to occur.