What is a star?

The basic difference between a star and a planet is that a star emits light produced in its interior by nuclear 'burning', whereas a planet only shines by reflected light.

The Pleiades (M45) The Pleiades (M45), a young star cluster.(Image: David Malin, Royal Observatory Edinburgh/Anglo-Australian Observatory)

There seem to be an enormous number of stars that are visible to the naked-eye at a really dark site but, in fact, the eye can only see about two thousand stars in the sky at one time. We can see the unresolved light of many thousands more when we look at the Milky Way, and the light of the Andromeda galaxy, which can be seen by the eye, comes from thousands of millions of stars.

The Sun is our own special star yet, as stars go, it is a very average star. There are stars far brighter, fainter, hotter and cooler than the Sun. Basically, however, all the stars we can see in the sky are objects similar to the Sun.

The Sun (and any other star) is a great ball of gas held together by its own gravity. The force of gravity is continually trying to compress the Sun towards its centre. If there were not some other force counteracting gravity, the Sun would collapse. Outward pressure is produced by the radiation from nuclear energy generation in the Sun's interior.