A star is a large ball of hot gas held together by gravity. Stars are so hot (a few million degrees Kelvin, or Celsius on the surface) and so dense inside, that hydrogen nuclei can combine together to form helium which releases energy – this is nuclear fusion. And this is the difference between stars and planets – stars have undergone nuclear fusion, whereas planets were never massive enough for nuclear fusion to begin.

False-colour image of the Sun showing its turbulent surface (credit: NASA-SDO)

The nearest star to the Earth is the Sun, the star that the planet Earth orbits around.