Nuclear furnaces
A star is a ball of gas held together by its own gravity and heated by nuclear fusion in its core. In the Sun, hydrogen nuclei fuse into helium, converting a little mass into energy according to E = mc². The outward pressure of hot gas balances the inward pull of gravity.
Colour, temperature and size
Hotter stars are bluer and cooler stars redder. Astronomers classify stars as O, B, A, F, G, K and M, from hottest to coolest; the Sun is a G star at about 5,772 K. A star's luminosity depends on both its temperature and its size: L = 4πR²σT⁴.
Plotting luminosity against temperature gives the Hertzsprung–Russell diagram. Most stars, including the Sun, lie on the main sequence while they fuse hydrogen. Red giants are cool but huge; white dwarfs are hot but tiny.
Planets
A planet does not fuse hydrogen. It shines by reflecting starlight, and its own heat glows in infrared. The IAU defines a planet in our Solar System as a body that orbits the Sun, is massive enough to be round, and has cleared its orbital neighbourhood — the criterion Pluto does not meet.