Stars in the sky are different colors because their temperatures are not the same. For example, white or blue stars are classified as hot stars, and orange or red-tinged stars are classified as cool stars. Scientists sometimes use blue, green and red filters to determine the color index of a star.
Stars come in many different sizes. The largest stars, with radii a thousand times bigger than that of the sun, are classified as supergiants. The smallest stars are classified as dwarfs. Binary stars are two stars that rotate around each other. Astronomers use Keplerian equations to determine the mass of both stars.
Stars' brightness, or luminosity, is determined by how much energy they emit, as well as how far from Earth they are. Luminosity is measured in magnitudes, measures of brightness of celestial objects, and is divided into apparent brightness (how bright the star looks from Earth), and absolute brightness, or the true brightness of a star.
Stars are also classified based on their age, or at what point in their life cycle they are. The youngest stars at beginning stages are called protostars. The sun is in a secondary or intermediate stage and is classified as a main sequence star. As stars reach the end of their life cycles, they swell in size and become red giants. After the red giant phase, stars shed outer layers to become small and dense and become white dwarfs. When stars become completely dark, producing no energy, they are classified as black dwarfs.
A few unusual stars do not age like other stars, and instead explode and are called supernovas. Supernovas leave behind a a small core that with time is classified as a neutron star if it is small in size, or a black hole if it is significantly larger.