A star is a large ball of hot gas, thousands to millions of kilometers in diameter, emitting large amounts of radiant energy from nuclear reactions in its interior. Stars differ fundamentally from planets in that they are self-luminous, whereas planets shine by reflected sunlight. Except for the Sun, which is the nearest star, stars appear only as points of light, even in the largest telescopes, because of their distance.

The brightest stars in Earth's sky have long been given names. Most of the familiar names originated with the ancient Greeks or with later Arab astronomers; an entirely different system was used by the Chinese, starting hundreds of years earlier, about 1000 B.C.. Polaris, the North Star, has a Greek name; Betelgeuse, a bright red star, has an Arabic name. Modern astronomers designate the bright stars according to the constellations they are in. For example, the star Polaris in the constellation Ursa Minor is called gamma Ursa Minoris. Variable stars (stars that periodically change in brightness) have lettered names, such as RR Lyrae in the constellation Lyra. Fainter stars are known by a catalog number. Thus, HD 12938 is the 12,938th star in the Henry Draper Catalogue.

Stars exist alone or in systems of two or more stars. Larger groupings are called star clusters. Except for a small percentage of isolated stars, stars and clusters are collected in very large groupings called galaxies. Our own very large grouping, often called the Milky Way, contains more than 100 billion stars. The total number of stars in the known universe exceeds a billion billion.