Stars

The distance to nearby stars can be measured using astronomical parallax. The apparent brightness of a star is proportional to its luminosity and inversely proportional to the square of its distance. The luminosity of a star depends on its temperature and surface area. Stars are primarily classified by their temperature. A Hertzsprung-Russell (HR) diagram plots luminosity versus temperature for stars. Most stars fall on a swath from upper left to lower right called the main sequence. Stars above and to the right of the main sequence are red giant and red supergiant stars. Stars below and to the left of the main sequence are white dwarf stars. The HR diagram is useful to describe the evolution of stars from their births to their deaths. More than half of all stars are double stars that are gravitationally bound to each other. These are called binary stars. Some binary stars can be resolved with telescopes. Others are detected through spectroscopic and photometric studies.