Sirius is a nearby binary star containing a luminous main-sequence star and a much fainter white dwarf. Their coexistence makes a useful lesson in stellar appearance: a hot object can be faint because its radiating area is small. The resolved Hubble Space Telescope image separates the compact companion from the bright primary's surrounding light.[ref]
The Dog Star, Sirius A, and its tiny companion by European Space Agency. Image description: Sirius A and its small, faint white-dwarf companion
The common orbit provides dynamical information, whereas spectral classification and luminosity constrain the two bodies' physical states. The white dwarf records prior stellar evolution; its current faintness does not mean it was always the less massive or less luminous member. Sirius therefore connects binary star measurements with white-dwarf cooling and the interpretation of the Hertzsprung-Russell diagram.
