It is estimated that a third of the stars in the Milky Way are part of a binary (two) or multiple (three upwards) star system, with more than one star orbiting a common centre of mass, or barycentre.
Depending on the mass of each star and the conditions of their formation, they can be quite close together or millions of miles apart, and the time it takes for them to orbit varies from hours to millennia. Binary star systems are particularly useful to astronomers because they can accurately determine the mass of the stars by analysing their orbits; this then enables them to estimate the mass of similarly bright lone stars.
Some binaries can be seen through a telescope, but many are only detected indirectly, either when one star eclipses another, or when the wavelengths of light emitted vary as the stars circle around their barycentre.
If the stars are close enough together, their gravitational pull enables them to exchange matter; this can be seen as a bright disc around the recipient star. If the recipient is a white dwarf, hydrogen received from its companion can be compressed by the intense gravity at the core and undergo nuclear fusion. This process releases huge amounts of energy, which can be seen as a nova. In some cases the energy can be so great that it triggers a supernova event, destroying the star.
Binary star systems can also drift apart, resulting in the formation of single stars. The breakup of multi-star systems can also occur due to close interaction with neighbouring celestial bodies, causing dramatic fluctuations in gravitational pull and leading to stars being thrown out of a system. These ‘runaway stars’ have been seen hurtling through space at speeds of up to 30 kilometres (18.5 miles) per second.