Some stars that look like partners only line up by chance from Earth
Two stars side by side in the night sky may have nothing to do with each other. Astronomers separate true star systems, bound by gravity and circling one another, from optical doubles that merely line up from Earth's point of view. Telling the two apart takes patience and clever tricks.
A star system is a small group of stars locked in orbit by mutual gravity. Two stars make a binary, three a ternary and four a quaternary. Large gravitationally bound crowds are usually called clusters or galaxies instead, and open clusters, with 100 to 1,000 members and messier dynamics, are a step up from star systems. Planetary systems are something else again, since they include planets and comets.
Pairs are by far the most common arrangement beyond solitary stars, and the numbers drop off steeply as membership rises. Research suggests binaries and multiples make up about a third of star systems in the Milky Way, while two-thirds are single. In the 1999 revision of Tokovinin's catalogue of physical multiples, 551 of 728 systems were triples, though selection effects make such figures hard to read. Sky views can mislead too: Beta Cephei is a real binary plus an unrelated optical companion, and Gamma Serpentis is, unusually, a triple made entirely of chance alignments.
Several methods expose genuine partners. Observing six months apart reveals parallax differences, useful only for nearby stars. Astronomers can watch stars orbit each other or an apparently empty point hiding a dim star or neutron star. Dips in brightness from eclipses work if Earth lies in the orbital plane, and light can also flicker as stars reflect each other or pull each other out of shape.
Left undisturbed, two stars trace stable ellipses around their shared centre of mass, as in Sirius, Procyon and Cygnus X-1, which pairs a star with a black hole. Bigger systems come in two types. Most are hierarchical, with close pairs treated as single bodies orbiting other pairs, so each level behaves predictably. Trapezia, with two to four stars, have tangled, chaotic orbits that resist such tidy analysis.
Source: Star system
