double star

Pair of stars that appear close together. Many stars that appear single to the naked eye appear double when viewed through a telescope. Some double stars are physically associated: they orbit each other, forming a genuine binary star. Other double stars are at different distances from Earth, and lie in the same line of sight only by chance. Through a telescope both types look the same.

Double stars of the second kind (which are of little astronomical interest) are referred to as ‘optical pairs’, those of the first kind as ‘physical pairs’ or, more usually, ‘visual binaries’. They are the principal source from which is derived our knowledge of stellar masses.