MULTIPLE STARS

Cecelia Payne-Gaposhkin was a pioneer of modern astronomy. She devoted much of her research to the study of multiple star systems and coined a comic adage to describe one of the basic tenets of that work: “Three out of every two stars are in a binary system.” By this she meant to illustrate that roughly half the stars in the sky have companion stars in orbit. If you were to look closely at half the stars you would find that there are two stars, where a more casual examination would have revealed only one point of light. Many people know that the nearest star to the Sun is Alpha Centauri. Less well known is that Alpha Centauri has a companion in wide orbit, known as Proxima Centauri. A closer examination shows that Alpha Centauri itself is not a single star but has a closely orbiting companion as well. Of the “two” stars closest to the Sun, three are in the same mutually orbiting stellar system.

Stars occur in many combinations. Single stars and pairs are most common, but some systems contain four or five stars in mutual orbit. In this chapter, we will concentrate on the systems with a pair of stars, double stars, or, somewhat more technically, binary stars (but we try to refer to the phenomenon of duplicity, not the word “binarity” born of mangled jargon that has crept into the literature). Binary stars come in two basic classes: wide and close.