What is a binary star?

As it turns out, the majority of stars are members of binary systems, or even multiple systems. The term binary in the stellar context was coined in 1802 by William Herschel only a few years after he introduced the term planetary nebula, as mentioned in Section 9.7. The first telescopic discovery of a double star, Mizar, is attributed to Giambattista Riccioli in 1650, just 41 years after Galileo's first telescope. Other stellar pairs were found by the mid-eighteenth century, but little effort was devoted at the time to their study.

A binary system consists of two stars revolving around their common centre of mass, as shown in Figure 11.1, and is defined by three parameters: the masses of its member stars and the distance d between their centres. The distance is not necessarily constant in time; it may vary periodically or change secularly. The masses, too, may change in the course of time. So perhaps a better characterization should be: initial masses and separation, and current age. Each parameter spans a wide range of values and their combinations are innumerable. In most cases, however, the members are so far apart that their individual structures and evolutionary courses are barely affected; they are thus no different from single stars, except that their dynamics as point masses is more complicated.

Binary stars are born together as a bound system; in principle, a star may capture another, in the presence of a third body, into a bound (negative energy) state, but the chances for that to happen are small even in a dense star cluster.