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Physics LibreTexts

14.4 Binary and Multiple Stars

The question of pinpointing the number of binary and multiple star systems is a challenge to observers. The nearest stars are easiest to observe but give too small a statistical sample to be reliable. At greater distances there are more stars, but faint companions might not be detected. Spectroscopic binary statistics are biased toward pairs with small separation distances, because according to Kepler's laws these have the fastest velocities and greatest Doppler shifts, thus being the most likely to be discovered. Visual binary statistics are biased toward wide separation distances, which make the two stars easier to resolve. All these biases, which tend to make the data unrepresentative of the whole population, are called selection effects.

Study of the 25 stars nearest the Sun shows that about half are single stars. Specifically, 48% are single, 36% are binaries, 12% are triple systems, and 4% are quadruple systems. Studies of larger samples show that only about 1% of stars inhabit systems with five members. With systems of six or more stars, definitions become hazy. It is unclear whether close groupings like the Trapezium in the Orion Nebula should be counted as multiple systems. Without detailed orbital information, it's not clear if the stars are gravitationally bound. Multiple star systems would present an interesting spectacle. In fact, the percentages quoted are really lower limits — it is likely that many of the seemingly single stars have companions too small and dim to detect. Astronomers estimate that most stars like the Sun have at least one companion. If true, this would imply that our own Sun is slightly unusual. However, recent research shows that low mass stars tend to have fewer companions. So for Sun-like stars, roughly 2/3 are in binary or multiple systems, while for red dwarfs the fraction is closer to 1/3.

The conclusion that our Sun might be unusual changes our view of stars entirely. The idea of the night sky full of single, separate stars is wrong. The lives of many stars are intertwined. Astronomers must understand the origin and evolution of systems of two, three, four, and more stars in order to claim any understanding of stars in general. It turns out that binaries go through some fascinating detours along the road of stellar evolution.