How Common Are Binary Star Systems?

Multiple star systems are much more common than single star systems. According to studies, approximately one-third of all star systems in the Milky Way are binaries or even higher-multiple systems, although only around 100,000 have been classified in star catalogues.

Prevalence of Binary and Multiple Star Systems

Research indicates that up to 85% of stars are in binary systems, with some stars belonging to triple or even higher-multiple systems. It’s not uncommon for stars to share a binary or multiple stellar partners. In fact, it’s estimated that more than half of all stars are part of binary or multiple systems. This prevalence can be attributed to various factors during the formation process.

Angular Momentum and the Formation of Multiple Star Systems

Understanding the role of angular momentum in the formation of these systems is crucial. Most of the angular momentum in the universe is found in planets, rather than in the much more massive stars. Cold clouds of gas and dust contain angular momentum, which needs to be reduced to allow these clouds to collapse under gravitational forces. Gravitational collapse releases energy, contributing to the birth of stars.

The cloud forms a core and an accretion disc. The disc takes on a ring-like or spiral shape to stabilize the structure, minimizing high-energy collisions. There are several ways this can be achieved:

Breaking up of the clouds to form multiple stars. Formation of planetary systems from accretion discs. Bipolar flows from particles ejected along the axis of rotation during particle-particle collisions as the gas heats up due to the release of gravitational energy.

Binary and Multiple Star Systems Among Different Star Types

The frequency of binary and multiple star systems varies among different types of stars. For example, among stars similar in size to the Sun, or larger, binary systems are common. However, it becomes less common among stars smaller than the Sun:

Solar-Type Stars: According to surveys, more than 50% of Sun-like stars are part of multiple systems. For more massive stars, like O- and B-type stars, this percentage can be as high as 80%.

Red Dwarfs: These are dim and have only recently been studied in detail with improved technology. Only about 25% of red dwarfs have stellar companions. A study concluded that upwards of two-thirds of all star systems in the galaxy are single red dwarf stars. This finding was detailed in a recent Astrophysical Journal Letters.

Other Star Types: It appears that being significantly larger makes it more likely for a star to have a companion. However, claims that the Sun has a companion are unfounded, and this idea has been largely dismissed by astronomers.

Conclusion

Binary and multiple star systems are quite common, and this prevalence is closely related to the role of angular momentum in the formation and evolution of these systems. Whether a star is a Sun-like star, a more massive star, or a red dwarf, understanding these systems is crucial to our understanding of the universe as a whole.