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Did you know? Binary Stars

The dictionary defines binary as “composed of or involving two things”, so a binary star is a system composed of two stars which orbit each other due to their mutual gravity. You can read Sydney City Skywatcher, Andrew Wood's, observations of double or binary stars here.


Binary systems can be formed of stars with different masses and ages, for example, two Sun-like stars, or a giant star and a Sun-like star, or two white dwarfs etc. The most massive binary system known is R144. The lightweight star in this system is about 95 times the mass of our Sun, and its heavyweight partner weighs as much as 205 Suns!

The long and short of binary stars – Some binary stars orbit each other at large distances. For example, Mira, is a system composed of a giant star and a white dwarf. Their separation is about 70 times the distance between Earth and the Sun taking them about 500 years to complete one orbit. On the other hand, HM Cancri, comprising two dense white dwarf stars, orbit each other once every 321.5 seconds (in this system the "year" is only 5.4 minutes!) at an estimated distance of only 80,000 kilometres apart (about 1/5 the distance between the Earth and the Moon).

As one of the stars grows older it starts losing its material through stellar winds. Its nearby companion can then siphon it in by gravity giving fresh fuel to rejuvenate the companion.

Observing binary star systems enable astronomers to analyse the characteristics of various stars. They are able to calculate the masses of stars and observe their spectroscopy as they influence one another.

Some binary stars can be invisible. In some instances, the second star of a binary system might be a dim brown dwarf or neutron star, only making itself apparent through its gravitational influence on its partner.