Sigma Coronae Borealis

Sigma Coronae Borealis is a quintuple star system in the constellation Corona Borealis, the main components of which are three sunlike stars. Appearing as a single star of apparent magnitude 5.3 to the unaided eye, Sigma can be separated into 6.6-magnitude Sigma¹ and 5.64-magnitude Sigma² when observed with a telescope. The two take 726 years to orbit each other. The orbit of Sigma¹ is eccentric, with an average distance of 128 AU, and a minimum of 31 AU and maximum of 225 AU. Sigma¹ is a yellow main sequence star of spectral type G1V and weighing around 1 solar mass. Sigma² is a spectroscopic binary system composed of two stars very close (6 solar radii) together that orbit each other every 1.14 days. These stars are 1.14 and 1.09 times as massive as the Sun, with spectral types F9V and G0V respectively. Each has a diameter around 1.2 times that of the Sun. They rotate each other very rapidly, and are very active as a result, with prominent star spots and are classed as RS Canum Venaticorum variables. The system emits radio waves. Much further away, at a distance of 14,000 astronomical units (AU), are a pair of red dwarf stars that take 52 years to rotate each other. This pair has a combined apparent magnitude of 12.24. Another pair of faint stars that appear nearby were found to be unrelated.

The system is thought to be anywhere from 100 million to 3 billion years old, from measurement of lithium in its spectrum. Its parallax was measured in 1999, yielding a distance of 74.2 light-years.