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Sigma Coronae Borealis, TZ Coronae Borealis, HD 146361, BD+34° 2750, HIP 79607, GC 21863, SAO 65165.
Sigma Coronae Borealis is a multiple 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 Sigma and 5.64-magnitude Sigma when observed with a telescope. The two take 726 years to orbit each other. Sigma is a yellow main sequence star of spectral type G1V and weighing around 1 solar mass. Sigma 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. 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 an apparent magnitude of 12,24.
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.
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