Sigma Coronae Borealis

Sigma Coronae Borealis (σ CrB) is a star system in the constellation of Corona Borealis. It is a quintuple star system containing three sunlike main-sequence stars and two other low-mass stars. The combined visual magnitude is 5.3 and the system lies 72 light years from Earth. σ CrB A is the variable star TZ Coronae Borealis.

The brightest components of Sigma Coronae Borealis form a visual binary first resolved in the 19th century, and are designated σ Corona Borealis A and B. More recently, the designations σ² and σ¹ Corona Borealis have come into use. Somewhat confusingly, the brighter component A is referred to as σ² because it has the higher right ascension. A third component, while being separated 635&Prime (translating to a minimum distance of 14,000 au), has a similar proper motion to the main system and is physically associated. It is known in the Washington Double Star Catalog (WDS), a compilation of observations of double stars, as component E, but it is usually called Sigma Coronae Borealis C.

Sigma¹ Corona Borealis is a G-type main-sequence star like the Sun, and has similar parameters: a mass roughly equal to that of the Sun, and an effective temperature of 5950 K. A visual orbit has been calculated, with a period of about 730 years and a high eccentricity of 0.72.

Sigma² Corona Borealis itself is a close binary. Here, the two stars are extremely close and orbit fairly quickly, every 1.14 days. This tiny separation of only 0.0279 au has allowed the two stars to exert tidal forces on each other, leading to synchronization of their rotation. They have also been classified as RS Canum Venaticorum variables (RS CVn)—young, active stars that show variability in their apparent magnitude due to starspots on their surfaces.