Cor Caroli

Lore & Background

The name Cor Caroli, meaning 'Charles's Heart', was coined in 1660 by Sir Charles Scarborough, physician to Charles II. Scarborough claimed the star shone exceptionally brightly on the night of Charles II's return to England. There has been uncertainty whether the name honors Charles I, executed in 1649, or Charles II, who restored the monarchy in 1660. The name first appeared on a star map in 1673, on Francis Lamb's chart, which labelled it Cor Caroli Regis Martyris ('the heart of Charles the martyred king'), indicating it referred to Charles I. In 2016, the IAU Working Group on Star Names approved Cor Caroli for α2 Canum Venaticorum.

The system lies about 110 light-years from the Sun and marks the northern vertex of the asterism known as the Great Diamond or Diamond of Virgo. The two stars are separated by an estimated 650–670 astronomical units and orbit a common center of mass with a period around 8,000 years. The brighter star, α2 Canum Venaticorum, is a chemically peculiar Ap/Bp star with a strong magnetic field about 5,000 times that of Earth. Its atmosphere shows overabundances of silicon, mercury, and europium, thought to result from some elements sinking under gravity while others are elevated by radiation pressure. This star is the prototype of the α2 Canum Venaticorum variables, whose brightness varies due to enormous starspots caused by the strong magnetic field. Its apparent magnitude varies between 2.84 and 2.98 with a period of 5.47 days. The fainter companion, α1 Canum Venaticorum, is an F-type main-sequence star with an apparent magnitude of about 5.60.

Reader's Guide

Cor Caroli is notable as the prototype of the α2 Canum Venaticorum class of variable stars, which are chemically peculiar Ap/Bp stars with strong magnetic fields. The brightness variations of these stars, caused by enormous starspots, make them important for studying stellar magnetic activity and chemical stratification. The system itself is a wide binary with a long orbital period of about 8,000 years, and its two components are easily resolved in small telescopes due to their 19.6-arcsecond separation. Historically, the star's name reflects 17th-century English political history, with competing attributions to Charles I or Charles II. The name first appeared on a star map in 1673, and the IAU officially recognized it in 2016. The star also has a Chinese name derived from the asterism Imperial Guards, and a U.S. Navy Crater-class cargo ship was named Cor Caroli after the star. As the brightest star in Canes Venatici and a vertex of the Great Diamond asterism, it serves as a navigational and observational landmark in the northern sky.