Star of the month

Cor Caroli, a star named in honour of King Charles II

Cor Caroli (Alpha (a) Canum Venaticorum) is the brightest star in the small, but deepsky rich constellation of Canes Venatici, the Hunting Dogs. Shining at mag. +2.9, it is complemented by mag. +4.2 Chara (Beta (b) Canum Venaticorum). On charts, Canes Venatici is typically depicted as a single line joining both stars together.

The name Cor Caroli literally means “Charles’s Heart” a name bestowed on it by Edmund Halley to honour King Charles II. It’s easy to locate because it sits to the south of the handle of the Plough asterism. If you view the handle of the plough as part of a circle, Cor Caroli marks the approximate position of the centre of the circle.

Through a telescope, Cor Caroli appears double. Alpha-2 (a2) Canum Venaticorum is the brighter component showing variability over a 5.5–day period between mag. +2.8 and +3.0. Alpha-1 (a1) Canum Venaticorum has a visual magnitude of +5.6. The

Alpha-2 and Alpha-1 are labelled this way because the western, and in this case dimmer, component is numbered first.

The apparent separation between both stars is 19.6 arcseconds.

Alpha-2 is a special type of star that shows a peculiar composition and has a strong magnetic field estimated to be around 5,000x stronger than Earth’s. The Sun, by comparison, has a field, which is just a few times stronger. Alpha-2 is a dwarf star of spectral type A0pSiEuHg. This means it’s a white star (‘A0’) with a peculiar spectrum (‘p’) showing strong lines for elements silicon, europium and mercury (‘SiEuHg’). Alpha-2’s strong magnetic field causes extreme examples of sunspots on the star’s photosphere and these swing into and out of view causing variations in its apparent brightness.

Alpha-1 has a spectral type of F0V, which means it’s a yellow-white main sequence dwarf type star. It’s also rich in iron. The stars are separated by around 675 AU and orbit one another over a period of around 8,300 years.