Brightest Stars

Cor Caroli, named for the heart of a king

Star chart showing relationship between Big Dipper and Canes Venatici.
Look for the tiny constellation Canes Venatici the Hunting Dogs – and its brightest star Cor Caroli – behind the handle of the Big Dipper.

Cor Caroli, Heart of Charles

Cor Caroli is the brightest star in the constellation Canes Venatici the Hunting Dogs. So this star is also called Alpha Canum Venaticorum. This star and Chara, Canes Venatici’s second-brightest star, are probably the only two stars you’ll ever come to know within the boundaries of this tiny constellation. You can pick out the pair easily if your sky is dark enough. Though not bright, Cor Caroli and Chara have a relationship on the sky’s dome to the famous Big Dipper asterism, which is ascending in the northeast on spring evenings. The two stars of Canes Venatici parallel the two end stars in the handle of the Dipper.

Cor Caroli means Heart of Charles. Some say the star was named to honor King Charles I of England, who was beheaded in 1649 during the English Civil War. These sources claim Cor Caroli was labeled on old star charts as Cor Caroli Regis Martyris, or Heart of Charles the Martyr King.

Not everyone agrees, however. Others say the star was named for Charles I’s son, Charles II. Sir Charles Scarborough, physician to Charles II, is sometimes given credit for having coined the name. It’s said Scarborough claimed the star shone with exceptional brilliance on the night of Charles II’s return to England in 1660 to restore the monarchy.

If you’re familiar with the constellation Leo the Lion, you can star-hop to Cor Caroli by drawing an imaginary line from the star Alkaid of the Big Dipper to the Leo star Denebola. See the chart below.

Star chart of Big Dipper and Leo with arrows from Dipper pointing to Cor Caroli and Leo.
Use the Big Dipper and constellation Leo the Lion as guides to finding the Hunting Dogs, Canes Venatici.

It’s really 2 stars

A small telescope reveals Cor Caroli to be a double star. So it’s easy to imagine father and son peacefully reunited in the heavens, after all their tumultuous years on Earth.

Cor Caroli doesn’t just appear double. It’s a true binary star, consisting of two stars revolving around a common center of mass. The pair lies some 110 light-years away. This is slightly farther away from us than the stars in the Big Dipper.

One orbital period may take as long as 8,300 years. In fact, since the year 1830, the fainter star has not appeared to change much in distance and direction in relation to the brighter star. The fainter star remains 19 arcseconds away and to the southwest of the brighter star. In reality, the stars are about 5 light-years apart, with the brighter star closer to us than the fainter star. The brighter star varies in brightness every 5.47 days from magnitude 2.84 to 2.98. The fainter companion shines at magnitude 5.6.

You can see this double star for yourself on the next clear evening. Binoculars won’t magnify enough to split the star into its two components. But a small telescope using at least 40 power will show the object as two stars. A large telescope using even more magnification will reveal the colors of the two stars. The brighter star is often described as white or blue-white. The fainter star is more difficult to discern, but some observers see it as mild lilac in color.

Circle showing cardinal directions around double star in middle.
This beautiful drawing is by Jeremy Perez. It shows Alpha Canum Venaticorum, aka Cor Caroli. A telescope reveals it as 2 stars. See more of Jeremy’s astronomical drawings at his website The Belt of Venus.

The 2 stars are very different

The fainter star is Alpha-1 and the brighter star is Alpha-2. One would think that the brighter star would be #1 and the fainter star would be #2. But the stars are numbered from west to east and the fainter star is west of the brighter star.

Alpha-2 is a hot star, spectral type A, with a temperature of 11,450 kelvins or 20,150 degrees Fahrenheit (in contrast with our sun’s surface temperature of 6,000 kelvins or 10,340 F). In other words, Cor Caroli shines 100 times brighter than our sun and is nearly three times larger than our sun. This bright variable star is the namesake for a certain class of stars known as Alpha2 Canum Venaticorum variables. These are heavy metal stars with very strong magnetic fields. “Metals,” in a star, refers to elements more complex than hydrogen or helium. In Cor Caroli, the metals are concentrated in sunspots, and are not evenly distributed in the star’s atmosphere. And so as the star rotates its brightness varies from our point of view.

The fainter of the two stars in Cor Caroli, Alpha-1, is nearly as cool as our sun at 6,785 kelvins (11,750 F). It’s only six times brighter than our sun, and nearly twice as large. Its spectral class is F.

Two fuzzy dots close together, one white, the other yellowish.
A photo – through a telescope – of Cor Caroli, showing its 2 component stars. Image via F. Ringwald, Fresno State.
Star chart with stars black on white background of constellation Canes Venatici.
Star map of the region around Cor Caroli, via International Astronomical Union/ Wikimedia Commons.

Bottom line: The star Cor Caroli, or Alpha Canum Venaticorum, is a binary star and the brightest star in the northern constellation Canes Venatici the Hunting Dogs. Both components can be seen in a small telescope.

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April 10, 2022
Brightest Stars

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Bruce McClure

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