Cor Caroli

Dark spots sometimes dot the surface of the Sun — magnetic storms that can last for days or weeks. But the storms on other stars can make those on the Sun look puny. They can be so monstrous that they can change the star’s brightness by quite a bit.

An example is one of the stars of Cor Caroli — the Heart of Charles. As seen by the eye alone, it’s the brightest star of Canes Venatici, the hunting dogs. It’s to the right of the handle of the Big Dipper as night falls, and wheels above the dipper later on.

Cor Caroli consists of two stars in a wide orbit around each other. One of the stars is about half again as massive as the Sun, and several times brighter. The other is about three times the Sun’s mass, and if you add up all forms of light, more than a hundred times brighter.

But over a period of about five and a half days, the star’s brightness at visible wavelengths varies by about 15 percent. And that’s because of the starspots.

The star’s magnetic field is hundreds of times stronger than the Sun’s. That produces giant spots — far bigger than anything ever seen on the Sun. The spots are much cooler than the surrounding gas. And they contain large concentrations of heavy elements that have been dredged up from deep inside the star. The combination makes the spots dark. As the star rotates, the spots move in and out of view, causing the star to get fainter and brighter — a big change for the Heart of Charles.

Script by Damond Benningfield