EX Hydrae

Just because a star is small doesn’t mean it’s boring. In fact, two tiny stars in the constellation Hydra are among the most active in our region of the galaxy. One of the stars takes hot gas from the other, which triggers occasional outbursts.

EX Hydrae is about 200 light-years away. Its main star is a white dwarf — the dead core of a once-normal star. It’s about a third as massive as the Sun, but only about as big as Earth. It’s so dense that its surface gravity is quite powerful — strong enough to pull material off the surface of its companion. That star is a red dwarf — a bare cosmic ember just one-tenth as massive as the Sun.

The stolen gas falls on the white dwarf in a couple of ways. Most of it forms a wide, thin disk around the white dwarf. The disk gets extremely hot, so it produces lots of X-rays. But the white dwarf also generates a strong magnetic field. Some of the stolen gas follows the lines of magnetic force and flows onto the star’s poles.

Every few years, so much g