Arcturus Stream

One of the brightest stars in northern skies travels through the galaxy with a few friends. But astronomers have had a hard time figuring out if they’re locals, or if they came from beyond the Milky Way.

Arcturus is low in the east as darkness falls, and climbs high across the sky later on. It shines yellow-orange — the result of its relatively cool surface. The star is probably at least seven billion years old. It’s leaving the prime of life, so it’s puffing up — getting bigger, brighter, and redder.

Arcturus travels through the galaxy with more than 50 known companions. Together, they form the Arcturus stream. The stars are all about the same age, and they move in the same speed and the same direction.

But that direction is different from most of the stars in the Milky Way. While most stars orbit in the plane of the galaxy’s flat disk, those of the Arcturus stream orbit perpendicular to the disk. And they move much faster than most of the other stars around us — about 275,000 miles per hour.

That suggested that the stars were born in a smaller galaxy that once orbited the Milky Way. The Milky Way captured the galaxy and stripped away its stars.

Research in the last few years, though, shows that the composition of the stars in the stream matches that of stars born in the Milky Way. So the stars of the Arcturus stream may be home-grown, and pushed into an odd path by the Milky Way itself.

Script by Damond Benningfield