Lopsided Halo

We may be living inside a football. A recent study measured the Milky Way Galaxy’s halo — a shell of stars that surrounds the galaxy’s bright disk. And it found that the halo is shaped like a lopsided football or zeppelin. That could be the result of a collision billions of years ago.

The stellar halo extends hundreds of thousands of light-years from the center of the galaxy. It contains lots of stars. But they’re widely spread, so the halo is faint. That makes it hard to map.

The new study used telescopes in space and on the ground to measure the motions of thousands of halo stars. That was enough to reveal the shape. It also revealed that the halo is tilted compared to the Milky Way’s disk. That off-kilter alignment suggests the halo is locked in place by a halo of dark matter. We can’t see the dark matter, but its gravity reveals its presence.

The stellar halo may have formed billions of years ago, when another galaxy slammed into the Milky Way. That splattered many of its stars like droplets from a water balloon thrown against a brick wall. In this case, the stars entered wide orbits around the Milky Way — forming the lopsided halo.

The Milky Way’s faint disk arcs high across the sky this evening. It stretches from Sirius, the brightest star in the night sky, in the southeast, over to W-shaped Cassiopeia high in the northwest, and down to the Northern Cross just above the horizon. The halo is nowhere in sight.

Script by Damond Benningfield