The Milky Way forms a glowing arch in the western sky on November evenings. It stretches from Aquila and Cygnus in the west, through W-shaped Cassiopeia high in the northeast, and down near the bright stars Aldebaran and Capella.

The latter section aims away from the Milky Way Galaxy’s dense heart, and toward its thinly populated outer regions. But no one knows just how far those regions extend, or how thinly populated they really are.

The galaxy consists of two main parts. There’s a wide, thin disk that contains beautiful spiral arms. And there’s a “halo” of old, faint stars around the disk. It extends for hundreds of thousands of light-years.

Most studies have put the diameter of the disk at about 100,000 light-years, with our solar system roughly halfway between the center and the edge. And it’s certain that the vast majority of the stars in the disk lie within that span.

Yet there’s no fortress wall at the “edge” of the disk. So stars trickle from the disk out into the halo. Several studies have found stars out to about 60,000 light-years from the center of the Milky Way, making the disk 120,000 light-years across.

And one study found quite a few stragglers out to about 85,000 light-years from the center — and some that could be a hundred thousand light-years. That would make the disk 200,000 light-years in diameter — a giant family of stars spinning through the universe.

Script by Damond Benningfield

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