thick disk

Imagine two crowds sharing the same dance floor. One crowd hugs the floor tightly, gliding in smooth circles; the other ranges higher, bobbing well above and below, moving a little more chaotically and more slowly around the room. The Milky Way's disk turns out to be two such overlapping populations of stars. The thin disk is the low, orderly crowd of younger stars; the thick disk is an older, puffier crowd that reaches much farther above and below the galactic plane. They occupy the same region of the galaxy but tell very different stories.

The thick disk is a flattened distribution of older stars whose layer is roughly 3,000 light-years tall — several times thicker than the thin disk's few hundred light-years. Its stars are typically 8 to 12 billion years old, noticeably metal-poorer than thin-disk stars, and they orbit the galactic center a bit more slowly, with larger up-and-down and in-out wanderings. It was identified in the 1980s when star counts perpendicular to the disk could not be fit by a single thin layer — a second, more extended layer of stars was needed to match the numbers, especially far from the midplane.

The thick disk is a fossil record of the galaxy's turbulent youth. Its old, metal-poor stars formed early, and their hotter, more scattered orbits may have been heated up by ancient mergers with smaller galaxies, or these stars may have been born thick in a chaotic early disk. Either way it stores chemical and orbital clues to how the Milky Way assembled. A caveat: the line between thick and thin disk is not sharp. The two populations blend, and astronomers still debate whether they are truly distinct structures or two ends of one continuous, smoothly aging disk.