Thick disk
geographic region of a galaxy
A thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a distinct galactic structure in the Milky Way, different from the thin disk and the halo.
It is thought to dominate the stellar number density between 1 and 5 kiloparsecs (3.3 and 16.3 kly) above the galactic plane and, in the solar neighborhood, is composed almost exclusively of older stars. For the Milky Way, the thick disk has a scale height of around 0.6–1.1 kiloparsecs (2.0–3.6 kly) in the axis perpendicular to the disk, which is 3-4 times larger than the thin disk, and a scale length of around 1.9–2.3 kiloparsecs (6.2–7.5 kly) in the horizontal axis, in the direction of the radius.
Its stellar chemistry and stellar kinematics (composition and motion of its stars) also set it apart from the thin disk. Compared to the thin disk, thick disk stars typically have significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.
The thick disk is a source of early kinematic and chemical evidence for a galaxy's composition and thus is regarded as a very significant component for understanding galaxy formation.
With the availability of observations at larger distances away from the Sun, it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all distances from the galactic centre. Instead, it is metal-poor inside the solar radius, but becomes more metal-rich outside it.
Begin with the source’s own compact description: “Thick disk” is geographic region of a galaxy. The dossier treats that line as a proposition to test through Thick, disk and geographic, not as a finished interpretation.
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The phrase “geographic region of a galaxy” supplies a clear boundary for inquiry. It also exposes the unanswered questions: who defined that boundary, when it became stable and which sources sit outside it.
The strongest evidence will usually combine a dated visual record with documents produced by the authority responsible for the place. The source revision retrieved here is dated Aug 31, 2026. The linked authority identifier is Q3030613. None of the 0 selected statements returned an explicit reference.
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Contemporary maps, plans, listed-building records, estate papers and the responsible local or national archive.
Three-step research path
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This entry incorporates text from “Thick disk” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.