Milky Way disk is 700 to 3,000 light-years thick

The Milky Way’s thin disk is about 700 light-years high, its thick disk is about 3,000 light-years high, and its stellar halo reaches to roughly 100,000 light-years in radius. The reason published numbers sound different is simple: astronomers are often measuring different parts of the galaxy, and sometimes they mean scale height rather than total visible thickness.

That is why one source can say the Milky Way is a few hundred light-years thick while another says several thousand, and both can be right. The galaxy is not one flat sheet; it is a layered system with a thin disk, a thick disk, and a surrounding stellar halo.

Milky Way disk thickness

The cleanest public summary comes from the European Space Agency’s Gaia overview: the thin disc is some 700 light-years high and the thick disc is about 3,000 light-years high. If you searched for “How thick is the Milky Way disk?”, that is usually the answer you want.

More technical papers often quote scale heights instead. A Gaia-based 2021 study measured the thin-disk scale height near the Sun at 260 parsecs and the thick-disk scale height at 693 parsecs. Using the standard conversion of 1 parsec = 3.26 light-years, that is about 848 light-years and 2,260 light-years, respectively. That does not contradict the ESA figures; it is a different way of describing how star density falls off with height above and below the galactic plane.

A quick way to think about it: “thin” and “thick” are not two metal plates stacked in space. They are overlapping stellar populations with different vertical spreads. The 2024 Milky Way disk review notes that, especially after Gaia, the disk looks like a non-equilibrium system with complicated stellar kinematic structure and multiple ways to define thin and thick disks.

That means the Milky Way disk is really both thin and thick. The thin disk contains most of the familiar flattened structure of the galaxy, while the thick disk is a puffier component extending farther above and below the galactic plane.

What counts as the halo

The stellar halo is a roughly spherical cloud of stars surrounding the Milky Way’s disk. ESA’s Gaia anatomy page puts that halo at about 100,000 light-years in radius, while a NASA visualization describes it as roughly 300,000 light-years across. Those are effectively the same size, just stated as radius versus diameter.

A newer DESI-based study mapped the stellar halo from 8 to 200 kiloparsecs. Since 200 kiloparsecs is about 652,000 light-years, that sounds much larger, but here again the definition matters. The study is tracing halo structure across a very extended range, and it found that the inner halo is oblate while the outer halo is prolate. In other words, the halo is not a neat, featureless ball. It has shape, substructure, and no razor-sharp outer wall.

That is also why asking how “thick” the halo is is a bit awkward. For the disk, thickness means height above and below the plane. For the halo, astronomers usually talk about radius, diameter, or the range over which halo stars are observed.

Why the numbers differ

Different Milky Way thickness numbers usually come from one of three differences:

There is also a more subtle issue: the Milky Way does not come with labeled edges. The post-Gaia view of the disk is messy in a productive scientific way. Populations overlap. Density fades gradually rather than stopping cleanly. Different studies select different stars, different radii, and different mathematical definitions.

One useful derived comparison makes the scale clearer: using ESA’s figures, the thick disk at 3,000 light-years is a little over 4 times the height of the thin disk at 700 light-years. So when people say “the Milky Way is thin,” they usually mean the galaxy is very wide compared with either disk component, not that it has only one narrow layer.

And yes, the galaxy can contain both grand structural facts and odd chemical trivia at once, like the claim that the Milky Way smells like raspberries and rum, or the longer explanation of why the Milky Way smells like raspberries and rum. Galaxies are annoyingly unwilling to fit into one tidy fact.