Galactic Plane Measurements Put the Milky Way Disk at 1,000 Light-Years Thick

BySarah Fraser
The Milky Way arching across a dark night sky above a landscape

The Milky Way’s thin disk is about 700 to 1,000 light-years thick, while its thick disk is about 3,000 light-years thick. The part people usually mean by “the Milky Way disk” is the thin disk, the flatter layer that contains most of the galaxy’s gas, dust, and younger stars, as described by ESA’s Gaia overview.

The Milky Way’s stellar halo is much larger but far more diffuse: ESA says it extends to roughly 100,000 light-years in radius. That means the halo reaches about twice as far from the center as the Milky Way’s bright disk, which spans more than 100,000 light-years across.

Milky Way disk thickness

ESA’s Gaia material gives the clearest split between the Milky Way’s two disk components: the thin disk is about 1,000 light-years thick, and the thick disk is about 3,000 light-years thick. NASA’s Webb explainer uses the same broad picture, describing the Milky Way’s thin disk as roughly 1,000 light-years thick and the thick disk as around 3,000 light-years thick.

ESA also gives a slightly narrower figure for the thin disk in one place, calling it about 700 light-years thick. That is not really a contradiction. It is the same order of magnitude, and it reflects a common astronomy problem: galaxy parts do not have hard edges, so “thickness” depends on exactly how the measurement is defined.

The practical difference between the two disks is not just size. ESA says the thin disk contains a large share of the Milky Way’s gas and dust and many younger stars, while the thick disk is a puffier, older stellar component that sits above and below the Galactic Plane, the galaxy’s central midline as seen in ESA’s Gaia guide. If you picture the Milky Way as a vinyl record, the thin disk is the flatter playing surface; the thick disk is a broader, fuzzier layer wrapped around it.

Halo size and what it means

The Milky Way’s stellar halo extends to about 100,000 light-years in radius, according to ESA. This is a different kind of structure from the disk: it is not a dense pancake of stars but a sparse, roughly spherical population surrounding the galaxy.

That large radius matters because it shows how much of the Milky Way is not in the bright, obvious band we see across the sky. ESA’s Gaia material describes the halo as containing older stars and globular clusters, while NASA’s WMAP education page similarly describes a thin stellar disk embedded in a much larger halo. NASA’s Scientific Visualization Studio also notes that the Milky Way sits inside an even larger dark matter halo, which is a separate structure from the stellar halo and often a source of confusion in casual explanations.

One useful way to keep the scales straight:

The disk is enormous in width and surprisingly slim in height. That mismatch is the whole shape of a spiral galaxy.

Why sources give different numbers

The short answer is that different sources are usually measuring slightly different things, not disagreeing about the galaxy itself.

Some pages describe the thin disk only. Others say “the Milky Way disk” and mean the combined stellar disk structure. Some quote a rounded classroom number like 1,000 light-years; others use a more specific estimate like 700 light-years. Halo numbers can also shift depending on whether the source means the stellar halo or the much larger dark matter halo, which are not the same thing, as NASA’s SVS page and WMAP overview make clear.

ESA’s article on the Milky Way’s early history also helps explain why astronomers separate the thin disk, thick disk, and halo at all: they are distinct structural components with different ages and formation histories, not just one blob with blurry boundaries, according to Gaia findings on the thick disk and halo.

So if you want the clean search-answer version, it is this: the Milky Way’s thin disk is about 700 to 1,000 light-years thick, its thick disk is about 3,000 light-years thick, and its stellar halo extends to about 100,000 light-years in radius.

Key Takeaways

Further Reading

Frequently Asked Questions

How thick is the Milky Way galactic disk in light years?

The usual answer is that the Milky Way’s main thin disk is about 700 to 1,000 light-years thick. Some sources round that to 1,000 light-years, while others separately note a thicker stellar component about 3,000 light-years thick.

What’s the difference between the thin disk and thick disk?

The thin disk is flatter and contains much of the Milky Way’s gas, dust, and younger stars, according to ESA. The thick disk extends farther above and below the Galactic Plane and is associated with older stars and an earlier phase of the galaxy’s formation, as described in ESA’s Gaia history article.

How thick is the Milky Way halo?

If you mean the stellar halo, ESA says it extends to about 100,000 light-years in radius. That is usually given as a radius rather than a “thickness,” because the halo is a diffuse, roughly spherical structure rather than a flat disk.

Why do sources disagree on Milky Way thickness?

They usually disagree because they are using different definitions. A source may mean the thin disk, the thick disk, the full stellar disk, the stellar halo, or even the separate dark matter halo, which are different structures.

References

Last reviewed: 2026-06