The Milky Way does not have a single fixed height, but near the Sun most disk stars lie within about 1,000 light-years of the galactic plane. Astronomers describe this with a thin-disk scale height of roughly 300 parsecs and a thick-disk scale height of about 900 to 1,200 parsecs, so the stellar disk is effectively a few thousand light-years thick when both components are included. Toward the center, the bulge swells to several thousand light-years in thickness, while the sparse stellar halo extends tens to hundreds of thousands of light-years above and below the plane.
What is the thickness of the Milky Way?
“Thickness” depends on which Galactic component you mean and how you define it. In the Solar neighborhood:
- Thin disk: scale height ≈ 300 pc (~1,000 ly). This contains most young and intermediate-age stars and most of what you see in the night sky.
- Thick disk: scale height ≈ 900–1,200 pc (~3,000–4,000 ly). This is an older, puffier component with fewer stars than the thin disk.
- Central bulge/bar: rises several thousand light-years above the plane, with a box/peanut shape mapped by red clump giants.
- Stellar halo: a very low-density, roughly spheroidal component that extends to ≳ 100,000 ly from the center.
For the disk, a practical rule of thumb is that the vast majority of disk stars are found within a few thousand light-years of the Galactic plane, while rare halo stars and globular clusters roam far beyond.
Representative measurements come from star counts and kinematics in large surveys. Classic SDSS-based work found thin- and thick-disk scale heights of ~300 pc and ~900 pc, respectively (Jurić et al. 2008). Review articles summarize consistent values and their uncertainties (Bland-Hawthorn & Gerhard 2016).
How is galactic thickness measured?
Instead of a hard “top” and “bottom,” astronomers model how stellar density falls with height z above the plane, typically using an exponential. The scale height is the characteristic distance over which the density drops by a factor of e. Because the density never truly reaches zero, thickness is quoted in terms of one or more scale heights:
- ~1 scale height encloses the highest-density layer near the plane.
- ~2–3 scale heights include the large majority of stars in that component.
Thin disk: hz ≈ 300 pc; Thick disk: hz ≈ 900–1,200 pc. The outer disk also flares, so the scale height increases with Galactocentric radius.
Modern data from ESA’s Gaia mission show the disk is not perfectly flat: it is warped, twisted, and exhibits vertical waves, especially in the outskirts.
Do stars orbit far above and below the galactic plane?
Yes. Even disk stars follow slightly inclined orbits and oscillate above and below the plane because the Galaxy’s mass is concentrated near the midplane. In addition:
- Thick-disk stars routinely reach thousands of light-years from the plane.
- Halo stars can have highly inclined, even retrograde, orbits with vertical excursions of tens of thousands of light-years.
A nearby example is Kapteyn’s Star, a halo star on a retrograde Galactic orbit, likely debris from a cannibalized dwarf galaxy. It is currently only ~13 ly away but belongs to a population that spends most of its time far from the disk.
Globular clusters, which are dense, old star systems orbiting the Milky Way, also follow high-inclination orbits. Many reach well above and below the plane, some to distances of tens of kiloparsecs.
What about the bulge, warp, and halo?
Bulge/bar: The inner Galaxy has a box/peanut-shaped bulge that is several thousand light-years thick. Red clump star maps indicate a half-height of roughly 1–2 kpc, implying a total vertical extent of roughly 6,000–13,000 ly across the bulge region (Wegg & Gerhard 2013).
Warp and flare: The outer stellar and gas disks are warped and flared. The warp tilts the disk away from the midplane at large radii, while the flare increases the local scale height outward. Gaia has mapped this in detail (ESA Gaia).
Halo: The stellar halo is a diffuse, roughly spherical component composed of old stars and streams from accreted dwarfs. It extends to at least ~100 kpc (~330,000 ly) and likely farther (Helmi 2020).
Where is the Sun relative to the galactic plane?
The Sun sits modestly above the midplane and oscillates through it.
- Height now: about 20–25 pc (~65–80 ly) above the local midplane (Bennett & Bovy 2019).
- Vertical speed: about 7 km/s toward the north Galactic pole relative to the local standard of rest (Schönrich, Binney & Dehnen 2010).
- Oscillation: period roughly 60–70 million years with a modest amplitude of a few hundred light-years peak-to-trough (Feng & Bailer-Jones 2013).
Note on reference planes: the Solar System’s ecliptic plane is tilted by about 60 degrees relative to the Milky Way’s galactic plane, so Galactic thickness is always measured perpendicular to the galactic plane, not the ecliptic.