Milky Way
The Milky Way is the barred spiral galaxy containing the Solar System, comprising roughly 100-400 billion stars arranged in a flattened, rotating disc with spiral arms, a central bar-shaped structure surrounding the supermassive black hole Sagittarius A*, and an extended halo of dark matter estimated to outweigh the galaxy's visible matter by a factor of roughly 5 to 1.
Curious
From the outside, the Milky Way would appear as a barred spiral galaxy: a flattened, rotating disc containing prominent spiral arms, with a bright central bulge crossed by an elongated bar-shaped structure of stars near the middle. The Solar System sits roughly 27,000 light-years from the galactic centre, out in one of the disc's spiral arms (the Orion Arm, sometimes called the Orion Spur) — a location that, among other things, means the Milky Way's own disc is what produces the faint band of light across the night sky that gives the galaxy its name.
Despite containing hundreds of billions of stars, the Milky Way's visible matter is actually the minority of its total mass — like most large galaxies, it is embedded in an extended halo of dark matter (see the Codex's dark matter entry) whose gravitational influence extends far beyond the visible stellar disc and accounts for roughly five times as much mass as all the galaxy's stars and gas combined.
Exploring
| Property | Approximate value |
|---|---|
| Stellar count | ~100–400 billion stars |
| Disc diameter | ~100,000 light-years |
| Sun's distance from centre | ~27,000 light-years |
| Sun's orbital period | ~230 million years |
Deep Dive
Harlow Shapley's 1918 study of the distribution of globular star clusters provided the first reasonably accurate determination that the Sun is located far from the Milky Way's actual centre — correcting an earlier, more geocentric-influenced assumption that the Solar System sat near the galaxy's middle. This period also saw the "Great Debate" of 1920, a public scientific discussion between Shapley and Heber Curtis over whether spiral nebulae (later confirmed by Hubble in 1924 to be entirely separate galaxies) were part of the Milky Way or independent structures — a debate settled decisively in Curtis's favour once Hubble's Cepheid variable star measurements established the vast distances involved.
Structure and rotation: The Milky Way's disc rotates differentially — stars at different distances from the galactic centre orbit at different rates, rather than as a single rigid body — and the precise pattern of this rotation, measured through radio and optical observations of gas and stars across the disc, was one of the earliest and most compelling pieces of evidence for dark matter's presence within our own galaxy, following the same rotation-curve logic Vera Rubin later applied more broadly to external galaxies (see the Codex's Vera Rubin biography).
The Milky Way belongs to a small collection of gravitationally bound galaxies called the Local Group, dominated by the Milky Way itself and the larger Andromeda Galaxy, alongside dozens of smaller satellite and dwarf galaxies. The Local Group is itself part of the larger Virgo Supercluster, which in turn forms part of the still-larger Laniakea Supercluster — a nested hierarchy of gravitationally associated structure extending the galaxy-formation picture (see the Codex's galaxy formation entry) up to the largest scales studied in modern cosmology.
Concept Relationship Map
Requires: galaxy formation, dark matter
Enables: large-scale structure, supermassive black holes (Sagittarius A*)
Commonly confused with: assuming the Milky Way is a static, finished structure rather than an actively growing, merging system
Sources
- Shapley, H. (1918). The Astrophysical Journal, 48, 154–181.
- Gaia Collaboration (2023). "Gaia Data Release 3: Summary of the Content and Survey Properties." Astronomy & Astrophysics, 674, A1.