The Milky Way, explained

At a glance

Key points

From Earth we see it edge-on as a band of light because we sit inside its disc.

• We orbit the centre once every ~230 million years
• Andromeda has roughly even odds of merging with us in 10 billion years
• Most stars are red dwarfs, cooler and dimmer than the Sun

Mapping it from the inside

Charting the galaxy from within its disc is like surveying a forest from one tree. In 1918 Harlow Shapley used the distribution of globular clusters to show that the Sun lies far from the centre, which sits towards Sagittarius, although his first estimate of the galaxy's size was too large. Radio telescopes later traced the spiral arms through the glow of hydrogen gas, which dust cannot hide, and ESA's Gaia has measured positions and motions for nearly two billion stars.

Its parts

At the centre a bulge and a long bar of old stars surround the black hole Sagittarius A*. The thin disc, about 100,000 light-years wide but only about 1,000 thick, holds the spiral arms, gas and young stars. Around it lies a sparse halo of old stars and globular clusters, and all of it is embedded in a far larger halo of dark matter that dominates the total mass, estimated at about 1–1.5 trillion Suns, though that figure is contested.

Where we are

The Sun sits in the Orion Spur, a minor arm, about 26,000–27,000 light-years from the centre, and circles it once every 230 million years or so. It has completed only about 20 such orbits since it formed 4.6 billion years ago. The nearest large neighbour is the Andromeda galaxy, 2.5 million light-years away, and smaller satellites such as the Large Magellanic Cloud orbit the Milky Way itself.

Common misconceptions

No spacecraft has photographed the Milky Way from outside: every face-on picture is an artist's concept or a map rebuilt from data. The pale band across a dark sky is the disc seen edge-on, and the dark lanes within it are clouds of dust, not gaps between stars. And from a light-polluted city the band is invisible, which is why many people have never seen it.

Related entries

• The Milky WayOur barred spiral home galaxy
• Sagittarius A*The black hole at the centre of the Milky Way
• Andromeda GalaxyM31 · the nearest large spiral

Sources

1. AIP: From our galaxy to island universes (opens in a new tab)
2. NASA Science: The Milky Way galaxy (opens in a new tab)
3. NASA Imagine the Universe: Ask an Astrophysicist (opens in a new tab)
4. ESA: ESA/Hubble heic2508 (opens in a new tab)
5. GRAVITY Collaboration 2019, A&A 625 L10 (arXiv) (opens in a new tab)
6. NASA Science: Sun facts (opens in a new tab)

Last reviewed 10 Oct 2026 against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.