The Milky Way is our home galaxy: a barred spiral of hundreds of billions of stars, including the Sun. It is an ordinary galaxy among hundreds of billions in the observable universe, but it is the only one we can study from the inside.
Size and structure
- Disk: about 100,000 light-years across (some estimates are larger) and only about 1,000 light-years thick, containing the spiral arms, gas and young stars.
- Bar and bulge: a bar-shaped concentration of older stars across the center.
- Halo: a faint sphere of old stars and about 150 globular clusters, see star clusters.
- Dark matter halo: an invisible envelope much more massive than all the stars, see what dark matter is.
- Central black hole: Sagittarius A*, about 4 million solar masses, see how black holes are detected.
Where are we?
The Sun sits about 26,000 light-years from the center, in a minor spiral arm called the Orion Arm or Orion Spur. It orbits the center at roughly 230 kilometers per second, completing one lap in about 230 million years. The band of light you see on a dark night is the disk seen edge-on from inside.
The Local Group
The Milky Way belongs to the Local Group, a gravitationally bound family of more than 80 known galaxies, most of them dwarfs. The largest members are the Andromeda Galaxy, about 2.5 million light-years away, the Milky Way, and the Triangulum Galaxy. The Large and Small Magellanic Clouds are satellites of our galaxy.
Larger structures
- The Local Group lies at the edge of the Virgo Supercluster.
- In 2014, astronomers mapped an even larger structure, Laniakea, about 500 million light-years across, which contains our supercluster and around 100,000 galaxies.
- On the largest scales, galaxies are arranged in a cosmic web of filaments and empty voids.
How we mapped our own galaxy
Mapping a galaxy from inside is hard because dust blocks the view. Radio and infrared observations reveal the structure behind the dust, and the European Space Agency’s Gaia mission has measured the positions and motions of more than a billion stars. See the different types of galaxies to compare the Milky Way with its neighbors.
