What is the Milky Way?

The Milky Way is the galaxy containing our solar system, a barred spiral roughly 100,000 light years in diameter and a few thousand light years thick through most of the disc, holding somewhere in the range of one hundred to four hundred billion stars. The sun sits about 26,000 light years from the center, in a minor spiral arm, and takes roughly 230 million years to complete one orbit. The hazy band visible from dark locations is the combined light of countless distant stars in the disc, seen edge-on because we are inside it, a fact Galileo established when he first resolved the band into individual stars with a telescope in 1610.

At the center sits Sagittarius A star, a supermassive black hole of about four million solar masses, whose existence was established by tracking the orbits of stars whipping around an invisible mass, work that earned a share of the 2020 Nobel Prize in Physics. The Event Horizon Telescope released an image of its shadow in 2022. Most of the galaxy's mass, however, appears to be dark matter in a halo extending well beyond the visible disc, inferred from the fact that stars in the outer regions orbit far faster than visible mass alone can explain.

Two points of context. The Milky Way is one of a large number of galaxies in the observable universe, and it belongs to the Local Group along with Andromeda and dozens of smaller galaxies. Andromeda is approaching and is expected to merge with the Milky Way in roughly four billion years, an event that will rearrange both galaxies without individual stars colliding, since the distances between stars are so vast. And light pollution has made the band invisible to a large majority of people in North America and Europe, which is a recent change on any historical timescale.