Structure of the Milky Way

Core Idea

The Milky Way is a barred spiral galaxy roughly 100,000 light-years in diameter containing 200–400 billion stars. It consists of a thin disk with active star formation in spiral arms, a thick disk of older stars, a central bar and bulge, a stellar halo of ancient globular clusters, and an extended dark matter halo. The Sun lies about 26,000 light-years from the galactic center in the Orion Arm. The galactic center hosts Sagittarius A*, a supermassive black hole of ~4 million solar masses, whose nature was confirmed by tracking the orbits of nearby stars accelerating around an invisible point mass.

How It's Best Learned

Trace each structural component of the Milky Way and understand the difficulty of mapping our own galaxy from the inside. Study the stellar orbit data around Sgr A* that earned the 2020 Nobel Prize in Physics and confirm the black hole's mass.

Common Misconceptions

• Images showing the Milky Way from above are artist reconstructions — we have never photographed our own galaxy from outside.
• The Milky Way is not static; it will merge with the Andromeda galaxy in roughly 4.5 billion years.

Explainer

From your study of galaxy morphology, you know that spiral galaxies have disk, bulge, and halo components, and that barred spirals feature an elongated stellar bar through the center. The Milky Way is one such barred spiral galaxy, roughly 100,000 light-years across, containing somewhere between 200 and 400 billion stars. Understanding its structure means learning to see the galaxy we live inside — a challenge, since we cannot step outside to photograph it. Everything we know about the Milky Way's shape comes from measuring distances to stars (using parallax and other methods you have already studied) and mapping their positions from our embedded vantage point.

The galaxy has several distinct structural layers. The thin disk, about 1,000 light-years thick, is where most star formation happens today; it contains young, metal-rich stars, gas, and dust concentrated in spiral arms. The Sun sits in one of these arms — the Orion Arm — about 26,000 light-years from the galactic center. Surrounding the thin disk is the thick disk, roughly 3,000 light-years deep, populated by older, more metal-poor stars on slightly more inclined orbits. At the center lies the central bulge, a dense concentration of mostly old stars surrounding a stellar bar that spans roughly 25,000 light-years. The bar funnels gas inward and shapes the spiral arm pattern.

Beyond the disk and bulge lies the stellar halo, a sparse, roughly spherical distribution of ancient stars and about 150 globular clusters — gravitationally bound balls of hundreds of thousands of stars that are among the oldest objects in the galaxy, dating back 10–13 billion years. The halo stars orbit on random, often highly elliptical paths, unlike the orderly circular orbits of disk stars. Their low metal content tells us they formed before the galaxy had enriched itself through many generations of stellar nucleosynthesis.

The most dramatic feature at the galaxy's heart is Sagittarius A* (Sgr A*), a supermassive black hole with a mass of approximately four million Suns. Its existence was confirmed by tracking individual stars orbiting an invisible point at the galactic center — some reaching speeds exceeding 7,000 km/s. Kepler's laws, applied to these orbits, yield the enclosed mass with extraordinary precision. Enclosing all of these visible components is an extended dark matter halo, inferred from the flat rotation curve of the galaxy: stars far from the center orbit faster than visible mass alone can explain, implying a vast reservoir of unseen mass extending well beyond the stellar halo. This dark matter halo contains roughly ten times more mass than all the galaxy's stars combined.