Get away from city lights on a clear night and you'll see a faint, patchy band of light arching across the sky. In 1610, Galileo turned his telescope on it and found that it was made of countless faint stars. That band is our galaxy, the Milky Way — and the reason it looks like a band is that we live inside a flat disk of stars. Look along the disk and you see stars piled up in every direction; look up or down out of the disk and you see far fewer.
That creates a real problem. Mapping a galaxy from the inside is like trying to map a forest while standing in the middle of it — you can't step outside to see the whole thing. Worse, the disk is full of interstellar dust that blocks visible light. In the late 1700s, William Herschel counted stars in every direction and concluded the Sun sat near the center of a small, flat system. He wasn't careless; the dust simply hid everything beyond a few thousand light-years, in every direction, so the Sun looked like it was in the middle.
Today we know the Milky Way is a barred spiral galaxy about 100,000 light-years across. It has four main parts:
| Part | Shape | What's there |
|---|---|---|
| Disk & spiral arms | Flat and thin — ~100,000 ly across, ~1,000 ly thick | Most stars, plus the gas and dust that make new ones |
| Bulge & bar | A crowded central bulge crossed by a bar of stars | Mostly old stars packed closely together |
| Nucleus | The innermost few light-years | A supermassive black hole and a dense star swarm |
| Halo | A huge, dim sphere surrounding everything | Ancient stars and about 150 globular clusters |
The Sun sits in the disk, about 26,000 light-years from the center — a bit more than halfway out — in a small arm called the Orion Spur. That's our galactic address.
A map of the Milky Way as it would look from outside. Click any part — or the Sun — to learn about it. Switch views to see it from above or from the side.
In 1918, Harlow Shapley found a way around the dust. He studied globular clusters — tight balls of hundreds of thousands of old stars. They're bright enough to see from far away, and most of them sit in the halo, above and below the dusty disk, where the view is clear. Using special pulsating stars inside them to measure their distances, Shapley mapped where the clusters were in three dimensions.
They weren't centered on the Sun. They formed a huge sphere centered on a point far away in the direction of the constellation Sagittarius. Shapley reasoned that the clusters must swarm around the true center of the galaxy — so the Sun was nowhere near the middle. His distance was too large, but the idea was right, and it moved us off-center, much as Copernicus had moved Earth away from the center of the solar system.
Mapping the rest took other kinds of light. Infrared passes through dust and reveals the bulge and bar. Radio waves pass through it too — especially the 21-centimeter radio signal given off by cold hydrogen gas, first detected in 1951. Measuring where that gas is and how it moves let astronomers trace the spiral arms across the galaxy for the first time.
The Milky Way as we see it across our sky, with the galaxy's center in the middle. Switch wavelengths.
The whole disk rotates around the center. The Sun travels at about 230 km/s and takes roughly 230 million years to complete one lap — a single "galactic year."
Rewind time and watch where the Sun was in its orbit around the galaxy.
That ties back to Unit 4. Every generation of stars enriches the gas with heavier elements — astronomers call everything heavier than helium "metals" — so a star's metal content hints at its age. Astronomers sort stars into two broad populations:
| Population | Where | Age | Metals |
|---|---|---|---|
| Population I | Disk and spiral arms | Young to middle-aged (the Sun is one) | Metal-rich — made from recycled gas |
| Population II | Halo, globular clusters, much of the bulge | Old — up to ~12 billion years | Metal-poor — formed before much recycling |
The center of the galaxy is hidden from our eyes behind 26,000 light-years of dust, but infrared and radio telescopes can see into it. At the very center is a compact radio source called Sagittarius A* (pronounced "A-star").
For more than 25 years, two teams — led by Reinhard Genzel and Andrea Ghez — tracked stars orbiting that spot in infrared light. One star, called S2, completes a full orbit in about 16 years and whips past the center at thousands of kilometers per second. Using those orbits and the same gravity rules that describe the planets, they calculated the mass inside: about 4 million Suns, packed into a region smaller than our solar system, giving off almost no light. The only thing that fits is a supermassive black hole. Their work earned the 2020 Nobel Prize in Physics, and in 2022 the Event Horizon Telescope released the first image of its shadow.