Milky Way
What is Milky Way?
Galaxies· Sun ~26,000 ly from the centre. The galaxy our solar system belongs to. Between 100 and 400 billion stars form a flat disc 100,000 light-years wide, and the Milky Way band across the night sky is that disc seen edge-on from inside.
📍 Visible from where you are right now — computed in your browser
Distance and scale
| Diameter | ~100,000 ly |
|---|---|
| Stars | 100–400 billion |
| Sun's place | ~26,000 ly out, in the Orion Arm |
| Centre | supermassive black hole Sgr A* |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
The galactic year: the Sun's 230-million-year trip around the Milky Way
The Sun is not standing still. It travels around the galaxy's centre at about 230 km per second. The galaxy is so large, though, that one lap takes about 230 million years. This span is called a galactic year. The Sun has gone around about 20 times since it formed. During the most recent lap, dinosaurs appeared and died out on Earth, and humans came later.
There is something odd about how the galaxy turns. In the solar system, the outer planets move more slowly than the inner ones. In the galaxy, stars far from the centre hardly slow down at all. The weight of the stars and gas we can see is not enough to hold on to stars moving that fast. So scientists think the galaxy is wrapped in unseen material called dark matter.
Seeing the Milky Way with the naked eye needs a dark sky. A world map of light pollution made from satellite data in 2016 found that about a third of all people cannot see it from where they live. For Europeans the figure is about 60 percent, and for North Americans about 80 percent. It cannot be seen from big cities such as London or New York. On a moonless night far from city lights, the band's curves and dark dust lanes appear.
The galaxy in depth
The Milky Way is a barred spiral galaxy about 100,000 light-years across. At its centre is a bar-shaped region crowded with stars, and spiral arms wind outward from it. It holds an estimated 100 to 400 billion stars. At the very centre is a black hole called Sagittarius A*, with about 4.3 million times the Sun's mass. The Sun lies in the Orion Arm, about 26,000 light-years from the centre.
Because we live inside the galaxy, there is no photo of the whole thing taken from outside. The hazy band that crosses the night sky is the flat disc seen edge-on from within. In 1610 Galileo used a telescope to show for the first time that this band is made of countless stars. The band looks brightest and widest toward Sagittarius, which is the direction of the galaxy's centre.
Until the 1920s, many people believed the Milky Way was the whole universe. That changed when Hubble showed that Andromeda is a separate galaxy beyond ours. The Sky view draws the Milky Way band in the background, so on a summer night you can turn toward Sagittarius and see the thick star clouds near the centre. On the size ladder, you can also compare the Milky Way with Andromeda or IC 1101.
Frequently asked
- How big is Milky Way?
- The diameter of Milky Way is ~100,000 ly.
- In one line, what is Milky Way?
- Galaxies — The galaxy our solar system belongs to.
- What can I see of Milky Way here?
- You can see the figures and story on this page.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.