The Milky Way: a travel guide to our home galaxy
You live in a spiral galaxy a hundred thousand light-years across, together with 200 to 400 billion stars. Every star you have ever seen with the naked eye belongs to it. The misty band of light arcing across a dark sky — the "milky way" the Greeks took for spilt milk — is simply the view straight through our own stellar disk. This is the travel guide to your home galaxy, from suburb to city centre.
The map
The Milky Way is a barred spiral: a flattened disk of stars, gas and dust with an elongated bulge (the bar) at the centre and spiral arms winding outward. The disk spans some 100,000 light-years yet averages only ~1,000 light-years thick — proportionally flatter than a pancake. Around it floats a spherical halo of ancient star clusters and, invisible but indispensable, an enormous cloud of dark matter holding the whole thing together.
Our address: the Orion Arm, a modest side street between the great Perseus and Sagittarius arms, about 27,000 light-years from the centre. A quiet suburb — thankfully: near the centre, supernovae and radiation make life considerably less cosy.
The centre: a sleeping giant
At the heart of the Milky Way, hidden behind thick dust clouds in Sagittarius, lives Sagittarius A*: a supermassive black hole of four million solar masses. For decades astronomers tracked stars whipping around it at thousands of kilometres per second — proof that something invisible and unimaginably heavy sat there (Nobel Prize 2020). In 2022 the Event Horizon Telescope photographed the beast itself.
Compared with the ravenous quasars of distant galaxies, our black hole is remarkably calm: it is barely feeding at present. But X-ray echoes in the surrounding gas betray a hearty meal just a few centuries ago.
A galaxy in motion
- We orbit along. The Sun races around the centre at ~230 km/s; one lap (a "galactic year") takes about 225 million years. Last time we were here, the first dinosaurs were walking.
- The Milky Way eats. Our galaxy grew by devouring smaller ones; the streams of those meals (like the Sagittarius Stream) still trail through the halo.
- Collision ahead. In ~4.5 billion years the Milky Way and Andromeda will merge into one giant galaxy ("Milkomeda"). Stars will almost never collide — the distances are too vast — but both spirals will deform into an elliptical cloud.
Frequently asked questions
How big is the Milky Way?
~100,000 light-years across, 200–400 billion stars, plus a far larger dark-matter halo.
Why is it called the Milky Way?
The Greeks saw spilt milk in the band of light: galaxias (gala = milk) — the root of the word galaxy.
Can you see it with the naked eye?
Yes, under a truly dark sky far from city lights — best in late summer towards the south.