Galaxies and the Milky Way

Zooming out from home

It is easy to feel that the Solar System is huge — and it is. Light from the Sun takes hours to reach the outer planets. But the Solar System is only a tiny speck inside something far, far bigger: a galaxy.

A galaxy is an enormous system of stars, gas, dust and dark matter, all held together by gravity. Our Sun is just one star among hundreds of billions in our galaxy, and our entire Solar System is a barely noticeable dot within it. Understanding galaxies means stretching your imagination to its limits — but it also helps you understand exactly where, in all the vastness of the Universe, we actually live.

Meet the Milky Way

The galaxy we live in is called the Milky Way. It got that name thousands of years ago, because the ancient Greeks and Romans saw a faint, milky band of light arching across the dark sky and imagined it as spilled milk.

That hazy band is not a cloud and not a single object. It is the combined glow of billions of distant stars in our own galaxy, too far away to see one by one. The reason it looks like a band is one of the most mind-bending facts in astronomy: we are inside the galaxy. The Milky Way is shaped like a flat disc, and we sit within that disc. When we look along the plane of the disc, we see star upon star piling up into a glowing strip. When we look up out of the disc, there are fewer stars, so that part of the sky looks darker.

The Milky Way is a barred spiral galaxy. That means:

• It is shaped like a flat, spinning disc with curved spiral arms sweeping out from the middle.
• Across its centre runs a straight bar of stars, which is where the "barred" part comes from.
• At the very heart sits a dense bulge of older stars and a supermassive black hole called Sagittarius A*.

The whole disc is around 100,000 light-years across — meaning light, the fastest thing in the Universe, would take 100,000 years to cross it. It holds somewhere between 100 and 400 billion stars, and the entire galaxy slowly rotates. Our Sun takes about 230 million years to complete a single orbit around the galactic centre. The last time the Sun was in this exact spot, dinosaurs had not yet appeared on Earth.

Where do we live in the galaxy?

For a long time people assumed Earth, or at least the Sun, must sit at the centre of everything. We now know that is not true at all. Our Solar System lies far out in one of the spiral arms, roughly two-thirds of the way from the centre to the edge, in a structure called the Orion Arm.

This is, in fact, a rather peaceful place to live. Near the crowded galactic centre, stars are packed tightly together and powerful radiation makes life unlikely. Out in our quiet suburb of the galaxy, stars are spread far apart and conditions are calm and stable enough for a planet like Earth to develop life over billions of years.

The three shapes of galaxies

The Milky Way is a spiral, but galaxies come in several shapes. Astronomers sort them into three main types:

1. Spiral galaxies — flat, rotating discs with graceful curved arms, like the Milky Way and Andromeda. The arms are bright with young, hot stars and glowing clouds of gas where new stars are being born.
2. Elliptical galaxies — smooth, rounded blobs of stars, shaped like a ball or an egg. They contain mostly old, reddish stars and little gas, so very few new stars form in them. The biggest galaxies in the Universe are giant ellipticals.
3. Irregular galaxies — galaxies with no clear shape, often small and messy-looking. Many have been pulled out of shape by the gravity of a larger neighbour passing nearby.

Sorting galaxies by shape helps astronomers understand their history. A galaxy's shape is a clue to how it formed, how old its stars are and whether it has collided with another galaxy.

A galaxy is not alone

Galaxies are social. They cluster together in groups, bound by gravity. The Milky Way belongs to a small cluster called the Local Group, which contains around 50 to 80 galaxies. Most are small, but two large ones dominate: our Milky Way and the Andromeda Galaxy.

Andromeda is the nearest large galaxy to us, about 2.5 million light-years away. Astonishingly, on a very dark night you can see it with your own eyes as a faint smudge — making it the most distant object most people can ever see unaided. The light reaching you from Andromeda left it 2.5 million years ago, around the time the very first humans were appearing on Earth.

Andromeda and the Milky Way are slowly drifting towards each other. In about four to five billion years they will gently merge into a single, larger galaxy. Because galaxies are mostly empty space, individual stars almost never collide — instead the two galaxies will swirl together over hundreds of millions of years, eventually settling into one.

And the Local Group itself is just a tiny part of something larger. It sits within a vast collection of clusters called the Virgo Supercluster, which is part of an even bigger structure. Zoom out far enough, and galaxies trace a cosmic web of glowing threads and dark, empty voids stretching across the entire Universe. The whole observable Universe is thought to hold at least two trillion galaxies.

Try it yourself: find the Milky Way

You can see our home galaxy with your own eyes — if you escape the city lights.

1. Choose a dark, moonless night. A bright Moon or city lights will wash the Milky Way out completely. The countryside, far from streetlights, is ideal. Summer evenings in the northern hemisphere give some of the best views.
2. Let your eyes adapt to the darkness for at least 20 minutes. Avoid looking at phone screens, which ruin your night vision.
3. Look for a faint, milky band of light stretching across the sky from one horizon towards the other. It may look like a thin cloud at first — but unlike a cloud, it does not drift or change.
4. Use binoculars to scan along it. The hazy band suddenly breaks apart into thousands of individual stars. In that moment, you are looking edge-on through the disc of your own galaxy.

When you find it, take a second to think about what you are seeing: the combined light of hundreds of billions of stars, including some whose light has travelled for thousands of years to reach your eyes — all part of the great star-city we call home.

Want to keep exploring? See Telescopes and Studying Space to learn how scientists study galaxies, and Planets of the Solar System to zoom back in to our own tiny corner of the Milky Way.