The Milky Way, for kids

Our galaxy: a slowly turning city of a few hundred billion stars, so wide that light needs a hundred thousand years to cross it. Every single star you have ever seen with your own eyes lives inside it — and on a dark night you can see the rest of it edge-on, as a band of haze.

What the Milky Way is

What it is

A barred spiral galaxy: a flat, slowly spinning disk of a few hundred billion stars held together by gravity. The Sun takes about 230 million years to go around it once -- the last time we were on this side, dinosaurs were new.

The centre

A crowded golden bar of old stars, and at its very middle Sagittarius A* -- a black hole with four million times the Sun's mass. The stars closest to it whip around it the way comets whip around the Sun.

The arms

The bright spiral arms are not separate streams of stars -- they are traffic jams, places where stars, gas and dust crowd together as they orbit. That is where new stars are being born, which is why the arms sparkle with young blue stars and pink star-forming clouds like the Orion and Eagle nebulae.

Where we live

The whole solar system -- Sun, planets, comets, spacecraft and all -- sits in the Orion spur, a short side-branch between two major arms, 26,000 light-years from the centre. The quiet suburbs: far from the crowded middle, in between the busy arms.

Dust and clouds

Between the stars drift huge clouds of gas and dust. Some, like the Coalsack, are dark and blot out everything behind them; others glow from the inside because new stars are switching on within.

When stars die

The Crab Nebula and the remnants of Tycho's and Kepler's supernovae are wreckage of exploded stars. Those explosions scatter the ingredients new planets are built from -- including the ones you are made of.

Neighbours

Smaller galaxies orbit ours the way moons orbit a planet. The Large Magellanic Cloud is the biggest of them -- close enough that in 1987 one of its stars was seen exploding with the naked eye.

Why it looks like a band and not a spiral

Every picture of a spiral galaxy is a picture of somebody else's galaxy, seen from outside. We are inside ours, about halfway out from the middle and almost exactly in the plane of the disc. Looking along the disc, you see stars piled behind stars for tens of thousands of light-years — that is the band. Looking up out of the disc, you see through it in a few hundred and the sky goes thin.

Where we live in it

The whole solar system sits in the Orion spur, a short side-branch between two of the major arms, about 26,000 light-years out from the centre. The Sun takes something like 230 million years to go round once — the last time it was in this part of its orbit, dinosaurs had not started yet.

The Orion Armpart of the galaxyNot observableThe Galactic Bar and Bulgepart of the galaxyNaked eyeSagittarius A*supermassive black holeNot observableThe Stellar Halopart of the galaxyNot observableThe Dark Matter Halopart of the galaxyNot observable

What is in it besides stars

Clouds where new stars are being made, the wreckage of old ones that exploded, tight knots of stars that formed together, and lanes of dust thick enough to blot out everything behind them. Several of these are within reach of a small telescope, and one of them — the Orion Nebula — is easy.

The Coalsack Nebuladark nebulaNaked eyeThe Helix Nebulaplanetary nebulaWith effortThe Orion Nebulaemission nebulaNaked eyeThe Dumbbell Nebulaplanetary nebulaSmall telescopeThe Ring Nebulaplanetary nebulaSmall telescopeThe Lagoon Nebulaemission nebulaNaked eyeThe Eagle Nebulaemission nebulaWith effortThe Witch Head Nebulareflection nebulaNot observableThe Pleiades (Seven Sisters)open star clusterNaked eyeThe Beehive Cluster (Praesepe)open star clusterNaked eyeThe Hercules Cluster (M13)globular star clusterNaked eye

The black hole in the middle

Sagittarius A*, four million times the mass of the Sun, sitting at the exact centre of everything here. We know it is there because we have watched stars whip around it for thirty years, and nothing else could hold them in those orbits. You cannot see it. You can look at the patch of sky it is behind, which is a stranger feeling than it sounds.

Sagittarius A*supermassive black holeNot observable

Our neighbours

Smaller galaxies orbit ours the way moons orbit a planet, and one big one is coming for us — Andromeda, in about four and a half billion years. Nobody will be hit: galaxies are so empty that they pass through each other.

Large Magellanic Cloudirregular galaxyNaked eyeThe Sagittarius Dwarf Galaxy and Streamdwarf galaxyNot observableAndromeda Galaxy (M31)spiral galaxyNaked eye

How anyone knows the shape of a thing we are inside

• How far away are the stars? — the Earth's own orbit as a measuring baseline
• How far away are the galaxies? — stars that pulse at a rate that gives away their true brightness

Then go outside

This is the one that genuinely needs a dark sky, so it is worth planning — a moonless night, away from town, and twenty minutes without looking at a phone so your eyes catch up. It is the oldest thing anyone has ever looked at with the naked eye.