You already know our neighbourhood. The Earth circles the Sun; eight planets, a scattering of moons, asteroids and comets travel with it; together they make the Solar System. It feels enormous — our fastest spacecraft take years just to cross it. But now we are going to keep zooming out, and the Solar System is about to shrink to a single, almost invisible speck.

Here is the first jolt. Our Sun — that blazing ball a million Earths could fit inside — is just one ordinary star. And it is not alone. It belongs to a colossal city of stars called the Milky Way galaxy, home to somewhere around 200 billion stars, most of them with worlds of their own. On this page we build up the true scale of things, from our one Sun all the way out to the whole observable Universe.

Our home galaxy: the Milky Way

A galaxy is a gigantic collection of stars, together with the gas and dust they form from, all held together by gravity. Ours, the Milky Way, is a slowly spinning disc — a flat, pinwheel-shaped swirl of stars wound into bright spiral arms, with a dense bulge of stars packed into the middle. From edge to edge it measures about 1\times10^{5} light-years across (we'll unpack that strange unit in a moment).

Just as gravity keeps the planets looping round the Sun, gravity keeps all those billions of stars bound to the galaxy — and the whole disc turns. Our Sun is not at the centre; it sits about two-thirds of the way out in one of the quiet spiral arms, and it drifts slowly round the galaxy's core, taking roughly 230 million years to complete a single lap. The Solar System has gone round only about twenty times in the entire history of the Earth.

On a really dark night, away from streetlights, you can actually see our own galaxy: a faint, milky band of light arching across the sky. That band is the combined glow of countless distant stars in the disc — you are looking edge-on, from the inside, at the Milky Way itself. That is how it got its name.

Billions of galaxies, with vast gaps between

Now zoom out again. The Milky Way is not the only galaxy — it is one of an estimated two trillion (2\times10^{12}) galaxies scattered through the observable Universe. Some are grand spirals like ours, some are fuzzy balls (ellipticals), some are ragged little dwarfs. Each one is its own island of hundreds of billions of stars.

Galaxies are not sprinkled evenly. Gravity gathers them into clusters — groups of tens, hundreds or thousands of galaxies travelling together. Our Milky Way belongs to a small cluster called the Local Group, whose biggest member is the great Andromeda galaxy. Between the clusters lie almost unimaginably huge empty voids — regions of near-perfect nothingness millions of light-years wide. The Universe is mostly gap.

Why we measure space in light-years

Kilometres are hopeless out here. The distance to the next galaxy written in kilometres would be a number with more than twenty zeros — impossible to picture or compare. So astronomers use a much bigger ruler: the light-year.

Light is the fastest thing there is, racing along at c = 3\times10^{8} metres every second. A light-year is simply the distance light travels in one year — and that turns out to be an enormous

Because these numbers are so vast, we write them in standard form — 9.5\times10^{15} is far tidier than 9\,500\,000\,000\,000\,000. Now the cosmos gets manageable. Light from the Sun takes about 8 minutes to reach us, so the Sun is 8 light-minutes away. The nearest star beyond the Sun is a little over 4 light-years off. Right across our galaxy is about 1\times10^{5} light-years. And the nearest big galaxy, Andromeda, is around 2.5\times10^{6} light-years away.

• To the Moon — about 1.3 light-seconds
• To the Sun — about 8 light-minutes
• To the nearest star (Proxima Centauri) — about 4.2 light-years
• Across the Milky Way — about 100,000 light-years (1\times10^{5})
• To the Andromeda galaxy — about 2.5 million light-years (2.5\times10^{6})
• Across the observable Universe — about 93 billion light-years (9.3\times10^{10})

Looking out into space is looking back in time

The light-year hides a beautiful secret. Because light takes time to cross those distances, the light entering your telescope tonight set out long ago — so you never see a distant object as it is now. You see it as it was when the light left it.

The Sun you see is 8 minutes old: if it winked out, you'd carry on seeing it shine for another 8 minutes. Look at Proxima Centauri and you see 4 years into the past. Look at Andromeda with the naked eye — it is the most distant thing a human eye can see unaided — and that faint smudge is 2.5 million years old: the light left it before humans even existed. The deepest telescope images show galaxies as they were billions of years ago, when the Universe was young. To look far out into space is to look far back in time. Astronomy is a kind of time machine.

(What all that ancient light reveals — that the galaxies are rushing apart, and the Universe began in a Big Bang — is the story of red-shift and the expanding Universe.)

See the scale for yourself

Step the slider from Earth outward. At each click the view zooms out by a huge factor, and what filled the whole picture a moment ago becomes a tiny dot — or vanishes entirely. Watch the distance label at the bottom leap from kilometres, to light-hours, to light-years, to billions of light-years. This is the true shape of your address in the cosmos.

Your cosmic address

Put it all together and you can write down where you live, from the doorstep outward:

You → Earth → the Solar System → the Milky Way galaxy → the Local Group → the observable Universe.

Every step out multiplies the size by a staggering amount, and at every step the thing you called "huge" the moment before shrinks to a speck. That is the real scale of the Universe: almost entirely empty, almost entirely dark, and far, far bigger than everyday words like "big" can carry.