The Milky Way is the galaxy we live in. Here is what is worth knowing about your own address.
Go outside tonight, look up, and pick any star. Any one at all. The brightest, the faintest, the red one, the one right above your house.
It is in the Milky Way. So is the next one. So is every star you have ever seen with your own eyes, in your whole life, from anywhere on Earth. Not one of them is outside.
The Milky Way is not something far away that you look at. You are inside it, and so is everything you can see up there.
On a really dark night, away from streetlights, a soft pale band crosses the whole sky. That band is the Milky Way — but not a different part of space. It is the same galaxy, seen the long way.
Our galaxy is shaped like a plate, and we live inside the plate. Look up or down and you are looking out through the thin part, and you see a few stars. Look along the plate and you are looking through the whole width of it, past millions of stars too far away to tell apart. They blur into a stripe of light. That stripe is the rest of your own galaxy.
The Milky Way has a few hundred billion stars. That is such a big number that it stops meaning anything, so here is something to hold it against.
Somebody went and counted the trees on Earth. Not one by one — they used satellites and a very large amount of arithmetic — and the answer came out at about three trillion.
So there are roughly ten trees on Earth for every star in the Milky Way. All those stars, the entire galaxy, every star anyone has ever wished on — and our one planet still has ten times more trees.
Next time you are somewhere with trees, try to count them. You will give up quickly, and that is exactly the point: you just gave up on a number smaller than the one you were staring at last night.
How far can you see, standing on the ground? →
The galaxy is not finished. In among the stars there are enormous clouds of gas and dust, and where a cloud gets thick enough, gravity squeezes it into lumps. Squeeze a lump hard enough and it gets hot enough in the middle to switch on. That is a new star.
The most famous place this is happening has a name that sounds made up and is not: the Pillars of Creation, three towers of gas light-years tall, with brand new stars glowing inside their tips.
See the Pillars of Creation → · The Orion Nebula — a star nursery you can find with your own eyes →
Stars do not last forever. How one ends depends entirely on how heavy it was when it was born, and there are two endings.
An ordinary star like our Sun runs out of fuel, swells up, and gently puffs its outer layers away into space — a glowing bubble with the bare core of the old star still shining in the middle. It is called a planetary nebula, which is a silly name (it has nothing to do with planets) given by astronomers with small telescopes who thought the round shapes looked planet-ish.
This is what our Sun will do, in about five billion years.
The Helix Nebula — a preview of our Sun's own ending → · The Ring Nebula →
A star much heavier than the Sun does not go quietly. It explodes — a supernova, bright enough for a few weeks to outshine every other star in its galaxy put together.
In the year 1054 one of these went off, and astronomers in China, Japan and the Arab world all wrote it down: a new star, bright enough to see in daylight, for weeks. The wreckage is still flying outwards today and you can find it with a small telescope.
The Crab Nebula — the wreckage of that 1054 explosion →
Those explosions are where the heavy stuff comes from. The iron in your blood, the calcium in your teeth, the oxygen you are breathing right now — none of it existed when the universe started. It was made inside stars and thrown out when they died, and then some of it ended up here.
You are made of the leftovers.