Three things: ours is not the only one, we can point at the
nearest one, and there are more of them than you are going to like.
1. The Milky Way is not the only one this was news, and recently
Our galaxy is a few hundred billion stars held together by gravity, turning
slowly. For most of human history, people thought that was everything —
that the Milky Way was the universe, and outside it there was nothing at all.
That turned out to be wrong, and it turned out to be wrong within living
memory of your great-grandparents. There are other galaxies. Lots of them. Each
one is another few hundred billion stars, and each one is a very long way from
every other one.
spiralellipticalirregularedge-on
They come in shapes, and the shape tells you the story. Spirals like ours are
still busy making new stars in their arms. Ellipticals are old and quiet, with
almost no gas left to make anything from. Irregular ones usually got that way
because something big went past and pulled them out of shape.
2. The nearest big one has a name and a place in the sky you can find it
It is called the Andromeda Galaxy, and it is the furthest
thing a human eye can see without any help at all. On a dark autumn night, if you
are north of the equator, it is a small faint smudge — and that smudge is a whole
other galaxy with a trillion stars in it.
The light hitting your eye left Andromeda two and a half million years
ago. It has been travelling that entire time, across everything in
between, to end up in you.
(There are a couple of much smaller, scruffier galaxies that are nearer —
little ones orbiting ours, like moons around a planet. Andromeda is the nearest
big one, and the only one most people will ever spot.)
How anyone worked this out
For a long time astronomers could see fuzzy patches in their telescopes and
had no idea what they were. Some were clouds of gas inside our own galaxy —
that part was clear. But some of them were spirals, and nobody could tell
whether those were small and nearby or enormous and impossibly far away.
In 1920 two astronomers had a public argument about it in Washington, which
people still call the Great Debate. One said the Milky Way was
everything and the spirals were inside it. The other said the spirals were
whole separate galaxies, unthinkably far off. Neither could prove it, and they
both went home.
Three years later Edwin Hubble settled it. Pointing the
biggest telescope in the world at Andromeda, he found a particular kind of star
that brightens and dims on a regular beat — and the speed of that beat tells
you how bright the star truly is. Compare that with how bright it
looks, and you get the distance.
The answer came back enormous. Far too far away to be inside our galaxy.
Andromeda was not a cloud in the Milky Way; it was another Milky Way, entirely
of its own. The universe got very much bigger that night, and it has been
getting bigger ever since.
3. How many there are hold on to something
Astronomers pointed a telescope at a patch of sky that looked completely
empty — no stars, nothing, a piece of black the size of a grain of sand held at
arm's length — and left it collecting light for days.
It was full of galaxies. Thousands of them, in the gap between the stars.
Counting the whole sky that way gives roughly two trillion
galaxies. That is about the same number as there are trees on
Earth — one galaxy for every tree, and every single one of those galaxies
holds hundreds of billions of stars of its own.
So what is there more of?
Stars in our galaxy: a few hundred billion.
Trees on Earth: about three trillion — ten times more
than that.
Galaxies in the universe: about two trillion — roughly
one per tree.
Stars in the universe: every galaxy's worth, all added up. That is more
stars than there are grains of sand on every beach and every desert on
Earth, and it is not close.
So there are far, far more stars than galaxies — each galaxy is a whole
bagful of them.
One more thing
Andromeda is coming towards us. In about four and a half billion years the
two galaxies will meet and merge into one bigger one.
Nothing will crash. Galaxies are mostly empty space, so the stars will drift
straight past each other. The sky will just slowly, over millions of years,
become a different sky.
