Galaxies

Just 100 years ago astronomers seriously argued about whether anything at all exists outside the Milky Way. Today we know: our galaxy is one of many billions. Galaxies are the islands of the universe, vast collections of stars, gas, dust and dark matter, separated by millions of light years of almost perfect emptiness. Each one is a world of its own, with its own shape, its own history and billions of suns.

Islands in an ocean of emptiness

A typical galaxy contains between a few million and several hundred billion stars, held together by their shared gravity. Between the galaxies, however, yawns vast emptiness. Our neighbouring galaxy Andromeda is around 2.5 million light years away: its light travelled for 2.5 million years before reaching your eye. So you see it the way it looked when the first members of our genus Homo were living in Africa.

Spirals, ellipticals, dwarfs

Galaxies come in different shapes. Spiral galaxies like the Milky Way or Andromeda are rotating discs with sweeping arms in which new stars are constantly forming. Elliptical galaxies are smooth, huge balls of stars with no arms and no disc, full of old, reddish stars. They often form when smaller galaxies merge with each other. Then there are irregular galaxies with no clear shape and countless dwarf galaxies circling the large ones as small companions.

Andromeda is coming, no need to panic

Andromeda and the Milky Way are moving towards each other at about 110 km per second. In around four and a half billion years the two galaxies will probably meet, though newer calculations say this is not certain. That sounds like a catastrophe, but it is astonishingly peaceful: the distances between individual stars are so enormous that virtually no two stars will collide. The galaxies will pass through each other, gravity will swirl their shapes, and one to two billion years later a single large elliptical galaxy will most likely be left.

Billions of islands

When the Hubble telescope stared at a seemingly empty, tiny patch of sky for days, it found thousands of galaxies there. Scaled up to the whole sky, that gives many hundreds of billions of galaxies in the observable universe alone. Each with millions to hundreds of billions of stars. There are far more stars than there are grains of sand on all the beaches of Earth. It is fine to feel dizzy at numbers like these, astronomers feel exactly the same.