Galaxies and the universe

Our galaxy is a flat, turning disc of a few hundred billion stars. We live inside it, about half way out, and have never seen it from outside.

The space between the stars holds thin gas and a little dust. It is emptier than any vacuum made on the Earth, and it is what every star is made from.

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The arms of a spiral galaxy are not fixed strings of stars. They are patterns that stars pass through, lit up by the young stars born inside them.

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The middle of the Milky Way is hidden behind dust. Seen in other kinds of light, it is a crowd of stars whirling round a black hole of four million Suns.

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Galaxies come as spirals, smooth ellipticals and ragged irregulars. They gather into groups and clusters, strung across space like a web.

Galaxies grow by pulling in their neighbours. A collision takes hundreds of millions of years, hardly any stars touch, and both galaxies come out a different shape.

Galaxies were not always as they are now. They began small and ragged, grew by pulling in gas and swallowing each other, and made most of their stars long ago.

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In about one galaxy in ten, the centre outshines all the stars put together. The light comes from gas heating up as it falls towards a giant black hole.

Some stars brighten and fade on a steady beat, and the beat gives away how bright they truly are. That is how the distances to other galaxies were first measured.

Galaxies are not scattered evenly. They gather into groups, clusters and long threads, with huge empty spaces between, like the walls of a foam.

A cluster is hundreds or thousands of galaxies held together by gravity. Most of what is in one is not galaxies at all: it is hot gas and dark matter.

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Distant galaxies are all moving away from us, and from each other. Run the film backwards and you arrive at a hot, dense beginning 13.8 billion years ago.

In its first few minutes the universe made its hydrogen and helium. After 380,000 years it turned clear, and the light set free then is still arriving.

A faint glow of microwaves comes from every direction in the sky. It is the light of the young universe, and its pattern holds the age and make-up of everything.

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Nobody watched the universe begin. Four separate lines of evidence, found in different ways by different people, all point to the same hot, dense start.

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13.8 billion years, from the first light to this evening, squeezed into a single calendar year so that the gaps between events can be felt.

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Everything we can see adds up to about 5 per cent of the universe. The rest is known only by what it does, and nobody knows what it is.

There is an edge to what can be seen, 46 billion light years away in every direction. It is not the edge of the universe, and nobody knows how far the rest goes.

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The Sun will die, the galaxies will run out of gas, and the last stars will go out. What happens after that depends on something nobody yet understands.

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