Thursday, September 29, 2011

Long way to the chemist’s: a rough guide to distances in the universe

Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

Douglas Adams, The Hitchhikers Guide to the Galaxy

We all know the universe is large, very large, but is it possible to really comprehend just how large it really is? Sit down, take a deep breath, and we can give it a go.

In my previous scale article, we considered the sizes of stars, and finished by imagining the sun being the size of an orange. On this scale, the nearest star to the sun, also the size of an orange, would be 2,300 kilometres away.

Even through stars can be immense on human scales, they are dwarfed by the distances between them.

On a clear night, away from the lights of civilisation, we may be able to pick out a few thousand individual stars as mere points of light.

The smooth swathe of light that accompanies them, however, is the combined light of many more distant stars. How many? It turns out the Milky Way is home to more than 200 billion stars, lots of stars like the sun, a few spectacular giants, and many, many faint dwarfs.

To get a handle on the size of the Milky Way, let’s pretend the distance across it is 3,000km, roughly the distance between Sydney and Perth.

On this scale, the separation between the sun and its nearest neighbour would be about 100 metres, whereas the diameter of the sun itself would be about a tenth the thickness of a human hair. Other than a bit of tenuous gas, there’s a lot of empty space in the Milky Way.

For much of human history, we have prided ourselves on being at the centre of the universe, but as Douglas Adams pointed out, we live in the “unfashionable end of the Western Spiral arm of the Galaxy”.

If the small town of Ceduna in South Australia, sitting roughly midway between Sydney and Perth, was the centre of the Milky Way, our sun would be orbiting 850km away, somewhere beyond Mildura in north-western Victoria (and, no, I’m not suggesting Mildura is unfashionable!)

So the Milky Way is huge, and light, traveling at 300,000 kilometres a second, takes 100,000 years to cross from side to side.

But we know that we share the universe with many other galaxies, one of the nearest being a sister galaxy to our own, the large spiral galaxy in Andromeda.

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