The Magic’s in the Milky Way

This week, my parents are in New Zealand and are about to go look up at the stars. I am, unsurprisingly, quite jealous. I’ve seen a somewhat dim and wispy Milky Way from dark parts of England and France, but I imagine this is going to be rather better. It was only relatively recently that I really had a good picture of why it looks the way it does, and I wanted to share it with them. So this is for my parents.

The galaxy we live in – the Milky Way – is a huge collection of stars and dust and gas and planets, and it probably has a black hole in the middle. It is, we believe, a huge spiral of stuff in space – but we’ve only ever seen it from the inside. It’s so enormously vast that we and our cameras could probably never get outside it to look back at our galactic home. Still, we think it looks like a pretty classic spiral galaxy, with a big bulge of stars in the centre, and arms that spiral out from the middle.

(I promise to improve these pictures very very soon! Please accept my shoddy sketches for now.)

mw1

The question, then, is how we fit into this picture. Where should we drop our “You are here” pin? Much to our chagrin, we actually live in something of a galactic backwater. We’re stuck out on one of the spiral arms, millions of miles from the hip-happening centre of the galaxy.

mw2

Odds are, if you’ve ever seen an artists impression of the Milky Way, you’ll have seen a beautiful spiral of stars (rather better than these splodgy sketches I hope!). But the Milky Way is only going to look like that from the top and bottom – it can’t be a spiral from all angles. In fact, from the side it’s probably pretty flat – a sort of bulbous disk that bulges in the middle.

mw3

So if the galaxy is side-on to you, it’s going to look something like this. (In fact we see other galaxies in the sky that look like this, so it’s a solid bet.)

As we said, we live in one of the spiral arms of the galaxy, which means we’re fairly near one edge of the galaxy. That means that most of the stars are on one side of us. So what you see in the sky depends on whether you are looking towards the centre of the galaxy or away from it. And because we effectively see the galaxy side-on (we live in the Milky Way, so we couldn’t see it from the top or bottom, we are looking in from the outskirts), we see a streak of stars and dust and gas across our sky in only one place. If we look in any other direction, we’re looking out of the galaxy. We can still see plenty of stars of course – our arm of the galaxy is full of them, and we’ll always be surrounded by those. But the big mass of stars that make up most of our galaxy all sit in that one particular direction.

It’s like living on the outskirts of a city. From where you are, most of the buildings are in one direction. If you look in that direction, you can see the skyline of the city, stretched out across the horizon. But if you look in any other direction – at the sky or ground or simply the opposite direction – you’re looking “out” of the city and you don’t see the highrise mass of buildings. There are, of course, still plenty of buildings around you that you can see – you do live in the city, after all. Swap buildings for stars and the city centre for our galaxy, and there you have it. Happy stargazing!

 

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