When I was a kid I used to love the nights when it was clear enough to see the Milky Way as an arch across the sky, a bit like this:

(source)

I remember wondering why it was lumpy and not smooth.

Now with recent photos I've finally realised that the blotchiness is caused by dust clouds blocking the light in patches. In a new panoramic image of the whole thing you can see it very clearly:

(click for high-res)

You can see that the glowing mass of stars behind is a perfect streak of light, with patchy black clouds in front. The Milky Way is a spiral galaxy of course but because we're in it ("far out in the uncharted backwaters of the unfashionable end of the western spiral arm") it looks like a line, with a bulge in the middle at the centre of the galaxy.

You can see the clouds here too in another galaxy:

(click for high-res)

This post was originally on LiveJournal.