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You look up in the sky at night and you see mostly stars. You look up at the stars at night and you see a lot of stars but if you know where to look, and this is really worth doing, you look in the region of Andromeda and Perseus. If you know where to look and it is a really dark night and you are away from the city, there is a fuzzy patch up there called the Andromeda Galaxy. It used to be the Andromeda nebula and now it is Andromeda galaxy. (That is a significant name change.)

image Although this image was taken through a telescope, you can see this galaxy the unaided eye. There it is; it's a fuzzy spot, you can't feel the structure with your eye but the fuzzy spot up there, and this was a great mystery at the time of Edwin Hubble. It was previously thought to be a big blob of gas. You look at all these other stars here, there are stars all over, and it was assumed that this big blob of gas was in front of those things. What Hubble discovered was that this was not a big blob of gas but in fact was made up of a huge collection of stars, individual stars. He could resolve the individual stars in this thing. That meant that the blob was not in front of the stars, but it was background, it was behind them. You look at this, see these stars here, well that is not a star, but look at these stars here they are all in the foreground and this thing is behind them and it consists of a huge number of stars. After Hubble's discovery, this collection of stars was initially dubbed "an island universe."

The Milky Way galaxy is very much like this: a bunch of stars shaped like this. We are sitting in the edge of the Milky Way galaxy, and as we look out what we see mostly are the stars within our own galaxy, but if we look beyond we can sometimes see distant galaxies and this is a distant galaxy. There are about as many stars in this, somewhat more than in our own Milky Way galaxy.

image Look with the Hubble space telescope. Take one pixel from this image, not here but look over here, look in the background and what do you see? Well you will still see the foreground stars from our own Milky Way galaxy, but what you will see, actually this is just gorgeous. What you see is there are some foreground stars, here's one, it's a foreground star, where's another foreground star? I don't think there are any others in this picture. What are the rest of those things? They are galaxies. There are more galaxies in the universe then there are stars in the Milky Way. How many stars are there in the Milky Way? About 10 billion, we estimate, and as you go out you see all these galaxies, going way back, every where you look there are more and more and more, the universe is still with these galaxies.


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