Galaxies come in a lot of shapes and sizes: huge ellipticals, big spirals, weird squishy irregulars. There is a sub-class called "dwarf galaxies" which are smaller than usual. We actually think they dominate the Universe by number, but because they have fewer stars - a few billion or so tops, compared to the hundreds of billions of a big one like our Milky Way - they fade rapidly with distance. Only a handful are close enough to study well. One of these is DDO 190, a nice little dude something like 9 million light years away. That's close enough to resolve individual stars in the galaxy, as you can see in this really pretty Hubble image of it:
[Click to galactinate, or grab the cosmic 3700 x 2600 pixel version.] DDO 190 is small, but not tiny: about 15,000 light years across. That's about 1/6th the size of our galaxy. It's also well outside our Local group of nearby galaxies (the Andromeda galaxy is less than 3 million light years distant from us, for comparison) and is thought to be part of the M94 galaxy group. But if true it's fairly isolated even from the others on its team; the nearest neighbor appears to be another dwarf galaxy several million light years away from it. This image is pretty nifty. For one thing, you can see lots of far more distant background galaxies, some right through DDO 190, which always gives me a kick. But the dwarf galaxy itself has some surprises. The bluish fuzzy regions are clouds of gas lit by young, hot stars. These stars don't live long (a few million years or so), meaning there's still some star birth going on in the little guy. That blue patch at the bottom is the brightest of them - it looks a bit like a more distant galaxy, but don't be fooled. Interestingly, it has two different populations of stars in it. The younger ones I mentioned (100 million years or younger) tend to be close in to the center, while older ones (4 billion years or more) are located in the outskirts. This is common in dwarf irregular galaxies. The older stars may be showing us what the primeval galaxy looked like, but now a burst of star birth has occurred near the center, making the galaxy look more condensed. Since the vast majority of galaxies in the Universe are dwarfs like this, we think bigger ones like ours get to their size by gravitationally colliding with and absorbing dwarfs. In fact, we know the Milky Way is eating several right now! Galaxies are cool, and pretty, and magnificent, but they're also cannibals. DDO 190 is isolated enough that it may be safe from that fate for quite some time. But the Universe is young, and galaxies patient. In a trillion years or so, we'll see who has whom over for dinner.
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