We live on the Earth. That’s just one planet orbiting a typical main sequence star called the Sun. And the Sun is a member of a pretty typical spiral galaxy called the Milky Way. But there are many other galaxies out there. Some are larger and older than the Milky Way, and others are smaller and younger. And some galaxies just defy description entirely. Let’s take a look at the different kinds of galaxies astronomers have discovered.
As I mentioned above, we live in a spiral galaxy, measuring about 100,000 light-years across. Spiral galaxies are all about the same size as the Milky Way. Some are a little larger, and some are a little smaller, but they all have that beautiful spiral shape. Examples of other galaxies with the spiral shape include Andromeda galaxy and the Whirlpool Galaxy.
Another kind of galaxies are the elliptical galaxies. These can be small dwarf galaxies with a fraction of the size and mass of the Milky Way. The smallest known galaxies, the ultra-compact dwarf galaxies have this elliptical (or egg like) shape. But the largest galaxies in the Universe are also elliptical. The massive galaxy M87, with a diameter of 120,000 light years is thought to contain several trillion times the mass of the Sun.
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Other galaxies just defy classification entirely. The irregular galaxies don’t seem to have any spiral or elliptical shape. They might have started out with a more recognizable shape, but after a few devastating gravitational interactions with other galaxies, and they got completely reshaped into a unique structure.
The closest other galaxy to the Milky Way is the Canis Major Dwarf Galaxy, located only 25,000 light-years away from Earth.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.