Galaxy Disk
Written by Fraser Cain
NGC 253
A grand spiral galaxy, like our own Milky Way, is shaped like a spinning pancake (or record). For example, the Milky Way measures 100,000 light-years across, but the galactic disk is only 1,000 light-years thick.
The most dramatic feature of a galaxy disk are the spiral arms. These start near the center of the galaxy, and then wrap around the galactic disk like a pinwheel to the galaxy's outer edges. Some galaxies have 4 or more spiral arms passing through the disk; the latest evidence is that our Milky Way probably only has two. The spiral arms are a region of greater density in the galaxy, and this is where most of the galaxy's star formation is happening.
You might be surprised to know that a galaxy's spiral arms are constantly changing. They're a region of greater density that sweeps around the galaxy. At various times, different regions of the galaxy are caught up in the spiral arms. As the density wave moves in, gas and dust collects together and new stars form. And then the density wave sweeps past and triggers a different region into furious star formation.
Why do galaxies form into disks? Because they're spinning. When the galaxy first came together billions of years ago, conservation of momentum of all the material that made the galaxy set it spinning. This is the same reason that the Solar System's planets are aligned along a disk.
The outer edge of the galactic disk is typically a ring of stars called the outer ring. Astronomers think these contain the stellar remnants of other galaxies that merged or collided.
Above and below the galactic disk is a galaxy's halo; a spheroid halo of old stars and globular clusters that can extent out twice the size of the disk. There are also tidal streams of stars surrounding the disk in all directions. These are the remnants of dwarf galaxies torn into long streams by gravity.
We have written many articles about galaxies for Universe Today. Here's an article about a galaxy with spiral arms that rotate backwards.
Want more resources on galaxies? Here's a link to the Messier catalog's section on Galaxies. And here's NASA's World Book on galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.
Filed under: Astronomy
