A short description…
- Spiral galaxies and the galactic disk are quite thin, dense, and rotate relatively fast. (Our Milky Way galaxy is believed to be a barred spiral.)
- The majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and might not interact through any means except gravity.
- Halo stars are typically much older and have much lower metallicities (that is to say, they are almost exclusively composed of hydrogen and helium) than disk stars.
- Many disk galaxies have a puffed up outer disk (often called the “thick disk”) that is composed of old stars.
- Globular clusters are typically old and metal-poor as well, but there are a few that are not nearly as metal-poor as most, or have some younger stars.
- High-velocity clouds, clouds of neutral hydrogen are “raining” down on the galaxy, and presumably have been from the beginning (this would be the necessary source of a gas disk from which the disk stars formed).
Galaxies come in a great variety of shapes and sizes (see the Hubble sequence), from giant, featureless blobs of old stars (called elliptical galaxies) to thin disks with gas and stars arranged in highly ordered spirals.
- The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of our Sun. The black hole mass is tied to properties of its host galaxy.
- Many of the properties of galaxies (including the galaxy color–magnitude diagram) indicate that there are fundamentally two types of galaxies. These groups divide into blue star-forming galaxies that are more like spiral types, and red non-star forming galaxies that are more like elliptical galaxies.