What types of galaxies do we find in the universe?
At the moment, we consider common types of galaxies and we ask about
characteristics of galaxies that you can see with visible light.
There are four main classes (as classified by Hubble).
- Spiral galaxies.
- Disk + central bulge.
- M51 Whirlpool Galaxy [type Sc].
- M31 Andromeda Galaxy [type Sb].
- M77 in Cetus [type SBp].
- M104 Sombrero Galaxy [type Sa].
- M85 a ``lenticular'' galaxy (on left)
-- mostly bulge, a little disk [type S0].
- NGC5866 a lenticular galaxy, mostly bulge [type S0].
- Barred spiral galaxies.
- Disk + central bulge with bar.
- M83 in Hydra [type SBc if you
think it has enough of a bar].
- M91 in Virgo Cluster [type SBb].
- M95 [type SBb].
- Elliptical galaxies.
- All bulge, elliptical shape, no disk; stars but no gas.
- M32 dwarf elliptical
galaxy, satellite of M31.
- M60 giant elliptical galaxy
in Virgo Cluster (on right, with NGC4647) .
- M87 giant elliptical galaxy,
the dominant galaxy in Virgo Cluster.
- Irregular galaxies.
How did galaxies get that way?
- The simplest explanation is that
- if all the gas is made into stars
before the gas has time to form a disk, then you get an elliptical galaxy.
- if the gas has time to stabalize into a disk before it is all used
up, then you get a spiral galaxy.
- Or perhaps some of the elliptical galaxies are made from merging of
other types of galaxies.
- Observations of distant galaxies indicates that spiral galaxies
were more common in the past than they are today.
- So maybe yesterday's spirals are todays ellipticals.
- This is an active research area. One problem is that if most of the
mass in galaxies is unaccounted for, we have a hard time understanding
the dynamics of galaxy formation.
Galaxies often come in interacting groups
One nice example consists of M81, M82, and NGC3077. Recall that we
looked at the irregular galaxy M82
earlier. Here is M81 by
itself. Note the big arm going down to the left, where NGC3077 is.
Here is a picture of M81 and M82 .
Here is a picture of the M81 group
taken with radio wavelengths. Notice the gas connecting the three galaxies.
Galaxies often come in clusters
Here is the
Here is a
close up view.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA