Celestial Tour: Types of Galaxies
These are some of the types of galaxies that exist in our universe.
Galaxy Structure: Video Segments
Produced by the Space Telescope Science Institute’s Office of Public Outreach.
All images, illustrations, and videos courtesy of NASA, ESA, and STScI except:
- NGC 6822 image courtesy of the Local Group Galaxies Survey Team/NOAO/AURA/NSF
- Taurus constellation drawing from Firmamentum Sobiescianum sive Uranographia by Johannes Hevelius, courtesy of the United States Naval Observatory
- NGC 4565 image courtesy of the European Southern Observatory
- Small Magellanic Cloud image courtesy of F. Winkler/Middlebury College, the MCELS Team, and NOAO/AURA/NSF
- Written by Vanessa Thomas
- Designed by John Godfrey
- Music courtesy of Association Production Music
Text, Types of Galaxies
A swirl of stars. Text, This is a classic example of a spiral galaxy.
Bright, curling arms of stars, laced with lanes of dust and clumps of gas, wrap around a bright central core.
These arms of stars, gas, and dust lie in a remarkably flat disk that stretches across tens of thousands of light-years.
Brown and gold galaxy, oriented diagonally. Text, If we could tilt the galaxy and look at it from the side, we would see something like this.
From this angle, it's not the bright stars that define the galaxies disk, but the thick, dark dust, made of different carbon and silicate compounds, within it.
Escaping the bounds of the disk, though, is a flock of stars around the galaxy's center.
These stars swell out of the disk, forming a central bulge.
Another fringe group of stars strays even farther from the disk.
Although you can't see it here, surrounding this galaxy is a large, spherical region called the halo.
The halo contains enormous and ancient clusters of stars, called globular clusters for their globe-light shape.
Galaxy halos do not contain as many stars as in the disk, so they are usually hard to see.
This halo, however, is different, due to the many globular clusters it contains - nearly 2,000, compared to only about 150 for our own Milky Way Galaxy.
The clusters appear as small white dots sprinkled above and below the galaxy's disk.
A halo of stars, a bright core or bulge, and arms that lie in a disk are the classic characteristics of a spiral galaxy.
But some galaxies have more or less than others.
Many spiral galaxies have an additional key feature; a bar of stars, gas, and dust streaming across the center of the galaxy.
This is a barred spiral galaxy.
In a barred spiral, the arms curl away from the ends of the bar instead of from the galaxy's core.
Curiously, some barred spiral galaxies also have a tight ring of stars encircling their heart.
There are some galaxies that look like spirals at first glance, but something about them isn't quite right.
They have a flat disk, a bright core, and a halo, just as a spiral does.
But they have no starry spiral arms.
Astronomers call them lenticular (or lens-shaped) galaxies.
Other galaxies have a long, flat disk, a bright core, and a halo, but they seem to lack a central bulge.
Meanwhile, some galaxies are nothing but bulge.
These, which are round or egg-shaped, and contain a bright core, are called elliptical galaxies.
But they do not have disks.
And then there are galaxies that have neither a disk nor a bulge.
They have no arms and no bright core.
In fact, they have no regular shape at all.
Naturally, they're called irregular galaxies.
Many irregular galaxies might have once been spiral or elliptical galaxies that were torn apart by the gravitational might of another, larger galaxy.
Our telescopes reveal many disruptive dances taking place between galaxies.
Close encounters between galaxies can demolish their structures and distort them into something quite bizarre.
Even classic spiral galaxies are not immune.