Astronomy
Normal Galaxies
Weblecture
Galaxies
Normal Galaxies
HUBBLE and Types of Galaxies
When Edwin Hubble first determined that Andromeda was a massive star system separate from the Milky Way Galaxy, only several hundred objects in space were identified as "non-stellar" objects. Some of these were gas clouds, like the Ring Nebula. The status of others was debatable, as getting completely clear images in different EM ranges was difficult. Hubble was able to classify galaxies into two kinds, spiral and elliptical, by determining whether they had arms or not. Further classification indicated how compact the galaxy was, and if it was densely or sparsely populated.
Modern long range radio and near infrared telescopes and space-borne telescopes like the Hubble have greatly expanded our understanding of the universe, and complicated it. More recently, astronomers have proposed an expanded galaxy classification system based in infrared imaging in ranges just beyond visible light. This division places three "tines" on Hubble's fork, recognizing that galaxies can form bars, and that some spirals are an intermediate form between those with bars and those with spherical cores, while some galaxies fit neither the elliptical nor spiral classes.
Dr. T. H. Jarrett (Caltech) from the Wikimedia Commons under the terms of the GNU free documentation license.
By combining key characteristics of galaxies we can classify them. Irregular galaxies are very small compared with spiral or elliptical galaxies, while elliptical galaxies have a greater range of masses and luminosities than either spiral/barred or irregular galaxies.
To get a sense of the variety of galaxies we have already observed, take a look at the Hubble Telescope archive, and more closely examine the following examples:
| NGC 1015, a galaxy with both bar and ring. |
 ESA/Hubble |
| t The Antennae Galaxies, NGC 4038 and NGC 4039, two spiral galaxies in collision. |
 ESA/Hubble |
| The Starburst Galaxy, Messier 94, showing new star formation in its outer ring. |
 ESA/Hubble |
Galaxies in Space
Use the Atlas of the Universe to explore the Local Group and other nearby galaxies by changing the scale of the map. Set the map to the Satellite Galaxy setting (the universe within 500 000 LY) and read the descriptions of the major satellite galaxies orbiting the Milky Way. Zoom out to The Virgo Supercluster setting. How many galaxy groups, large galaxies, and dwarf galaxies are there within this region of space?
In January, 2019, astronomers announce they had identified a small, isolated dwarf galaxy only 3000 light years across about 30 million light years from the Milky Way during a Hubble Survey of white dwarfs conducted in September, 2018. If you ignore the bright stars in the region within the dotted oval, you will see an open galaxy of much fainter stars behind them.
NASA, ESA, and L. Bedin (Astronomical Observatory of Padua, Italy)
Discussion Questions
- How can the formation of bright new stars help us determine forces shaping the structure of a galaxy?
- What forces can disrupt the structure of a galaxy?
- What is the likely outcome of a collision like the one shown for the Antennae Galaxies?
- What is the likelihood that many more small dwarf galaxies lie within the Local Group?
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