Types of Galaxies

Key points: Basic types of galaxy - spiral, barred, elliptical, irregular; types vs. distance (how young galaxies differ from those around us)




Spiral galaxies are the most intriguing visually,

But in fact galaxies come in many shapes and sizes.











Color image of spiral M51

Galaxy shapes have 3 broad categories, based on the role of the bulge (the round distribution of stars at the center) and the disk (the flat distribution that includes the spiral arms).

  spiralsbuttonbook.jpg (10323 bytes), ellipticals, irregulars

Ellipticals are basically all bulge with no disk. They can range from spherical to elongated, football-like shapes

M87, a spheroidal elliptical M86, another elliptical
Picture of an elliptical galaxy (M87, the giant elliptical at the center of the nearby Virgo cluster of galaxies (about 15 million parsecs away): A more "elliptical" elliptical, M86

Irregulars -- catchall type for galaxies without symmetric shapes, and no clear bulge or disk

Picture of an irregular galaxy, the Large Magellanic Cloud, at 55 thousand parsecs one of the two closest galaxies to the Milky Way (and possibly in the process of merging with us!) APOD, W.-H. Wang, IfA, Hawaii, http://antwrp.gsfc.nasa.gov/apod/ap060510.html

Note the following systematic trends:

1) Interstellar material ranges from essentially none in ellipticals to substantial quantities in some irregulars

2) Ellipticals have low to zero rotation rates while spirals have relatively high rotation rates; that is, the stars in ellipticals have a wide variety of orbits - a little like a swarm of gnats - whereas in spirals the stars have orbits that lie in the plane of the disk

3) The most massive galaxies are ellipticals with some irregulars being very small ("dwarfs"). Spirals tend to be intermediate in size.

The most distant galaxies - are they the same types as nearby ones?

animation, zooning in on the Hubble Deep Field Under the leadership of former UA professor Bob Williams, the Hubble Space Telescope spent many days observing a small region just above the Big Dipper. The "Hubble Deep Field", or "HDF", is just a point to our unaided eyes. Here we zoom in by a factor of 1000 to get a look. The total area of the HDF is about 1% that of the full moon. (From STScI)
A portion of the Hubble Deep Field At first glance, the HDF shows more and more faint galaxies just like the nearby ones. If we look at the ones at the highest redshift, they seem subtly different -- many are small, like pieces of the large nearby galaxies. This behavior reminds us of the models of the early Universe, where galaxy fragments formed first and only later merged into large galaxies.

                          In the 18twrightmilkyway.jpg (12411 bytes)h Century, Thomas Wright proposed that the Universe was filled with groupings of stars like the Milky Way, from http://homepage.mac/com/kvmagruder/bcp/milky/shape.htm

sirtflaunch.jpg (4413 bytes)

mwmiztec.jpg (34782 bytes)


Mixtec personification of the Milky Way, from http://garnet.acns.fsu.edu/~dco2511/

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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