by E.H. Shepard from A. A. Milne's Winnie-the-Pooh books

Anatomy of a Galaxy

Barbara Jo Mattson

Galaxies come in three basic types: spiral, elliptical and irregular.  Spiral galaxies are the one most people think of when they think of a galaxy - they make the prettiest pictures, and we happen to live in one.  Ellipticals appear as just blobs of light in most pictures and irregulars are those that don't seem to fit in either of the other categories (i.e. the "miscellaneous" or "other" category found so often when people are classifying things).
 

Spiral Galaxies

Spiral galaxies are made of three main (luminous) parts: halo, disk and bulge.  The cartoon below shows these basic parts in a spiral galaxy viewed edge on.

The disk is where the spiral structure lies and contains most of the interstellar medium (stuff like gas and dust between the stars).  The central bulge or nucleus contains old stars, and is the bright center seen in many images of galaxies (see the pictures below).  Finally, the halo contains globular clusters (clusters of old stars) and field stars (stars not associated with any clusters).

The following image shows the spiral structure and central bulge quite well in NGC 1232.


Credit: FORS1, 8.2-meter VLT Antu, ESO
(Click on the image for the APOD page with more information.)

The next image shows the Sombrero Galaxy, which is another spiral galaxy, but we don't see the spiral structure.  Why not?  This one happens to be tilted so that we see it edge on instead of face on as we see with NGC 1232.  The disk can still be seen as the dark line through the middle of the galaxy.


Credit: Peter Barthel (Kapteyn Inst.) et al., FORS1, VLT ANTU, ESO
(Click on the image for the APOD page with more information.)

Elliptical Galaxies

Elliptical galaxies have a far simpler structure than spirals - basically they are all bulge.  The stars orbit the center of the galaxy in a more random fashion than in spirals - meaning they are not confined to a disk, but can orbit along any elliptical path about the center.

Below is an image of the giant elliptical galaxy, NGC 1316.


Credit: VLT ANTU, ESO
(Taken from  Joshua E. Barnes' site.)

Irregular Galaxies

This is the "catch-all" category for galaxies that show no symmetrical or regular structure.  Many irregular galaxies show evidence of exhibiting unusual behavior.  Others appear to be pairs of galaxies interacting through gravitational forces.  Some may even be in the throes of a merger.

The following image is of the irregular galaxy, Sextans A.


Credit: D. Hunter (Lowell Observatory), Z. Levay (STScI)
(Click on the image for the APOD page with more information.)

There are finer classifications of galaxies.  For example, spirals can be classified further by how big the bulge is compared to the disk and how tightly wound the spiral arms are.  Elliptical galaxies are further classified by how they appear in the sky - from nearly circular to elongated along one axis.  Even irregulars are sub classified by whether or not they show young stellar populations or not.  See the web resources below for more information on galaxy classifications.

In addition to the luminous parts of the galaxy (those discussed above), it is believed that galaxies have a halo of dark matter.  The presence of the dark matter is inferred from the rotation curve of the galaxy - the velocity of stars in orbit around the galaxy's center as a function of distance from that center.  If the the only matter that was present in a galaxy was the luminous matter, then we would expect the velocity drop as we observe the stars in the outer regions of the visible galaxy, since the amount of luminous matter drops fairly quickly in those outer regions.  Instead, however, we see the velocity level off as a function of distance from the center - this indicates that there is a large amount of matter that we do not see located outside the luminous matter.

Web Resources

Created: 03 July 2001