Galaxies 101 - A Primer to the Types of Island Universes

Spring is the time of year when our night sky is enriched with a plethora of galaxy types. On a dark night, we can often see a "hazy" band of light that stretches across the sky. This band is part of our own Milky Way galaxy, a gigantic collection of stars, gas, and dust. Far beyond the Milky Way, there are billions of other galaxies, some similar to our own and some very different, scattered throughout space to the very limits of the observable universe.

M101 - Spiral Galaxy in Ursa Major Imaged on T11 by Michael Petrasko and Muir Evenden

Types of Galaxies:

Astronomers classify galaxies into three major categories. Spiral galaxies look like flat disks with bulges in their centers and beautiful spiral arms. Elliptical galaxies are redder, more rounded, and often longer in one direction than in the other, like a football. Galaxies that appear neither disk-like nor rounded are classified as irregular galaxies.

Spiral Galaxies:

Spiral galaxies usually consist of three components: a flat disk, an ellipse-shaped formed bulge and a halo. The disk contains a lot of interstellar gas and dust, and most of the stars in the galaxy. The gas, dust, and stars in the disk rotate in the same direction around the galactic center at hundreds of kilometers per second and are often arranged in striking spiral patterns. The bulge is located at the center of the disk and consists of an older stellar population with little interstellar matter. The near-spherical halo surrounds the disk and is thought to contain copious amounts of dark matter: matter that acts gravitationally like "normal" matter but that can't be seen! Astronomers infer the presence of this dark matter by the motions of stars and gas in the disk of the galaxy, as well as older stellar populations in the halo-like globular clusters. The young stars in the disk are classified as stellar population I, and the old bulge and halo stars as population II.

M104 - "Sombrero Galaxy" in Virgo - The image was taken on  iTelescope.com's T21 by Sam P. and Tom M. from PCIS.

Astronomers classify spiral galaxies according to their appearance by using the Hubble scheme. Those with pronounced bar structures in their centers are called "barred spirals" and are classified "SB" (examples are given in brackets). Galaxies with conspicuous bulges and tightly wound spiral arms are called "Sa" (Sombrero galaxy) or "SBa" (NGC 3185). Galaxies with prominent bulges and pronounced spiral arms are classified as "Sb" (M31, M81) or "SBb" (M95, NGC 4725). Other spirals with loose spiral arms and a small bulge are classified as "Sc" (M33, M74, M100) or "SBc" (M83, M109).

There are some galaxies like M84, M85, and NGC 5866 that are disk galaxies without any spiral structure. These galaxies are called "S0" or lenticular galaxies. Though the origin of lenticular galaxies is still debated the most plausible explanation to date is that the gas and stars that would reside in the galaxy disk have been stripped by interactions with the hot gas in clusters and groups of galaxies. From their appearance and their stellar contents, they look more like ellipticals rather than spirals and have often been misclassified due to this fact. For instance, misclassification has occurred for both the Messier object examples listed above.

Elliptical Galaxies:

Elliptical galaxies are ellipsoidal agglomerations of stars, which usually do not contain much interstellar matter. Photometric studies of elliptical galaxies suggest that they are triaxial (all three axes of the ellipsoid are of different sizes). Unlike spiral galaxies, ellipticals have little or no global angular momentum, so different stars orbit the center in different directions and there is no pattern of orderly rotation. Normally, elliptical galaxies contain very little or no interstellar gas and dust and consist of old population II stars only. Elliptical galaxies are classified according to the Hubble scheme into classes "E0" to "E7", in increasing order of ellipticity. Thus E0 galaxies appear round like M89 while E6 galaxies like M110 and NGC 3377 are almost cigar-shaped.

The largest galaxies in the universe are giant elliptical galaxies. They contain a trillion stars or more and span as much as two million light-years - about 20 times the width of the Milky Way. These giant ellipticals are often found in the hearts of galaxy clusters. For example, the giant elliptical galaxy M87 is found in the heart of the Virgo Cluster.

Elliptical galaxies also constitute some of the smallest galaxies in the universe. These galaxies are called dwarf elliptical galaxies and dwarf spheroids. Relative to normal ellipticals they are very faint and are often found in galaxy clusters or near large spiral galaxies. For instance, there are 9 dwarf spheroids like Leo I which are satellites of our Milky Way galaxy.

Irregular Galaxies:

A small percentage of the large galaxies we see nearby fall into neither of the two major categories. This irregular class of galaxies is a miscellaneous class, comprising small galaxies with no identifiable form like the Magellanic clouds (the Large Magellanic Cloud and Small Magellanic Cloud are two satellite galaxies of the Milky Way) and "peculiar" galaxies that appear to be in disarray like NGC 1313. There is no discernable disk in these systems, although they often have copious amounts of gas as well as high rates of star formation. Irregular galaxies are often found to be gravitationally interacting with galaxies nearby, which often accounts for their ragged appearance.