I. Classification of Galaxies

I. CLASSIFICATION OF GALAXIES

A galaxy is a system of stars and other interstellar matter, (the matter and gases between the stars,) which make up the universe. Few galaxies, except for the Milky Way, are readily viewable by the naked eye, usually appearing to be fuzzy stars. They were first cataloged and categorized by Edwin Hubble (after whom the Hubble Telescope was named), an American astronomer working at the Mount Wilson Observatory in California during the first half of the 20th century. As galaxies vary according to size, structure, composition, and activity, Hubble categorized them by their shapes. These shapes are (1) elliptical, (2) spiral, (3) lenticular, and (4) irregular.

Figure 1. The Hubble sequence: classification of galaxies. Credit: Wikipedia.

Elliptical Galaxies

Elliptical, or spherical, galaxies are characterized by the random movement of the stars within them. They also contain very little interstellar matter, young stars, or open star clusters. Open clusters are loosely scattered stars in a cloud of gasses. If you look through a telescope, the elliptical galaxy would look like a bright blob in the night sky. Typically, they are redder than other galaxies, and shaped vaguely like footballs. Different stars orbit around the galaxy’s center in different directions, so there is no orderly rotation. A typical elliptical galaxy is M49, shown in figure 2. Figure 2. Elliptical Galaxy M49. Credit: Nicole Peterson, Calvin Observatory, Calvin College

1 Elliptical galaxies can be further subdivided into two forms, boxy giant ellipticals resulting from anisotropic motion (i.e., motion in random directions), and disk-like ellipticals, which have nearly isotopic (one-directional) motion but appear flattened out due to rotation. Giant ellipticals are the largest form of galaxy in the universe. Often made up of nearly a trillion stars, they are most commonly found in the center of galaxy clusters. Figure 3 shows the giant elliptical galaxy NGC 1316 in the constellation Fornax. Figure 3. Giant elliptical galaxy NGC 1316. Credit: NASA

Spiral Galaxies

A spiral galaxy is probably the most familiar to us, as the Milky Way, our home galaxy, falls into this classification. Spirals are easily identified by the long arms of stars spiraling out from a bright mass, or bulge, of stars at the center. The two main characteristics of this class of galaxy are (1) a central bulge surrounded by a flat disk of stars, and (2) a great deal of momentum. Outside of the main galaxy itself is a halo of stars that are slowly being caught in the spiraling momentum of the galaxy. Spirals are considered the most beautiful of all the galaxies and have the smallest range of mass and size. Over 70 percent of the known galaxies in our universe fall into the spiral category. Spiral galaxies are sorted by their basic characteristics into S0 (“S” for Spiral), Sa, Sc, Sb, and B galaxies.

S0 galaxies. S0 galaxies have a very bright nucleus with lots of gas and dust but no visible spiral arm structure, such as the galaxy NGC 3115, shown below. S0s are considered “normal” spiral galaxies, meaning it has a spherical halo and a nuclear bulge, although they lack the clearly defined arms of the other spiral galaxies. Figure 4. S0 galaxy NGC 3115. Credit: NASA.

Sa galaxies. Sa galaxies, like the galaxy NGC 3623 shown in figure 5, have a very bright nucleus and tightly wound arms. These are considered the first spiral galaxies. The arms are smooth and have no clear delineation of stars. Figure 5. Sa galaxy NGC 3623. Credit:: Anglo-Australian Observatory

2 Sc galaxies. Sc galaxies have a small nucleus and open arms. They are sometimes referred to as “the grand design” because they are so easy to recognize (see figure 6). These spirals have the highest proportion of gas and space dust of any of the spiral types. Figure 6. Sc galaxy GC 3184. Credit:: the University of Chicago.

Sb galaxies. Sb galaxies are intermediate between the Sa and Sc galaxies. They have very definite and pronounced spiral arms, which are tightly wound. The Sb spirals are further divided into two categories – (1) those that have large, amorphous nuclear regions, and (2) those that have small nuclear regions with an intermediate spiral pattern. Figure 7 shows an Sb galaxy of the former variety. Figure 7. Sb galaxy NGC 1068. Credit: Students for the Exploration and Development of Space.

B galaxies. Spiral galaxies with bars have the same classification as other spiral galaxies, but are given the designation B (“B” stands for Barred) as well. Thus, a barred spiral galaxy with a small nucleus and very open arms would be classed as an SBc galaxy, such as NGC 1097, shown in figure 8. Figure 8. Barred spiral galaxy NGC 1097. Credit: NASA.

Lenticular Galaxies

A lenticular, or biconvex lens shaped, galaxy is an intermediate galaxy between an elliptical galaxy and a spiral galaxy. Lenticular galaxies are disc galaxies (like spiral galaxies) which have used up or lost their interstellar matter (like elliptical galaxies). Because of their ill-defined spiral arms, if they are inclined face-on, it is often difficult to distinguish between them and elliptical galaxies. They are also sometimes referred to as “armless spiral galaxies” because they have the characteristic center bulge of a spiral galaxy, but no apparent “arms.” Unlike the other galactic categories, lenticulars do not have any sub-classifications.

3 Figure 9. Lenticular galaxy NGC 936. Credit: Sloan Digital Sky Survey

Irregular Galaxies

The final classification of galaxies is the “irregular” galaxy. As the name suggests, these galaxies do not fall neatly into any of the earlier categories. They often seem chaotic, with no bulge or arm structure. It is believed that these galaxies make up approximately 25 percent of the known galaxies. Like the previous categories, with the exception of lenticular galaxies, irregular galaxies are further classified as either Irr-I galaxies, which have some structure but not enough for further classification, and Irr-II galaxies, which have no structure at all.

Figure 10. Irregular galaxy NGC 1569. Credit: University of Texas MacDonald Observatory.