The idea that some of the objects seen in the night sky might be galaxies outside of our own was first suggested back in the 18th century. But it was not until the beginning of the 20th century that a man by the name of Edwin Hubble was able to prove that the Andromeda "Nebula" was so far away that it could not be within the Milky Way galaxy. Hubble used Cepheid variables to determine the distance to the Andromeda galaxy. Today we now know that there are billions of galaxies outside the Milky Way.

In his study of galaxies, Hubble came up with the scheme to classify the galaxies. The Hubble classification scheme is strictly based upon the visual appearance of the galaxy. Hubble separated the galaxies into four types. These types are spiral galaxies, barred spiral galaxies, elliptical galaxies, and irregular galaxies.

Spiral galaxies have a flat, disk-like shape. At the center of the disk is a ball shaped distribution of stars referred to as the nucleus. The nucleus of the spiral galaxy and the halo of globular clusters surrounding the galaxy contain mostly old population II type stars. The spiral arms contain a mixture of old and new stars including population I type stars. There is also gas and dust within the spiral arms. And it is within this gas and dust that new star formation is taking place. Spiral galaxies were further classified by Hubble as Sa, Sb, and Sc. This classification is once again strictly based upon the appearance of the galaxy. The Sa type galaxies have a large central nucleus and tightly wound spiral arms. In contrast the Sc type galaxies have a small nucleus and loosely wound spiral arms.

A second type of galaxy classified by Hubble are barred spiral galaxies. Their characteristics are very much like spiral galaxies except that instead of the spiral arms emanating from spherical nucleus, the spiral arms emanate from a bar. This bar is an extension emanating from the spherical nucleus. The stellar populations are predominantly the same as those within spiral galaxies. Hubble used the same classification scheme that he used for spiral galaxies to classify barred spirals as SBa, SBb, and SBc.

NGC 1365: A Nearby Barred Spiral Galaxy
Credit: FORS Team, 8.2-meter VLT Antu, ESO

The third type of galaxy is elliptical. Elliptical galaxies are very different from spiral galaxies in shape and stellar population. Elliptical galaxies are spherical to elongated egg-shaped collections of stars. Elliptical galaxies contain mostly population II type stars and little to no gas or dust. Star formation is not taking place in elliptical galaxies. Hubble classified them as E0 through E7 where E0 ellipticals have a round spherical shape and E7 ellipticals have an elongated football-like shape. It should be noted that this classification is strictly based upon how the galaxy looks from the Earth. If the galaxy looks like a football but is viewed along the nose it will appear round. Elliptical galaxies can be the largest galaxies in the known universe (called giant elliptical) or very small galaxies (called dwarf elliptical).

Elliptical Galaxy NGC 4881 in Coma
Credit: W. A. Baum (U. Washington), WFPC2, HST, NASA

The combination of spiral, barred spiral, and elliptical galaxies comprise what is referred to as the tuning fork diagram of galaxies. Spirals and barred spirals create the tines of this fork and the elliptical galaxies create the handle. At the joint between the handle and the tines is a type of galaxy called a lenticular galaxy. This galaxy has a flat disk shape but no spiral structure in the disk.

The final type of galaxy is irregular. These galaxies basically have no distinctive shape or they may appear something like the other galaxies but with some characteristic that does not allow them to be classified with the others. In many cases the lack of shape may be due to the gravitational tides created by neighboring galaxies.

M82: An Irregular Galaxy
Credit: The Electronic Universe Project

In his study of distant galaxies, Edwin Hubble discovered that the farther away a galaxy is from the Milky Way the faster it is moving away from us. The speed of the galaxy is determined by red shift of the spectra. The distance to the galaxy is determined by several different methods and is generally less accurate the farther away the galaxy is from the observer. The relationship Hubble discovered is referred to as the Hubble law. The Hubble law states mathematically that the relationship between distance and speed is linear, i.e. v = H*r. Where v is the speed of recession, and r is the distance to the galaxy. The slope of this line is H, which is referred to as the Hubble constant. Since it is very difficult to determine the distance to far off galaxies the exact value of H is not well known. Current research places this value between 50 and 100 km/sec/Mpc. If an accurate value for H can be determined than accurate distances for far off galaxies can be found. In addition, the value for H can lead us to the age of the universe. More on that later.

Studies of galaxies in their positions in universe show that galaxies tend to form in groups called clusters of galaxies. These clusters may be classified as rich or poor depending on the number of galaxies within the cluster. Rich clusters will contain thousands of galaxies. Clusters may also be classified as regular or irregular depending on the overall shape of the cluster. Rich, regular clusters generally contain mostly elliptical galaxies whereas irregular clusters will have a much more even mix of galactic types.

The Coma Cluster of Galaxies
Credit: O. Lopez-Cruz and I. K. Shelton (U. Toronto), Kitt Peak National Obs.

Further studies of the clusters of galaxies indicate that these clusters may also form in groups referred to as superclusters. Three-dimensional maps of the universe tend to show some interesting patterns. These maps indicate that clusters of galaxies and superclusters form around large voids in which no galaxies seem to exist. These voids seem to form in shapes like large soap bubbles with the galaxies forming on the soapy film.

This grouping of galaxies has a side effect that should be noted. As the galaxies tend to form close to each other gravity will oftentimes cause the galaxies to collide. These collisions may influence the rate of star formation within the galaxies. This would be due to the gravitational forces causing collapse in the gas clouds of the galaxies. In many cases the galaxies will be so close together that the overall shape of the galaxies will be altered. In the extreme cases the galaxies will merge together as one. If one galaxy is much larger than neighboring galaxies, then the large galaxy may actually consume its neighbors. This process is referred to galactic cannibalism.

This page was last updated on 06/28/01 .