Seen through a telescope, the night sky reveals tiny points of light and slightly larger, hazy splotches. The former are stars in our own galaxy—the Milky Way—and the latter are nebulae, stellar clusters or other galaxies, which are complete stellar systems far beyond our own.
There are two broad classes of galaxies: elliptical (probably the most common) and spiral. Spiral galaxies are further divided into normal, which constitute the majority of spirals, and barred spirals. About 3 per cent of galaxies do not, however, fit into any of these categories and are termed irregular galaxies.
Seyfert galaxies comprise about 2 per cent of the spiral groups. Their central regions, or nuclei, are extremely bright and include regions of very high temperatures in which gas moves around violently at speeds of thousands of miles per second. The frequent outward movement of this gas suggests the occurrence of explosions in the galaxy center.
The shapes of galaxies
The various classes of galaxies are classified by the Hubble system, according to their degree of flatness. The elliptical galaxies range from EO to E7—from an almost spherical shape to a flattened disk. The normal spirals are classified as SO (with a large nucleus and little or no arm structure), Sa and Sb (with progressively less tightly-coiled arms and progressively smaller nuclei), and Sc (which have virtually no nucleus and wide open arms). A similar progression occurs with barred spirals: they again range from those with a large nucleus to those with a small nucleus and open arms trailing from the bar, designated as SBa, SBb, and SBc, respectively. The shape of a galaxy, from spherical to flat with only a small central bulge, results from its mass and the amount of angular momentum. The shape is partly a measure of the intensity of rotational motion of the system. It is thought that elliptical galaxies were formed out of masses of dust that had only a small, overall angular momentum and therefore have no arms.
In spiral galaxies, stars in the nucleus revolve about the center in the same direction as the arms, which trail behind. The origin of the spiral arms is not known, although the spiral pattern is explained by some astronomers in terms of a “density wave.” They suggest that a wave of increased density in a spiral shape results in compression of gas and sweeps around the galaxy, initiating the formation of stars in a spiral distribution. Spiral galaxies are thought to be younger than elliptical ones, which contain red, “older” stars and much less nebular material—from which stars are born.
Galaxies vary in size from dwarfs to giants. Andromeda Nebula, in the local group, is one of the largest spiral galaxies known and has a diameter of 60,000 parsecs. Most spiral galaxies have diameters of from 10,000 to 30,000 parsecs. Elliptical galaxies are usually much smaller, and a dwarf elliptical galaxy may only be 2,000 parsecs in diameter.
A galaxy’s mass is calculated in terms of solar masses—for example, Andromeda Nebula is thought to have 3 X 1011 solar masses. Spiral galaxies are estimated to have masses between about 109 to more than 1011 solar masses.
Spectra of galaxies
Despite the distances involved, the electromagnetic spectrum of a galaxy can be quite extensively interpreted. The movements of a galaxy—whether it is rotating and whether it is approaching the earth or receding—are revealed by its spectrum. The spectral lines of many distant galaxies show a “red shift”: they are displaced towards the red, long-wavelength, end of the spectrum. The farther a galaxy is from the earth, the faster it appears to recede (a phenomenon described by Hubble’s law). Also, because of the Doppler effect, the faster a galaxy moves away from us, the longer the wavelength becomes—and the redder the light. This spectral shift can therefore give an indication of how far away a galaxy is. The ratio of the distance of a galaxy to the speed with which it is moving away is known as Hubble’s constant, Ho.
Galaxies can be counted, like stars, in every direction of the sky—except in the Milky Way, where clouds of dust conceal those beyond. This area is known as the “zone of avoidance.” There are about 150 galaxies near enough to enable us to see stars within them. Beyond them are millions and perhaps billions of galaxies and supergalaxies (clusters of galaxies).