Clusters, Galaxies, Black Holes and Stars 279
discover an even wider range of different types of galaxies. In order to
sort out the different types of galaxies, Hubble set up a classification
scheme for galaxies based on their shape. He divided the galaxies into
four major classes: 1) normal spirals, 2) barred spirals, 3) ellipticals, and
4) irregulars. Normal spirals are galaxies with spherical galactic centers
whereas barred spirals have bar-shaped centers. The elliptical galaxies
are elliptically-shaped with no spiral arms at all. This class also includes
spherical galaxies since the sphere is a degenerate ellipse. The irregular
galaxies have amorphous shapes with no distinguishing features. The
spiral galaxies are further subdivided according to how tightly coiled the
spiral arms are wrapped around the galactic centers. The elliptical
galaxies are also further subdivided according to the degree of their
ellipticity.
Elliptical galaxies form almost geometrically precise ellipsoids. They
rotate in such a manner that their stars orbit the galactic centers in highly
eccentric elliptical orbits. The stars of the elliptical galaxies are very old.
There is no evidence for new star formation. In the spiral galaxies, on the
other hand, new stars are formed in the spiral arms. The irregular
galaxies are also rotating systems in which new stars are forming but
there is no evidence of spiral arms.
There is a great deal of variation of size within any given class or
subclass of galaxies. The largest galaxies belong to the elliptical class of
galaxies. Elliptical galaxies can be up to 30 times more massive than the
spiral galaxies. They also can be smaller as is the case from some of the
elliptical galaxies in our Local Group. The irregular galaxies tend to be
smaller than the spirals but here, too, there are exceptions.
Some astronomers believe that the various types of galaxies form an
evolutionary sequence so that a single galaxy within its lifetime passes
through the various subclasses described above. It has been suggested
that irregular galaxies develop spiral arms and evolve into spiral
galaxies. The spiral galaxies then evolve through tighter and tighter
spiral forms because of their rotation until their arms finally merge into
their nuclei. They are now full-fledged elliptical galaxies with a very
high degree of eccentricity. At this point the gravitational interaction of
the stars in the galaxy produce a more spherical distribution until the
galaxy evolves into a perfectly spherical galaxy.
Other astronomers argue that since a large number of elliptical
galaxies are 30 times larger than the largest spiral galaxies, it would be
impossible for a spiral galaxy to evolve into one of the larger elliptical