Elliptical Galaxies
Many galaxies
take the form of ellipsoids, with no spiral structure or flattened disks.
These elliptical galaxies exhibit little
recent evidence for star formation or the nebulae and H II regions that accompany star formation.
Because they consist of relatively old stars, ellipticals have a red or yellow tint relative to
the bluer color of spirals.
Examples of Elliptical Galaxies
The adjacent image shows an example, the
giant elliptical galaxy
M87 in the center
of the
Virgo cluster.
Some other examples of elliptical galaxies include
M32, which is an
E2 dwarf elliptical near the Andromeda Galaxy,
and the E6 elliptical galaxy
M110, another satellite of the Andromeda Galaxy.
The Hubble Classification
In the Hubble
sequence E0, E1, E2, ... E7, the number is related to how flattened the ellipse
appears to be, with E0 corresponding to no flattening and E7 to a very elongated
ellipse. The Hubble classification
scheme uses the apparent
ellipticity, so it refers to the projection of the
galaxy's shape on the celestial sphere, not its actual shape. The actual shape for particular ellipticals
might be spherical, ellipsoidal, or flattened ellipsoidal.
Properties of Elliptical Galaxies
The masses of elliptical galaxies range
from about
107
up to 1013 solar masses. The corresponding range of
diameters is about 1/10 kpc up to about 100 kpc, and
the absolute blue
magnitude varies over a correspondingly large range
from negative 8 to negative 23.
Thus, the smallest of the elliptical galaxies, which are called dwarf
ellipticals, may be only a little larger than globular clusters. Conversely, the
giant elliptical galaxies like M87
are a thousand times larger than dwarf ellipticals, placing them
among the largest galaxies in the Universe. This is a much greater
range in size than is found for the spiral galaxies.
As we shall consider in more detail later, there is evidence that the giant ellipticals have
gotten to be giants in a most inhospitable way. They have probably
eaten their galactic neighbors!