An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties:
The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies.
This traditional portrait of elliptical galaxies paints them as galaxies where the star formation was over after the initial burst, now shining only with their aging stars. No significant star formation was thought to happen. In general they appear yellow-red, which is in contrast to the distinct blue tinge of a typical spiral galaxy, a colour emanating largely from the young, hot stars in its spiral arms.
There is a wide range in size and mass for elliptical galaxies: a small as a tenth of a kiloparsec to over 100 kiloparsecs, and from 107 to nearly 1013 solar masses. The smallest, the Dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. The single largest known galaxy, M87 (which also goes by the NGC number 4486), is an elliptical. This range is much broader for this galaxy type than for any other.
It was once thought that the shape of ellipticals shape varied from spherical to highly elongate. The Hubble classification of elliptical galaxies ranges from E0 for those that are most spherical, to E7, which are long and thin. It is now recognized
that the vast majority of ellipticals are of middling thinness, and that the Hubble classifications are a result of the angle
with which the galaxy is observed.
There are two physical types of ellipticals; the "boxy" giant ellipticals, which are dominated by random motion (which
is greater in some directions than in others), and the "disky" normal and low luminosity ellipticals, which have nearly isotropic
random velocities but are flattened due to rotation.
Dwarf elliptical galaxies are probably not true ellipticals at all; they have properties that are similar to those of irregulars and late spiral-type
galaxies. Many astronomers now refer to them as "dwarf spheroidals" in recognition of this (note that this is still a topic
of some controversy).
Ellipticals and the bulges of disk galaxies have similar properties, and are generally regarded as the same physical phenomenon
Elliptical galaxies tend to lie in the cores of galaxy clusters and in compact groups of galaxies.
Some recent observations have found young, blue star clusters inside a few elliptical galaxies, along with other structures that can be explained by galaxy mergers. Current thinking is that an elliptical galaxy is the result of a long process where two galaxies of comparable mass, of
any type, collide and merge.
Such major galaxy mergers are thought to have been common at early times, but may carry on more infrequently today. Minor galaxy mergers involve two galaxies of very different masses, and are not limited to giant ellipticals. For example, our own Milky Way galaxy is known to be "digesting" a couple of small galaxies right now.