Elliptical galaxy
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. |
* The motion of stars is dominated by random motion, unlike spiral galaxies, which have very little random motion and are dominated by rotation.
* Very little
interstellar matter, few young stars, and few
open star clusters* Consist of old, so-called
Population II stars
* Larger elliptical galaxies typically have a system of
globular clusters, indicating an old population.
This traditional portrait of elliptical galaxies paints them as galaxies where
star formation has finished after the initial burst, leaving them to shine with only their aging stars. Very little
star formation is 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: as small as a tenth of a
kiloparsec to over 100 kiloparsecs, and from 10
7 to nearly 10
13 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. Most of these small galaxies may not be related to other ellipticals. 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 elongated. 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, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion), 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 phenomenonElliptical 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.
*
M32*
M49*
M59*
M60 (NGC 4649)*
M87 (NGC 4486)*
M89*
M105 (NGC 3379)*
M110*
Active galaxy*
Barred spiral galaxy*
Dwarf galaxy*
Dwarf elliptical galaxy*
Dwarf spheroidal galaxy*
Galaxy classification*
Galaxy formation and evolution*
Groups and clusters of galaxies*
Irregular galaxy*
Lenticular galaxy*
List of galaxies*
List of nearest galaxies*
Ring galaxy*
Spiral galaxy*
Starburst galaxy*
Seyfert galaxy*
Timeline of galaxies, clusters of galaxies, and large scale structure*
Elliptical Galaxies, SEDS Messier pages
*
Elliptical Galaxies
