elliptical galaxy

elliptical galaxy A type of galaxy with a smooth, featureless circular or elliptical appearance, no spiral arms, and little or no interstellar gas or dust; symbol E. Elliptical galaxies range from about 10⁷ solar masses and a few thousand light years in diameter for dwarf ellipticals to over 10¹² solar masses and over 100 000 l.y. in diameter for giant ellipticals. Elliptical galaxies are classified from E0 to E7 according to their apparent shape, E0 appearing circular from our viewpoint and E7 the most elliptical. The degree of ellipticity is calculated from the ratios of the major (a) and minor (b) axes, using the formula 10(a - b)/a. Dwarf ellipticals are given the designation dE, while giant and supergiant ellipticals are known as D galaxies and cD galaxies respectively; such massive examples are usually found at the centres of clusters.

 The stars in elliptical galaxies are mostly old (Population II), although some ellipticals also contain intermediate-age stars that formed more recently. The light from elliptical galaxies falls off in a characteristic way from the centre to the edge, except where the galaxy has been disturbed in some way, for example by tidal forces from a passing galaxy or the addition of a faint envelope by galaxy cannibalism, as occurs in D and cD galaxies. Relative to their luminosity, elliptical galaxies have the greatest number of globular star clusters of any type of galaxy; a large elliptical may possess several thousand of them. A high proportion of the bright galaxies in rich clusters of galaxies are ellipticals (40 %), whereas the general proportion of ellipticals outside rich clusters is much lower, around 10 %.

 The intrinsic shape of elliptical galaxies can be spheroidal (cigar- or discus-shaped) or truly ellipsoidal (triaxial) with different dimensions along all three axes. Some ellipticals appear to be rotating sufficiently quickly to explain their flattened shape, but many (particularly the large ellipticals) show very little rotation. The details of the origin of elliptical galaxies remain controversial. They could either be old systems which formed rapidly and then quickly used up or lost their interstellar gas, or they could result from the merger of spiral galaxies.