Galaxy evolution through Emission lines

Abstract

Wehave reviewed: the use of emission lines for understanding galaxy evolution, morphological classes and energy distribution of galaxy. Hubbles system divides galaxies into three basic categories: elliptical, spiral and irregular galaxies. Spirals have the a disc like appearance with well defined spiral arms originating from the center and extending throughout the outer regions. These spirals are classified Sa, Sb and Sc as a function of decreasing prominence of the bulge and increasing im portance of the spiral arms. Elliptical galaxies show various degrees of flattening, ranging from systems that are approximately spherical to those that approach the f latness of spirals. This class is further divided into sub-classes as a function of their observed flattening. Perfectly round elliptical are called E0 , whereas the most flat tened are the E7. Elliptical are red in color, and their spectra indicate that their light comesmostlyfromoldstars, especially evolved red giants