The classification of galaxies into the three main groups, elliptical, spiral and irregular, depends on their shape and composition. Most of the known galaxies fall under the elliptical category because, as the name suggests, they are spherical in shape, tapering at the edges. A source at Thinkquest (2001) discusses the contents of elliptical galaxies
Elliptical galaxies contain hardly any gas or dust and are made primarily of old, cool, Population II stars. In these galaxies, stars are grouped together in globular clusters and orbit almost randomly and haphazardly around the center.
Spiral galaxies contain their elements in a flat disc spiraling around a bulge at the center. The spiral arms contain mostly open clusters of young, Population I stars. Large amounts of dust and gas are found in the disc from which new stars form. The bulge at the center of the galaxy is filled with older stars. Spiral galaxies can be of two kinds ordinary spirals and barred spirals. The central bulge of the latter is elongated with the arms of the galaxy spinning the ends of the bar. Ordinary spirals have central bulges that are less elongated with the stars organized into a disc around it. They may be further classified according to the proximity of the arms to each other and the size of the central bulge.
Irregular galaxies are characterized by haphazard, distorted shapes. The central bulge and the arms of these galaxies are not easily detectable. Both old and new stars are found in irregular galaxies usually abundant in gas and dust. The formation of such galaxies may be the result of interactions between nearby galaxies.
Statemaster Encyclopedia (2005) explains the wavelength ranges in which active galaxies emit energy.
This energy, depending on the active galaxy type, can be emitted across most of the electromagnetic spectrum, as infrared, radio waves, UV, X-Ray and gamma rays.
Elliptical galaxies contain hardly any gas or dust and are made primarily of old, cool, Population II stars. In these galaxies, stars are grouped together in globular clusters and orbit almost randomly and haphazardly around the center.
Spiral galaxies contain their elements in a flat disc spiraling around a bulge at the center. The spiral arms contain mostly open clusters of young, Population I stars. Large amounts of dust and gas are found in the disc from which new stars form. The bulge at the center of the galaxy is filled with older stars. Spiral galaxies can be of two kinds ordinary spirals and barred spirals. The central bulge of the latter is elongated with the arms of the galaxy spinning the ends of the bar. Ordinary spirals have central bulges that are less elongated with the stars organized into a disc around it. They may be further classified according to the proximity of the arms to each other and the size of the central bulge.
Irregular galaxies are characterized by haphazard, distorted shapes. The central bulge and the arms of these galaxies are not easily detectable. Both old and new stars are found in irregular galaxies usually abundant in gas and dust. The formation of such galaxies may be the result of interactions between nearby galaxies.
Statemaster Encyclopedia (2005) explains the wavelength ranges in which active galaxies emit energy.
This energy, depending on the active galaxy type, can be emitted across most of the electromagnetic spectrum, as infrared, radio waves, UV, X-Ray and gamma rays.
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