Galaxies and Galaxy Groups

Abstract

Few galaxies are loners: almost all are members of groups of at least a dozen systems. The Milky Way is no exception. It is the second brightest in a group of three dozen, mostly dwarf, galaxies. The brightest member of the Local Galaxy Group is the Andromeda Spiral M31. The most luminous members of the Local Group, including all those visible with any type of binoculars or small richest-field telescope, are listed in the accompanying table. The three brightest systems in the Local Group are all large spirals: the Andromeda Galaxy M31, the Milky Way, and the Triangulum Galaxy M33. The Local Group also has three Magellanic spirals (irregular systems with incipient spiral structure)-the Large Magellanic Cloud (LMC), NGC 3109, and IC 10-and four dwarf ellipticals-M32, and NGCs 147, 185, and 205. However the majority of our Local Group members are either dwarf irregulars resembling, but even smaller than, the Small Magellanic Cloud (SMC), or extremely loose, low-luminosity dwarf spheriodals (none of which are visible in wide-field instruments). The Local Group does not have any lenticular (SO) or giant elliptical (E) systems: the nearest of these types of galaxies are in the Centau- rus (NGC 5128) Galaxy Group. The total mass of the Local Group is estimated to be 3.7 trillion Solar masses. Half of the total luminosity of the Local Group comes from the Andromeda Spiral alone.