Our backyard: the Local Group

Abstract

The Local Group contains roughly three dozen galaxies within a sphere about a megaparsec in radius, centred between the Milky Way and our nearest large neighbor, the Andromeda galaxy M31. Figure 4.1 shows the brighter members. The three most prominent are M31, the Milky Way, and M33; according to the classification of Section 1.3, these are all spiral galaxies. M31 is about 50% more luminous than the Milky Way, while M33 is only 20% as luminous. Between them, these three galaxies emit 90% of the visible light of the Local Group. The only elliptical galaxy is M32, a satellite to M31. The remaining systems are irregular galaxies, or the even less luminous dwarf irregulars, dwarf ellipticals, and dwarf spheroidals. Many of these smaller galaxies are in orbit either around the Milky Way or around M31. Table 4.1 lists known and probable members of the Local Group within a megaparsec of the Sun. The apparent brightness of each member is generally known to within 10%, except for the Milky Way, where our location within the disk presents special problems. Distances to Local Group galaxies are derived by picking out individual stars, measuring their apparent brightness, and estimating their true luminosity, using methods such as the period–luminosity relation for Cepheid variables. In this way, distances to the ten or so brightest galaxies can be measured to within 10%.