Our Milky Way Galaxy is a member of the Local Group of galaxies, a small cluster of galaxies lying within a radius of about one million parsecs (3,260,000 light-years) from the Milky Way. About 30 members are known. Others are presumed to exist but are discovered only slowlyÑthe most recent one, a dwarf galaxy named Antlia, in 1997Ñbecause of the difficulty in identifying dwarf galaxies that lie close to the plane of our Galaxy, with its heavy interstellar absorption of radiation from such objects. Even some accepted members in the Local Group have still not been adequately studied. For example, one dwarf elliptical galaxy first noted in 1994 and named Sagittarius is so obscured that not until 1997 was it identified as being probably our closest companion, three times nearer than the Large Magellanic Cloud.The two giant members of the Local Group are our own Galaxy and the Andromeda galaxy (M 31). There is a subclustering about these two giants: 14 galaxies in the Milky Way group and 9 galaxies in the M 31 group. The other galaxies lie outside both groups. Aside from the two giant galaxies, the Local Group also includes an average spiral (M 33), the Large Magellanic Cloud, and more than a half dozen dwarf Magellanic irregular galaxies, including the Small Magellanic Cloud and the Mini Magellanic Cloud.
The rest are dwarf elliptical galaxies. The range in absolute magnitude is from ç9 to ç21 (a range in absolute luminosity of 63,000). Most of the luminosity and mass of the Local Group are in M 31 and the Milky Way. The total mass is about 700 billion solar masses. One low-luminosity system, Leo II, is an extreme dwarf elliptical system with a mass of only a million solar masses, comparable in size to globular clusters. Even more extreme dwarf systems probably exist. Detection of these very faint systems continues by means of computer analyses of photographic plates.The Local Group is important because individual stars can be studied in detail within each member and because the most common type of galaxy, the dwarf elliptical, is best observed there.