The true number of members remains unknown, and there are three reasons for this. First, there are parts of the sky, especially those areas obscured by the Milky Way dust, that have not yet been searched for members. We know that no large spiral galaxy member lies hidden, because we could detect such a galaxy by its radio emission (especially its neutral hydrogen emission), even if its optical image were completely absorbed by Milky Way dust. However, an elliptical galaxy, perhaps one of low luminosity like the Sculptor dwarf, would not be easy to find because it would emit virtually no radio radiation. Considering the size of the area obscured by the Milky Way, we would expect no more than one or two hidden elliptical galaxies that might have been missed by searches thus far.

The second reason that the list may be incomplete is that there might be objects, like the extremely inconspicuous Ursa Minor dwarf, that are simply too faint and sparse to have been found. The most distant dwarf elliptical galaxies are And I, II, and III, which might not have been found easily if their discoverer, Sidney van den Bergh, had not been specifically searching the Andromeda area for them. Other such galaxies, in other parts of the sky and perhaps even a little farther away, might still await discovery. If these types of galaxies are approximately uniformly spaced in the Local Group, there could be as many as 50-100 of these objects within its boundaries. However, it is believed that there is a higher than average density of them near our galaxy, because of its large mass, and that there may be only a few undiscovered examples in the more distant parts of the group.

The third reason that the number is uncertain is the fact that the "boundaries" of the group are not clearly defined. There are several small galaxies, mostly irregular galaxies, that lie at distances of about 1 Mpc (3 million light years), and it is not always clear whether they are members of the group or merely field galaxies.

Types

Within the Local Group there are examples of all three main types of galaxies: spirals, ellipticals, and irregulars. The three spirals (the Milky Way, the Andromeda galaxy, and M33) are the most luminous galaxies of the group. The Magellanic Clouds and other irregular galaxies are also fairly bright. There are no giant, luminous elliptical galaxies, however, even though such galaxies are often conspicuous members of more populous clusters of galaxies. The elliptical members include some of intermediate brightness, like M32, and eight extremely faint dwarf ellipticals.

Size

The Local Group is very small, when compared to the famous galaxy clusters like those in Virgo and Coma, which span hundreds of millions of light years. From our perspective, the group has a diameter of approximately 3 million light years; that is, the most distant certain members are about that distance from the Milky Way galaxy. We are not yet sure, however, just where to draw the boundaries of the group, especially because the distances to some of the more distant dwarf irregular galaxies are not yet reliably known.

Dynamics

The best way to decide on membership in the Local Group has been to measure the velocities of all nearby galaxies and then to see which seem to be moving together in space. If a certain galaxy has a velocity that is very different from the rest, then it is probably an interloper. Our fellow members of the group do not seem to be participating in the Hubble flow (the general expansion of the universe), because they are all gravitationally held together, at least loosely, in the group. Various tests of the stability of the group have been made over the years, with somewhat uncertain results. It appears likely from these studies that the group is a fairly stable dynamical entity, but that it is held together principally by its interstellar dark matter. Most of the visible mass of the group is contained in just two members: the Milky Way and the Andromeda galaxy, and these are falling towards each other at a velocity of 300 km s^-1. The group is probably not collapsing, but rather these two galaxies are probably in highly elongated orbits around the group center of mass, which is somewhere between them. The velocities of all members are probably balanced by the distribution of dark matter in the group, about which we know very little.

Member of the Virgo Cloud

The Local Group is not a simple, isolated clump of galaxies, but is one of a large number of groups that belong at least peripherally, to a giant complex of galaxies called the Virgo cloud. The center of the Virgo cloud is the Virgo cluster, a large, massive, irregular cluster of many hundreds of galaxies. Studies of the velocities of expansion of the universe in our neighborhood show that the Local Group is falling toward the Virgo cloud with a velocity of a few hundred kilometers per second, which is probably caused by the large gravitational field of the Virgo galaxies. We are thus gravitationally attached to Virgo as one of its outlying members.

DESCRIPTION OF THE MEMBERS

Spiral Galaxies

The Milky Way galaxy and the Andromeda galaxy (frequently referred to as M31) are the most luminous and massive members of the group. M31 is a Hubble Sb type, with a luminous, large bulge of older stars, surrounded by a less-luminous disk of gas, dust, and younger stars, arranged in spiral arms. Its diameter is about 200,000 ly and its mass is approximately 700 billion times that of the sun. Among the many objects that have been studied within it are about 300 globular clusters, 400 open clusters, numerous dust clouds, gas clouds, stellar associations, supernovae remnants, and other components.

The Milky Way galaxy is of Hubble type Sb/Sc, with a somewhat less-conspicuous central bulge and a brighter disk and looser arms than M31. We do not know its total luminosity, because we cannot see it from a distant perspective and because so much of its visible light is obscured by the dust in the disk in which we are enveloped, but indirect evidence suggests that it may be roughly twice as bright as M31. The total mass is approximately 500 billion suns, though this is uncertain because of the unknown distribution of dark matter in our galaxy.

M33 is a Hubble type Sc galaxy; it is smaller and fainter than the other two spirals, being only about as bright visually as a few billion suns. It contains many blue luminous stars in its complex, thick spiral arms, and has several spectacular glowing gas clouds (giant HII regions).

Elliptical Galaxies

There are only four moderately bright elliptical galaxies in the Local Group and all four are companions to the Andromeda galaxy. Two very close companions are M32 and NGC 205, both of which are seen super- imposed on the outer parts of M31. M32 is a nearly circular galaxy with a population of exclusively very old stars, whereas NGC 205 is more elongated in shape and contains a small but remarkable population of young stars, with accompanying dust and gas. The other two companions, NGC 147 and NGC 185, are somewhat fainter and more distant from M31.

All other known elliptical members of the Local Group are classed as dwarf ellipticals; they are very low in density, faint in luminosity, and small in size, with typical brightnesses being about a million suns and typical diameters being only about 10 thousand light years. These objects contain primarily old stars, though some show evidence of star formation that took place not too long ago (only 7 or 8 billion years ago, compared to the oldest stars, with ages of 15 billion years).

Irregular Galaxies

The two best studied irregular galaxies in the Local Group are the two Magellanic Clouds, galaxies that are so bright that they were well known to the early explorers and were named after the great navigator Ferdinand Magellan. They are in the southern skies and can only be observed well from below the equator. The biggest of the two, the Large Magellanic Cloud, is only about 150 thousand light years away (some astronomers cite a distance of 160,000 or even 170,000 ly) and the Small Magellanic Cloud is not much farther. They can be studied in great detail because of this proximity. We have information about nearly every kind of star and interstellar object in these two galaxies and they have played an important role in helping our understanding of such fundamental issues as stellar evolution and the extragalactic distance scale. The Large Cloud has a truly giant HII region as one of its most conspicuous features, a huge complex of massive stars, stars being formed, gas, and dust, called 30 Doradus, popularly known as the Tarantula nebula. In 1987 a bright supernova explored near it, causing a sensation among astronomers by being the first nearby supernova in over 300 years. This star, Supernova 1987a, was observed in optical and radio wavelengths, as well as in x-rays, and even its neutrinos were detected, making it a veritable bonanza for our understanding of the supernova phenomenon.

The numerous other irregular members are dwarf galaxies, some only a thousand or so light years across. They show a surprising variety in their structure and in their histories, with some apparently having had recent bursts of star formation and others having been relatively quiet for most of their recent lives. Some are sufficiently far that we do not yet have good distances for them, and we are still uncertain as to whether they are members of the group. But the nearer ones are well studied and they continue to give astronomers clearer and clearer pictures of the nature of galaxy evolution and the behavior of stars in a variety of different environments.

Additional Reading

Baade, W.(1963). Evolution of Stars and Galaxies. Harvard University Press, Cambridge, MA.

Hodge, P.W.(1986). Galaxies;. Harvard University Press, Cambridge, MA.

Hodge, P.W.(1989). Populations in Local Group Galaxies. Ann. Rev. Astron. Ap. 27 139.

Hubble, E.P.(1936). The Realm of the Nebulae. Yale University Press, New Haven, CT.

Iwanowska, W.(1989). The Local Group. In From Stars to Quasars, S. Grudzinska and B. Krygier, eds. Nicolas Copernicus Nicolas Copernicus University, Torun, p. 55.

Mould, J.(1988). Review of the Local Group distance scale. In The Extragalactic Distance Scale, S. van den Bergh and C.J. Pritchet, eds. Astronomical Society of the Pacific, San Francisco, p. 32.

van den Bergh, S.(1979). Our galaxy as a member of the Local Group. In The Large-Scale Characteristics of the Galaxy, W.B. Burton, ed. Reidel, Dordrecht, p. 577.

Adapted from The Astronomy and Astophysics Encyclopedia, ed. Stephen P. Maran