The Local Groupthe Milky Ways neighborhood

The Milky Way is one of 10 billion galaxies in the universe—or maybe 100 billion . . . no one knows for sure. Viewed from a distance, the galaxies appear to link together to span space like soap suds fill a sink. They associate with other galaxies in groups and clusters that form extended sheets and surfaces of bubbles, outlining irregular dark voids.

The Milky Way belongs to the ''Local Group'' of galaxies, which in turn connects to other groups and clusters to form the ''Local Supercluster.'' The Local Supercluster is dominated by the Virgo Cluster of about 2500 galaxies, some of which one can see with binoculars or a small telescope. The Local Supercluster has a diameter of at least 100 million light-years.

The Local Group is spread over a more modest 3 or 4 million light-years. About 35 galaxies that we know of can claim membership; the exact number depends on how one defines the boundaries of the Local Group. Most of the galaxies are relatively faint, like smeared-out globular clusters, and escaped notice for a long time.

Astronomers discovered new ''dwarf'' or faint Local Group members as recently as 1990, 1994, 1997, and 1999, and the search 20

continues.

The Milky Way and the Andromeda galaxies, both large spirals, dominate the Local Group in terms of mass and luminosity. Most of the rest of the Local Group galaxies are companions to these, entrained by the larger systems' gravity. Even M33, the only other spiral in the Local Group, belongs to the Andromeda galaxy's domain.

Astronomers have a new outlook on dwarf galaxies since the early 1980s. Before then, it was easy to overlook them as insignificant members of the Local Group. But dwarf galaxies turned out to carry a surprising amount of dark matter. In fact, dark matter dominates their make-up. And while spiral galaxies like the Milky Way have a bright bulge in the middle and extended dark halos, the dwarf galaxies are dark even in their middles.

Some astronomers have now proposed a theory that gives dwarf galaxies center stage in the drama of galaxy formation. The early universe may have spawned very dark dwarf-type galaxies, rich in gas. These would have agglomerated to form larger systems, which became the spiral galaxies. The dark matter at the centers of the larger systems would have gravita-tionally attracted the gas, which eventually formed the luminous stars, leaving the halo regions dark. The Local Group dwarf galaxies that we currently see would generally be outlying remnants that took longer to participate in spiral-building.21

However the theory stands up to further investigation, it is clear at least that the Milky Way is currently merging with lesser galaxies such as the Sagittarius dwarf, and has stripped gas from other galaxies in the past. The Magellanic Stream provides evidence of an encounter about 100 billion years ago, for example. Astronomers have tracked a filament of hydrogen gas that is anchored in a cloud encompassing both the Small and Large Magellanic Clouds. As the Magellanic Clouds drifted near the Milky Way, the gas dragged behind, and now leaves a trail that arcs about one quarter of the way around our galaxy. Other evidence of past merger activity comes from observations by the Sloan Digital Sky Survey of galaxy detritus, left behind after the Milky Way had a ''snack'' on the Sagittarius dwarf galaxy a few billion years ago.22

Many astronomers now believe that mergers and interactions among galaxies help define their properties, so that a galaxy's ''environment'' is as important to its evolution as is the intrinsic factor of mass. The Local Group is too small to readily show the effect of environment, but studies of a number of larger clusters have revealed distinct patterns. Elliptical galaxies, stripped of most of their gas and populated by older stars, tend to congregate at the centers of rich clusters, leaving spirals at the edges. This segregation of galaxies makes sense if, as is supposed, mergers involving spiral galaxies lead to the formation of ellipticals. Spirals near the centers of rich clusters would have many opportunities to merge and transform, and so would be rarer than spirals in the quieter outer regions of the clusters.

Since the 1920s, when we first conceived of our stellar system as one of many island universes, our view of the Milky Way has undergone many transformations. We no longer think of it as lying at the center of the universe, nor even as a ''continent'' among islands. Our galaxy is large, but not outstandingly so; even within our small Local Group, the Andromeda galaxy is slightly more massive. Our galaxy has a supermassive black hole at the center, but not a particularly impressive one, and it is not stirring up enough gas around it to produce bright x-rays and other indications of activity. But this rather modest, temperate galaxy that we call home has enough wonders and curiosities, particularly in the bulge and halo, to keep astronomers busy for a long time. And our galaxy shares with others a mysterious origin that is somehow connected to dark matter, whose existence we were not even fully aware of until about 30 years ago. Considering that more than 90% of the universe consists of unknown dark matter, we may have to admit that the stellar system we have studied so long, and with so much success since the middle of the twentieth century, may add up to only a tiny part of the galaxy—as though we had been studying the foam on a breaking wave, and thought we understood the ocean.

Figure 1.1. The "Hubble Deep Field"—a view taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. As described in the text, most of the objects seen here are distant galaxies. A foreground star, within our own galaxy, has "rays" extending from it—an artifact of the imaging system. The view is actually a synthesis of separate images in red, green, and blue light. (Credit: Jeff Hester and Paul Scowen (Arizona State University), and NASA.)

Figure 1.1. The "Hubble Deep Field"—a view taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. As described in the text, most of the objects seen here are distant galaxies. A foreground star, within our own galaxy, has "rays" extending from it—an artifact of the imaging system. The view is actually a synthesis of separate images in red, green, and blue light. (Credit: Jeff Hester and Paul Scowen (Arizona State University), and NASA.)

Figure 1.2. The Milky Way in the northern and southern hemispheres (left and right panels, respectively). Mosaic assembled by Axel Mellinger from 51 wide-angle photographs taken over the course of three years. (Credit: Axel Mellinger. Reprinted with permission.)

Figure 2.5. The Pleiades. Only about half a dozen stars in the Pleiades open cluster are visible to the naked eye, but many more appear through telescopes or in long-exposure photographs. In telescopic views, one can see a blue veil near some of the brighter stars—a reflection nebula caused by dust. (Copyright Anglo-Australian Observatory/Royal Observatory, Edinburgh. Photograph from UK Schmidt plates by David Malin. Reproduced with permission of David Malin Images.)

Figure 2.7. The Orion nebula, the most famous example of a diffuse emission nebula. The nebula shrouds from view a stellar nursery, where stars are condensing out of hydrogen and helium gas. (Copyright Anglo-Australian Observatory/Royal Observatory, Edinburgh. Photograph from UK Schmidt plates by David Malin. Reproduced with permission of David Malin Images.)

Figure 2.8. The Cat's Eye nebula, an example of a planetary nebula. The name comes from the fact that the gas and dust surrounding the star looks, through a small telescope at least, like a disk or planet. (Credit: J.P. Harrington and K.J. Borkowski (University of Maryland), and NASA.)

Figure 2.9. The Horsehead nebula in Orion. The Horsehead nebula is a dark nebula, silhouetted against the brighter light from the emission nebula in Orion. (Credit: NASA, NOAO, ESA and The Hubble Heritage Team. STScI/AURA.)

Figure 10.4. "Grand Design" galaxy. The spiral arms are prominent and extend in an unbroken line from the center to the extremities in this example of a so-called "grand design" galaxy, M51, the "Whirlpool" galaxy. (The bright round patch shown in Lord Rosse's drawing at the end of one of the spiral arms (figure 6.2) is just off the top edge of this image.)

Figure 10.4. "Grand Design" galaxy. The spiral arms are prominent and extend in an unbroken line from the center to the extremities in this example of a so-called "grand design" galaxy, M51, the "Whirlpool" galaxy. (The bright round patch shown in Lord Rosse's drawing at the end of one of the spiral arms (figure 6.2) is just off the top edge of this image.)

Figure 10.7. The Milky Way galaxy from inside. This view of our galaxy taken in the infra-red portion of the spectrum using instruments on the Cosmic Background Explorer (COBE) satellite, shows our galaxy's thin disk of stars, and dust (which appears red or orange in this image) within the disk. Infra-red light penetrates dust and gas much better than visible light, so the image reveals much more of the central swath of the Galaxy than we could see with traditional telescopes. (Copyright Edward L. Wright. Used with permission.)

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