Origins

Deep-space images of the early universe suggest that galaxies were forming within one to three billion years after the Big Bang. However, as radio telescopes become ever more sophisticated, astronomers will likely revise the time of earliest galaxy formation.

How did the first proto-galaxies take shape in the conditions of a much smaller --about 5 percent the present size--and hotter universe? Computer simulations suggest that early star formation might not have produced the type of clustering that could give rise to galaxies. Instead, many of the first generation stars, which are thought to have been much larger than subsequent generations, may have lived brief lives as solitary giants, dying as monumental supernovas or gamma ray bursts. Eventually, star clusters and then galaxies were able to form; evidence today suggests that such development took place in densities of dark matter.

Galactic contents. The luminosity of galaxies, the product of billions of stars, is only a fraction of their content. The bulk of galactic mass is both very dark and very mysterious. Whenever the first galaxies emerged, they would have done so within patches of dark matter called halos. These invisible density pockets stabilized the incipient galaxies, preventing their stars from drifting away as the universe expanded. A dark halo acts as a kind of shell around an individual galaxy. In addition to the invisible halo, there is another dark entity in the center: a black hole. So far, most galaxies inventoried by astronomers contain a monster black hole at their core. While dark matter halos appear to be prerequisite for galaxy formation, the role of black hole cores is not yet clear. These invisible gravitational enigmas are among the most interesting phenomena in the universe.

Galactic Clusters

The gravitational pull of galaxies causes them to cluster, and eventually to form massive super clusters. The Milky Way is one of the larger members in a cluster of some 50-odd galaxies called the the Local Group, which in turn is on the periphery of the Virgo Super Group. Galaxy clusters are distributed on web-like filaments, strung out across the universe along the edges of massive voids. The filaments are thought to mark the scattered density patches from the infant universe.

The galaxy clusters also reflect the process by which larger galaxies cannibalize nearby small ones. A galaxy with an initial size advantage over its neighbors tends to absorb the smaller ones. Although galaxies are separated by distances measured in light years, they are inevitably pulled together by gravity. These collisions and absorptions are protracted events spanning millions of years. Examples are right at hand: the Milky Way is a large galaxy which is slowly devouring a number of smaller companion galaxies. Several billion years from now, the Milky Way and the Andromeda Galaxy, which are relatively equal in size, will collide and merge.