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Galaxy Formation

Space Science Short

National Aeronautics and Space Administration
NASA Headquarters Washington, D.C.
December 1994

Although astronomers have uncovered many of the details revealing the life cycles of individual stars, they still do not completely understand how galaxies, like our Milky Way, begin and end their lives.

The problem is that, although stars within the Milky Way may be seen in a variety of evolutionary stages, few examples of young galaxies are known, and their images come to us from the most distant edge of our visible universe. At these vast, multi-billion-light-year distances, it becomes increasingly difficult to determine what role environment plays in the formation of a galaxy.

Theoretical investigations indicate that galaxies formed from a diluted but lumpy mixture of hydrogen and helium gas - the primordial elements forged in the Big Bang. They also indicate that two vastly different scales of mass prevailed less than 100 million years after the Big Bang, which ultimately affected the formation of galaxies.

Matter either was clumped into vast collections more than a million times the mass of the Milky Way, or into small clumps one million times smaller than the mass of our Milky Way. Superclusters of galaxies may have evolved from the former. Globular clusters - spherical collections of very old, densely packed stars usually found in orbit around galaxies, like the Milky Way - may have evolved from the latter.

Could these globular clusters be the meager leftovers of an ancient, once-common population of small clumps as predicted by theory? This possibility now seems increasingly more likely. So the question then arises: What formed the vast majority of the galaxies?

Detailed ground-based and space-based images of distant galaxies are beginning to turn up some interesting insights into galaxy formation.

First, as we look deeper into the universe, galaxies appear to emit more of their light in the blue part of the visible spectrum. From studies of nearby galaxies, blue light is a sign that very young, massive and luminous stars are forming. Since we see these galaxies as they were between 5 and 10 billion years ago, we appear to be witnessing events that occurred within a few billion years after these galaxies were formed.

Astronomers also have noticed that as they examine the images of these distant blue galaxies, the images are frequently distorted or contain what appear to be multiple nuclei. The Milky Way seen at a similar great distance would look like a uniformed flattened disk, with a single bright nucleus - the galactic center. Nearby "multiple-nuclei" galaxies that have been studied show the cores of individual galaxies colliding and merging into one single system of stars and gas. These collisions are violent, and take millions of years to play out. But in at least some instances, such as NGC 1275, recently observed with the Hubble Space Telescope, galaxy collisions can actually trigger the formation of massive stars.

In the depths of space, we may be witnessing collisions between smaller galaxies triggering the formation of massive luminous stars. The images, rich in blue light, gives tantalizing evidence that "environment" may have been more important than cosmic "genetics".

Galactic cannibalism was far more common in the ancient past. Galaxies may have grown to their current size by consuming their neighbors. The ultimate building blocks may indeed have been the paltry million-solar-mass clumps that theoreticians believe were abundant before the universe was a few million years old.

For more information and pictures, link to Hubble Identifies Primeval Galaxies, Uncovers New Clues to the Universe's Evolution (HubbleSite - NewsCenter - Background Info, December 6, 1994)

Updated: December 28 '97

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