The Formation of Galaxies

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. (See the later discussion of dark matter and the formation of structure.)

Two Scales of Matter

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 such as M15 in the adjacent image may have evolved from the latter.

Results from Recent Observations

As we look deeper into the Universe and therefore back in time, galaxies appear to emit more of their light in the blue part of the visible spectrum. This blue light is a sign that very young, massive and luminous stars are forming (see the discussion of the spiral arms in spiral galaxies, for example). 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.

Cosmic Cannibalism

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.

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Source:
NASA Space Short
NASA Headquarters
Washington, D.C.
December, 1994