Evolution of

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. That is, because of the finite speed of light, observing galaxies at large distances is equivalent to looking back in time to the early history of the Universe, as the image shown above indicates (click the image for an explanation).

Galaxies: Snapshots in Time (Explanation)

Distant cluster of galaxies (Explanation)
For example, the image adjacent right shows a cluster of galaxies at such a distance that the light we see in the image was emitted when the Universe was about 2/3 of its present age. (Click on the image for a larger version with a description.) At these vast distances, it becomes increasingly difficult to determine what role environment plays in the formation of a galaxy.

The following image shows galaxies from an even earlier time.

Galaxies in the Young Universe (Explanation)

These galaxies are about 12,000,000,000 light years away; thus, they represent the Universe when it was only a few billion years old. Click on the image for an explanation of this figure. A discussion of the newest evidence for the Universe's early evolution may be found in this this NASA press release.

One of the most important ways that galaxies evolve is through interaction with other galaxies. The adjacent image taken using the 1 meter European Southern Observatory telescope, illustrates an extreme example of this. This false color image is of a distant cluster of galaxies called Abel 3827, which lies at a redshift of 0.1, corresponding to a distance of about 1.5 billion light years. In the enhanced image we see that the dominant central galaxy of the cluster (the bright central region) has swallowed 5 other galaxies in the cluster (the 5 large yellow blobs inside the central region) (Ref). In many other clusters of galaxies one finds a large galaxy near the center, suggesting that galactic cannibalism in clusters may be relatively common.

The figure shown below (Ref) is a Hubble Space Telescope image of a cluster of galaxies approximately 8 billion light years away (one of the most distant galaxy clusters known) in which there is evidence for a very large amount of current and recent interaction between galaxies. Among the hundreds of galaxies in the image, 13 have been identified as presently undergoing or recently participating in collisions (see the blowups in the right panels of the image).

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