Chapter 27: The Evolution and Distrubution of Galaxies

Chapters 27 and 28 are tightly connected and fall under the overall title of cosmology. Cosmology is the study of the universe as a whole, how it began, how it evolved, and how or if it will end.

27.1 Observations of Distant Galaxies

For very distant galaxies, our telescopes receive enough light to measure the overall color, but not the spectrum.

Clues to the Evolution of Galaxies

From the Hubble Deep Field Surveys we find that very distant galaxies differ from those nearby: they are smaller, more numerous, gas rich, filled with more blue stars, and forming stars at a higher rate.

27.2 The Evolution of Galaxies

The evolution of galaxies is dominated by collisions and mergers. A merger is a collision where both galaxies are roughly equal in size. See pictures and simulations showing evidence of collisions and mergers.

A collision can result in regions of rapid star formation, known as starbursts. These regions are easily discernable due to the large number of young, massive blue stars. These massive stars have very short lives, maybe 100 million years. In older regions of galaxies, the blue stars died out long ago, leaving mostly longer lived yellow stars.

Astronomers now have a rough model of the formation of spiral and elliptical galaxies based on collisions.

27.3 The Distribution of Galaxies in Space

How are galaxies distributed in space? Early measurements revealed that there is a roughly equal number of galaxies found in any particular patch of sky. This leads to something called the cosmological principle: that the universe is isotropic and homgeneous. Isotropic means that something is the same in all directions, and homogeneous means that something is everywhere of the same composition. Applied to the universe, this means that the universe looks basically the same in all directions, both in the numbers and types of galaxies that are seen.

The Local Group

The Local Group is the name for the set of galaxies within about 3 million light-years of the Milky Way. It is our little corner of the universe. The Local Group consists of about 40 galaxies.


Somewhat larger groupings of galaxies are known as clusters. We lie on the edge of the Virgo Cluster.

Superclustes and Voids

Nearby clusters are grouped into superclusters. Superclusters oftern consist of tens of thousands of galaxies.

Astronomers mapping the locations of galaxies noticed that they tend to clump together in a filamentery structure with large voids that contain few or no galaxies. This distribution wasn't expected, and needs to be explained by the model for the universe.

27.5 A Universe of (Mostly) Dark Matter?

Measurements of orbits around galaxies reveal the presence of large amounts of dark matter in other galaxies, as was found for the Milky Way. Mass can also be measured using gravitational lensing of distant galaxies and quasars by nearby galaxies. The results continue to find much more mass than can be accounted for with visible (giving off some detectable electromagnetic radiation) mass, suggesting the presence of a large amount of dark matter. The amount of dark matter required by the measurements is about 7 times greater than the amount of visible matter!

© Robert Harr 2005