Chapter 25: Galaxies

Galaxies are small agglomerations of stars, small with respect to the distances between them. When we look at the sky with our modern telescopes, we see an enormous number of galaxies at amazing distances from us. The opening picture of ch. 25 is a great example. This is a famous image from the Hubble telescope. A careful examination reveals more than 2500 galaxies in this picture. And there is nothing special about this region of the sky except that there are no stars or nearby galaxies to overwhelm the feeble light from the much more distant galaxies. The most distant galaxies in this image are more than 10 billion light years from us.

25.1 The Great Nebula Debate

The true character of galaxies is only apparent using modern telescopes. With early telescopes, galaxies had the appearance of fuzzy stars. What these were was debated, but without more data no conclusion could be made. In 1925, using the 100in. telescope on Mt. Palomar, Edwin Hubble was able to image individual stars in the Andromeda galaxy, our nearest neighbor. Some of the stars were Cepheids and RR Lyrae which allowed him to determine that the distance to Andromeda is about 900,000 LY. This showed that this collection of stars was well outside the Milky Way, marking it as a separate galaxy.

25.2 Types of Galaxies

Observations reveal that galaxies come in three types:

Do Galaxy Types Evolve?

This question is still under investigation. We do understand that at least some galaxies have changed over billions of years. Galaxies collide and merge, transforming into massive ellipticals. Isolated spiral galaxies will change over time as their clouds of gas are used up, new star formation slows, and the spiral arms become less conspicuous.

25.3 Properties of Galaxies

Skip except to say that galaxies have masses from 108 to 1013 Msun.

25.4 The Extragalactic Distance Scale

Galaxies can be billions of lightyears from Earth. It is important to know the distance to galaxies in order to calibrate their size and luminosity. How can astronomers measure such tremendous distances?

As is the case with measuring the distances to stars, the measurement of distances to galaxies involves several techniques that are useful for different ranges of distance. Some of the methods are listed in Table 25.2. For the nearest galaxies, we can resolve individual stars, and therefore we can look for variable stars to determine the distance. For more distant galaxies we can use Type I supernovae, or look for the brightest galaxy in a cluster. Redshifts can also be used, as explained in the next section.

Variable Stars

The period-luminosity relation of Cepheids and RR Lyrae variable stars is used to measure distances out to about 100 million LY. This work was subject to a major correction in the 1950's, as noted in the text, when astronomers realized they were mixing up the two types of variable stars. One of the tasks of the Hubble Space Telescope is to measure Cepheids in more distant galaxies to improve the overall accuracy of the extragalactic distance scale.

25.5 The Expanding Universe

Slipher's Pioneering Observations

From 1912 to 1932, Vesto Slipher made spectra of nearby galaxies (for Percival Lowell who was looking for extraterrestrial life -- recall the martians -- and thought the galaxies they were actually planetary systems in the process of formation). Slipher discovered that most of the spectra are redshifted, that is, most of the objects are moving away from us. If the motions are random, we expect about half of the objects to be redshifted and half to be blueshifted.

The Hubble Law

Edwin Hubble, the guy the telescope is named for, measured the recession velocity (redshift) and distance to galaxies. (How did he measure distances? Variable stars and the H-R diagram!) He saw a surprising relationship, objects are moving away from us and the speed they move with increases with their distance from us! The result is summed up by the Hubble law:

v = H×d
where v is the recession speed, d is the distance, and H is a number called the Hubble constant.

As you can see in the example Hubble diagrams, all but a few nearby galaxies are moving away from us. The fact is not a coincidence, it has now been confirmed for all the galaxies that have been measured.

Hubble's Law and Distances

The relation between recession velocity and distance gives us a new way to determine the distance of stars and galaxies. Once we accurately know the Hubble constant, we can use the measured redshift to determine the distance to a galaxy. This turns out to be very handy for astronomers.

Models for an Expanding Universe

What does it mean that all galaxies are moving away at speeds proportional to their distances?

It implies that the universe is expanding. But why do we seem to be at the center with everything moving away from us. It took a long time to discard the notion that the Earth is the center of the universe, and then that the Sun is the center of the Galaxy, and now here we come back to a situation where we seem to be at the center of universe!

It turns out that a feature of a uniformly expanding universe is that an observer at any location sees all the galaxies moving away from that location according to Hubble's law! The text discusses how this can occur using two analogies, an ant on a ruler, and raisins in raisin bread. I'll do a demonstration with a balloon.

In chapter 28 we will encounter one more implication of Hubble's law. As a preview, imagine going backward in time. That is, imagine a "movie" of the universe running backward from the present time. Then instead of moving away from each other, all the galaxies would move towards one another. If we go back far enough, all the galaxies would come together at one place. The implication is that at some point in the past, everything in the universe was at one location, and since then it has expanded to its present size. This idea initiated the big bang theory.

© Robert Harr 2002