CHAPTER 17
A Diversity of Galaxies
CHAPTER OUTLINE
17-1 The Hubble Classification
1. In 1924, Hubble found Cepheid variables in three spiral nebulae, showing that they were actually spiral galaxies. The evidence that galaxies existed outside the Milky Way expanded our appreciation of the size of the universe.
2. Hubble divided galaxies into three basic types: spiral, elliptical, irregular. Each major classification contains subdivisions. More recently, astronomers have discovered objects that fit none of Hubble’s categories, not even irregular.
Spiral Galaxies
1. Hubble divided spiral galaxies into two groups: ordinary spirals and barred spirals.
2. Ordinary spirals are designated with an S; barred spirals are designated with an SB.
3. A barred spiral galaxy is a spiral galaxy in which the spiral arms come from the ends of a bar through the nucleus rather than from the nucleus itself.
4. Each type of spiral galaxy is then further subdivided into categories a, b, c, depending on how tightly the spiral arms are wound around the nucleus. Galaxies with the most tightly wound arms are type a; they also have the most prominent nuclear bulges.
5. Up to 2/3 of all spirals contain bars. The bar system provides an efficient mechanism for fueling star birth at the center of an SB galaxy.
6. Galaxies that seem to have the nuclear bulge and disk of a spiral, but no arms, are called lenticular (or S0) galaxies.
7. Type c spirals contain more gas and dust than type a, resulting in a larger proportion of their mass being involved in star formation.
8. Most spiral galaxies are from 50,000 to 2,000,000 million light-years across and contain from 109 to 1012 stars.
Elliptical Galaxies
1. Elliptical galaxies are ellipsoids; they are classified from round (E0) to very elongated (E7).
2. Most of the galaxies in existence are ellipticals, but most of these are smaller than spiral galaxies.
3. A few giant elliptical galaxies have 2´1013 stars and are thus larger than any spiral galaxy.
Irregular Galaxies
1. An irregular galaxy is one that cannot be classified as spiral or elliptical.
2. Fewer than 20% of all galaxies fall in the category of irregulars, and they are all small, normally having fewer than 25% of the number of stars in the Milky Way.
3. Collisions between galaxies are not unusual because on average galaxies are separated by distances only about 20 times their diameter. On the other hand, stars in a galaxy rarely collide since they are separated by distances that are millions of time their diameter.
4. Because of their great distances, galaxies exhibit no proper motion. Evidence of past collisions has to come from present appearance.
5. Computer simulations show that colliding galaxies actually pass through one another with few collisions between individual stars. However, the large dust and gas clouds in the galaxies make them more likely targets, resulting in increased star formation rates.
6. Bursts of star formation may also occur as a result of collisions or tidal interactions among neighboring galaxies.
7. Galactic cannibalism often occurs as a result of collisions.
Hubble’s Tuning Fork Diagram
1. Hubble’s tuning fork diagram relates the various types of galaxies. In his plan, S0 galaxies form the connecting link, because they have characteristics of both elliptical and spiral galaxies.
2. Astronomers once also thought the diagram represented an evolutionary sequence, but this interpretation has been discarded as old stars have been found in all three types.
Historical Note: Edwin Hubble
1. Early in Hubble’s career he attended a presentation by Slipher with observational data showing spiral nebulae were galaxies in their own right. Following this Hubble began his own work on photographing nebulae.
2. He searched for variable stars in these nebulae, and observed several Cepheid variables in M31 (the Andromeda “nebula”) and M33. By comparing each star’s luminosity with his observations of the star’s apparent brightness, Hubble was able to deduce their distances which placed them outside of the Milky Way.
3. In the mid-1920s, Hubble started investigating the expanding universe hypothesis. By observing galaxies he combined their radial velocities with their measured distances, and deduced what is now called the Hubble law of redshifts.
17-2 Measuring Galaxies
1. Most important properties of a galaxy that we can measure are its distance, mass, and motion.
Distances Measured by Various Indicators
1. Cepheid variables are excellent distance indicators but can be seen in only relatively nearby galaxies, out to perhaps 200 million light-years.
2. Bright stars (giants, supergiants, novae) can also be used as distance indicators.
3. Large globular clusters and supernovae are of consistent brightness so they, too, can be used to determine distances to more distant galaxies.
4. These objects allow astronomers to determine distances out to about 1000 million light-years.
5. Starting with the period-luminosity relationship of Cepheids, astronomers are able to follow a chain of reasoning and observation that allows them to determine the distances to galaxies too far away for their Cepheids to be visible.
6. As one distance measurement builds on another in a series of steps, constant checks are always being made as new data arrive. Otherwise, an error in the first step will propagate up through the chain of steps and lead to wrong conclusions.
7. In this analysis we are assuming that galaxies in our neighborhood are basically the same as those farther away. This may seem reasonable but keep in mind that we are seeing distant galaxies as they were in the past, not as they are today.
The Hubble Law
1. In 1912, Slipher found that spiral nebulae had redshifted spectra indicating that they were moving away from us at tremendous velocity.
2. In 1920s, Hubble and Humason showed that there is a relationship between the recessional velocities of galaxies and their distances.
3. Hubble showed that the universe is expanding, and his work is the foundation for today’s theories of cosmology—the study of the nature and evolution of the universe as a whole.
4. The redshift that Hubble observed is not due to the Doppler effect.
5. The Hubble law states that a galaxy’s recessional speed (u) is directly proportional to its distance (d): u = H0d, where H0 is the Hubble constant (the proportionality constant in the Hubble law; the ratio of recessional velocities of galaxies to their distances).
6. The latest observations of the radiation left over from the hot big bang indicate a value of H0 = 70.4 ± 1.3 (km/s)/Mpc or about 21.6 ± 0.4 (km/s)/Mly.
7. The value of H0 changes with time. It is simply the slope of the line in the graph of recessional velocity of galaxies versus their distance as measured during this period of time in the universe’s life.
The Hubble Law Used to Measure Distance
1. For the most distant galaxies, most of our distance indicators can’t be seen. Therefore, the Hubble law can be used to determine their distances.
Other Relations
1. The Tully-Fisher relation holds that the wider the 21-centimeter spectral line, the greater the absolute luminosity of a spiral galaxy. Using the Tully-Fisher relation, astronomers can determine the absolute magnitude of a galaxy and use it as a distance indicator.
2. The Faber-Jackson relation is relationship between the luminosity and central stellar velocity dispersion of an elliptical galaxy. Using this relation, astronomers can measure the Doppler shift of the light emitted by the stars and therefore the spread of their velocities, and then estimate the absolute magnitude of the galaxy to use it as a distance indicator.
Historical Note: Milton Humason
1. Humason began working at the observatory on Mount Wilson as a mule driver to transport equipment up the steep slopes and then as a janitor at the observatory.
2. After learning to develop photographic plates and persuading observatory astronomers to teach him math, he was hired as a full-time night assistant. He worked closely with Hubble.
3. Later, his contributions in astronomy were so significant that he was awarded an honorary doctor’s degree from the University of Lund, Sweden.
4. He almost discovered Pluto and photographed it on two occasions, but in one case it was too close to a star to be visible, and in the other it happened to coincide with a flaw on the photographic plate.
Advancing the Model: Observations, Assumptions, and Conclusions
1. It is always important in science to separate observations from the conclusions that are based on the observations.
2. Scientists must be on guard to remember the difference between what is observed—the evidence—and the conclusions that they reach based on those observations. Between an observation and a conclusion lies one or more assumptions, and the validity of the conclusion is based not only on the accuracy of the observations, but on the validity of the assumptions.
The Precision of Science
1. Every measurement in science is to some degree an approximation. No measurement is absolutely exact. What scientists attempt to do is to be aware of how inexact their measurements are.
2. Calculating the likely error is a common practice in all natural sciences. Scientists thus attempt to be specific about their inexactness.
3. A scientist tries to be aware of the assumptions involved in each measurement.
17-3 The Masses of Galaxies
1. A galaxy’s mass can be determined by observing the rotation periods of some parts of it (using Doppler shift data) and then applying Kepler’s third law.
2. Another method is to use a pair of galaxies revolving around each other. The problem with this method is that it is difficult to determine the angle of the plane of revolution to our line of sight.
Clusters of Galaxies; Missing Mass
1. Most galaxies are part of clusters. A cluster of galaxies is a gravitationally linked assemblage.
2. The local group of galaxies is a cluster of over 54 galaxies that includes the Milky Way Galaxy, the Andromeda galaxy, the two Magellanic Clouds, and numerous dwarf galaxies.
3, A supercluster is a group of clusters of galaxies. Our local supercluster contains the local group and the Virgo cluster. Between superclusters are great voids with no galaxies.
4. It seems that matter in the universe forms a cosmic web in which galaxies are formed along filaments, and clusters are formed at the intersections of these filaments.
5. A third method of measuring the masses of galaxies takes advantage of their clustering. It uses the Doppler effect to find the speed (and thus period) of a galaxy at the outskirts of a cluster.
6. The cluster method gives mass values for clusters that are much greater than is accounted for by the visible stars within the galaxies in the cluster.
7. Missing mass is the difference between the mass of clusters of galaxies as calculated from Keplerian motions and the amount of visible mass.
8. For the Milky Way we can account for as little as 1/10 of the total mass of the Galaxy.
9. Several possibilities have been proposed for the nonluminous matter.
(i) Ordinary “nonluminous” matter; composed of ordinary matter but not easily observed (e.g., planets, brown dwarfs, very old white dwarfs, etc.)
(ii) Hot dark matter; neutrinos and other exotic particles (introduced by theories but not observed yet) moving at very high speeds
(iii) Black holes
(iv) Cold dark matter; an exotic form of matter, moving at relatively slow speed, which can be detected only by its gravitational interactions; it appears to be quite abundant throughout the universe.
10. It seems that the universe is only about 4% normal matter and 20% dark matter, the remaining 76% being dark energy (discussed in Chapter 18).
11. Dark matter is distributed in galaxies and clusters of galaxies in a way similar to visible matter, as shown by the rotation curves of galaxies.
12. Galactic halos may contain much of the missing matter.
17-4 The Origin of Galactic Types
1. Two modern theories—the cloud density theory and the merger theory—purport to explain why galaxies exist in various types.
The Cloud Density Theory
1. Elliptical galaxies formed from the densest gas/dust clouds. Rapid star formation then used up the gas/dust before a disk had a chance to form.
2. Clouds with lower density would have formed stars less frequently, and the dust and gas would have collapsed into a disk before star formation used it all up.
The Merger Theory
1. According to this theory, spiral galaxies formed before elliptical galaxies, and ellipticals are the result of mergers of spirals.
2. In clusters where galaxies are packed close together, ellipticals dominate, supporting the notion of frequent mergers. In loosely packed clusters of galaxies, ellipticals are fairly rare.
3. At this point neither theory explains irregular galaxies well. Some irregulars are seen to be pairs of galaxies in collision.
Look-Back Time
1. We have observed objects that may be as far away as 13 billion light-years. This means that the light we see left these objects 13 billion light-years ago.
2. Look-back time is the time light from a distant object has traveled to reach us.
3. The look-back time complicates our interpretation of galaxies because the farther out we look, the earlier in time we are seeing them. Our assumption that distant clusters are similar to nearby clusters may not be valid, since we have observed galactic cannibalism in large clusters of galaxies.