Download Chapter 17 - Astronomy

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 gal-
axy.
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 () is directly proportional to
its distance (d):  = 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 con-
clusion 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.
17-5 Active Galaxies
1. All galaxies emit radio waves; for a normal galaxy, radio waves constitute only about
1% of the galaxy’s total luminosity.
2. A radio galaxy is a galaxy having greatest luminosity at radio wavelengths. A typical
radio galaxy emits millions of times more energy in radio waves than does a normal galaxy.
3. Cygnus A, the first radio galaxy, was discovered in 1951 and has a double-lobed radio
source associated with the visible light image.
4. Most of the galaxies associated with double-lobed radio sources are either giant ellipticals or spirals.
5. The radio lobes are enormous and mark the positions where the outflows start interacting with the intergalactic medium.
6. Radio galaxies often appear unusual when viewed in visible light.
7. Radio galaxies are one type of a group of high-energy galaxies called active galaxies.
An active galaxy is a galaxy with an unusually luminous nucleus. Because the energy of
an active galaxy comes from its nucleus, astronomers often refer to active galactic nuclei
(AGNs) rather than active galaxies.
8. Jets seem to be a universal phenomenon. They are the natural byproducts of accretion
onto a compact objects, emanating at right angles to the disk that surrounds the object.
They are mostly well-collimated and transfer energy, matter, momentum, and magnetic
fields from the central region to the surrounding environment.
Quasars
1. In 1960 an unusual star-like object—3C 273—was discovered that emitted intense radio waves. The object appeared to be very small, it had a small jet protruding from it, and
the radio waves were emanating from the jet and the main body of the object.
2. The spectra of 3C 273 and 3C 48 (the second unusual object discovered in 1960)
showed emission lines, which could not be identified. Because of their star-like appearance and strong radio emission, the objects were named quasars.
3. A quasar (quasi-stellar radio source) is a small, intense celestial source of radiation
with a very large redshift.
4. In 1963, the unusual spectral lines found in 3C 273 and 3C 48 were shown to be highly
redshifted hydrogen lines. If the redshifts are caused by the Doppler effect, the quasars
are moving at 15% and 30% of the speed of light, respectively.
5. Since the early 1960s, more than 23,000 quasars have been discovered. Unlike 3C 273
and 3C 48, most quasars are not sources of radio waves. Most are blue-white objects and
X-ray emitters.
6. Many quasars vary in intensity in an irregular way, changing intensity in weeks or
months. This observation confirms their small size.
7. Redshifts for quasars range from 0.06 up to 6.41 for the farthest known quasar. The
latter quasar is receding at about 96% the speed of light.
8. At such great distances, a bright quasar must be 1000 times more luminous than a galaxy like ours, while being much smaller than a typical galaxy.
Competing Theories for the Quasar Redshift
1. The local hypothesis is a proposal stating that quasars are much nearer than a cosmological interpretation of their redshifts would indicate.
2. If quasars were local, then we would see at least some highly blueshifted ones, but we
don’t.
3. Most astronomers now agree that quasars’ redshifts do fit the Hubble law.
Seyfert Galaxies
1. Observations of quasars suggest that there are many similarities between quasars and
active galaxies.
2. A Seyfert galaxy is one of a class of spiral galaxies having active nuclei and spectra
containing emission lines.
3. It now appears that quasars may be at the nuclei of some type(s) of galaxies.
Quasars and Gravitational Lenses
1. Twin quasars were discovered in 1979, having the same luminosity, the same redshift,
and identical spectra.
2. According to the general theory of relativity, a gravitational lens is the phenomenon in
which the gravity due to a massive body between a distant object and the viewer bends
light from the distant object and causes it to be seen as two or more objects.
3. Since 1979 many examples of gravitational lensing have been found.
4. When the alignment between the viewer, distant object and massive body is perfect,
we observe a ring (called an Einstein ring).
5. Gravitational lenses are important not only because they provide another confirmation
of the general theory of relativity but also because they indicate that quasars are indeed
very distant.
6. A graph of the density of quasars as a function of distance shows that most quasars appear at a fairly specific distance from us. Since distance is proportional to time, this indicates that quasars existed during a relatively short period of time in the distant past.
Quasars, Blazars, and Superluminal Motion
1. In the early 1970s, astronomers discovered blazars (BL Lac objects). Blazars are especially luminous AGNs that vary in luminosity by a factor of up to 100 in just a few
months.
2. Radio observations of blazers indicated that they are double radio sources oriented in
such a way that one jet is coming straight (or nearly so) at us. This was supported by observations of superluminal motion, motion that appears to occur faster than the speed of
light.
3. Superluminal motion is also observed in some quasars. It is simply a projection effect,
but does indicate very high speeds for the material in the jets.
Advancing the Model: Galaxy Formation and Evolution
1. As part of its formation process, a large spiral should be immersed in a leftover halo of
hot gas. Such halos around spiral galaxies have been observed.
2. The presence of gas in spiral galaxies dampens out the disruptive effects of mergers,
allowing spirals to survive a violent collision.
3. Over its lifetime, a spiral can make many transitions between being barred or unbarred.
4. As a result of mergers between galaxies, matter gets spun off and forms small dwarf
galaxies.
5. The rate of star formation also depends on the galaxy’s mass. Early in the universe’s
life, massive galaxies formed stars early and rapidly, whereas less massive galaxies
formed stars over longer periods of time.
6. Star-forming activity has gradually decreased with time.
7. A byproduct of mergers is that collisions destroy the disks of spiral galaxies, send stars
into more chaotic orbits, and therefore produce football-shaped (elliptical) galaxies.
8. While these are part of the accepted theories, there are observations of a few galaxies
very early in the life of the universe that challenge them.
17-6 The Nature of Active Galactic Nuclei
1. According to present theory, the tremendous energy that comes from an AGN is
caused by an immense black hole at the nucleus of the galaxy. The black hole is surrounded by an accretion disk heated by infalling material.
2. The leading theory on the nature of AGNs holds that the different observed types of
AGNs are basically the same, and that they appear different depending upon their orientation with respect to us.
3. According to this unification theory, when an AGN is viewed edge-on we see it as a
radio galaxy (radio lobes and jets). When an AGN is viewed at a small angle, we see it as
a quasar. When the jet is aimed directly toward us, we see the AGN as a blazar.
4. In order to test this theory we must detect radiation from AGNs that is not blocked and
is not affected by the orientation of the dust torus surrounding the accretion disk around
the central black hole. Such requirement is fulfilled by far-infrared radiation.
5. In 2001, ESA’s Infrared Space Observatory showed that very hot and luminous quasar
cores are found even in weak radio galaxies at large distances.
6. AGNs are not found in our neighborhood because previous AGNs are the ancestors of
today’s galaxies.
7. A census of many nearby galaxies suggests that nearly all of them harbor supermassive
black holes that once powered quasars. The mass of the black hole is proportional to the
mass of the host galaxy and the number and masses of the black holes are consistent with
what would have been required to power the quasars.
8. It seems that galaxies have progressed from having quasars or blazars at their centers,
to Seyferts or radio galaxies, to normal spiral or elliptical galaxies.