Chapter 15 Normal and Active Galaxies

Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 15Normal and Active

Galaxies

Copyright © 2010 Pearson Education, Inc.

Chapter 15Normal and Active Galaxies


Units of Chapter 15Hubble’s Galaxy ClassificationThe Distribution of Galaxies in SpaceHubble’s LawActive Galactic NucleiThe Central Engine of an Active Galaxy



a) giant globular clusters in the halo.b) small irregular galaxies that orbit the

Milky Way.c) large molecular clouds in the disk of

our Galaxy.d) the brightest ionized hydrogen

regions in our Galaxy.e) spiral nebulae originally discovered

by Herschel.

Question 1

The Magellanic Clouds are


a) giant globular clusters in the halo.b) small irregular galaxies that orbit the

Milky Way.c) large molecular clouds in the disk of

our Galaxy.d) the brightest ionized hydrogen

regions in our Galaxy.e) spiral nebulae originally discovered

by Herschel.

Question 1

The Magellanic Clouds are


Spiral galaxies are classified according to the size of their central bulge.

Hubble’s Galaxy Classification


Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest.Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc progressively less tight, although the correlation is not perfect.



a) ongoing star formationb) a disk, bulge, and haloc) globular clusters in the halod) open clusters in the diske) all of the above

Question 2

What property is shared by spiral galaxies?


a) ongoing star formationb) a disk, bulge, and haloc) globular clusters in the halod) open clusters in the diske) all of the above

Question 2

What property is shared by spiral galaxies?


Similar to the spiral galaxies are the barred spirals.



Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of fewer than a million stars.Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation.Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy.



Ellipticals are classified according to their shape, from E0 (almost spherical) to E7 (the most elongated).



a) disk and spiral arms.b) halo.c) central bulge.d) open clusters.e) companion galaxies, the Magellanic

Clouds.

Question 3

Based on their shapes and stars, elliptical galaxies are most like the Milky Way’s


a) disk and spiral arms.b) halo.c) central bulge.d) open clusters.e) companion galaxies, the Magellanic

Clouds.

Question 3

Based on their shapes and stars, elliptical galaxies are most like the Milky Way’s

Like the stars and globular clusters in our halo, elliptical

galaxies contain little or no gas and dust to make new stars.


S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas.



The irregular galaxies have a wide variety of shapes. These galaxies appear to be undergoing interactions with other galaxies.



A summary of galaxy properties by type



Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning.



Cepheid variables allow measurement of galaxies to about 25 Mpc away.However, most galaxies are farther away then 25 Mpc. New distance measures are needed.

Type I supernovaeTully-Fisher

The Distribution of Galaxies in Space


Type I supernovae all have about the same luminosity, as the process by which they happen doesn’t allow for much variation.

Type I Supernovae


Apparent Vs. Absolute


The rotation of a galaxy results in Doppler broadening of its spectral lines.



Tully-Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity.

Tully-Fisher


With these additions, the cosmic distance ladder has been extended to about 1 Gpc.



Here is the distribution of galaxies within about 1 Mpc of the Milky Way.



There are three spirals in this group – the Milky Way, Andromeda, and M33. These and their satellites – about 45 galaxies in all – form the Local Group.Such a group of galaxies, held together by its own gravity, is called a galaxy cluster.



A nearby galaxy cluster is the Virgo Cluster; it is much larger than the Local Group, containing about 3500 galaxies.



Question 4

Hubble took spectra of galaxies in the 1930s. What did he find?

a) Most galaxies showed redshifts.b) All galaxies showed blueshifts.c) Galaxies showed about half redshifts and

half blueshifts.d) Galaxies showed no line shifts at all.e) Some galaxies showed a redshift that

changed into a blueshift at other times.


Question 4

Hubble took spectra of galaxies in the 1930s. What did he find?

Redshifts of galaxies indicate they are moving away from us.

a) Most galaxies showed redshifts.b) All galaxies showed blueshifts.c) Galaxies showed about half redshifts and

half blueshifts.d) Galaxies showed no line shifts at all.e) Some galaxies showed a redshift that

changed into a blueshift at other times.


Universal recession: All galaxies (with a couple of nearby exceptions) seem to be moving away from us, with the redshift of their motion correlated with their distance.

Hubble’s Law


These plots show the relation between distance and recessional velocity for the five galaxies in the previous figure, and then for a larger sample.

Hubble’s Law


The relationship (slope of the line) is characterized by Hubble’s constant H0:

recessional velocity = H0 distance

The value of Hubble’s constant is currently uncertain, with most estimates ranging from 50 to 80 km/s/Mpc.Measuring distances using Hubble’s law actually works better the farther away the object is; random motions are overwhelmed by the recessional velocity.

Hubble’s Law


a) more distant galaxies showing greater blueshifts.

b) distant quasars appearing proportionally dimmer.

c) more distant galaxies showing greater redshifts.

d) slowly varying Cepheid variables appearing brighter.

e) more distant galaxies appearing younger.

Question 5

Hubble’s law is based on


a) more distant galaxies showing greater blueshifts.

b) distant quasars appearing proportionally dimmer.

c) more distant galaxies showing greater redshifts.

d) slowly varying Cepheid variables appearing brighter.

e) more distant galaxies appearing younger.

Question 5

Hubble’s law is based on


Question 6

Hubble’s constant measures

a) the density of galaxies in the universe.b) the luminosity of distant galaxies.c) the reddening of light from dust clouds.d) the speed of a galaxy.e) the rate of expansion of the universe.


a) the density of galaxies in the universe.b) the luminosity of distant galaxies.c) the reddening of light from dust clouds.d) the speed of a galaxy.e) the rate of expansion of the universe.

Question 6

Hubble’s constant measures

Hubble’s law relates how fast galaxies are moving away from us

at different distances. A larger value for H0 implies a faster

expansion rate.

Velocity = H0 x Distance


a) the size of the universe.b) distances to galaxies.c) the speed of recession of galaxies.d) the density of matter in the universe.e) the temperature of the Big Bang.

Question 7

To calibrate Hubble’s constant, astronomers must determine


a) the size of the universe.b) distances to galaxies.c) the speed of recession of galaxies.d) the density of matter in the universe.e) the temperature of the Big Bang.

Question 7

To calibrate Hubble’s constant, astronomers must determine

Distances to galaxies are determined using a variety of “standard candles,”

including Cepheid variables, supernova explosions, model galaxies,

and model clusters.


This puts the final step on our distance ladder.

Hubble’s Law


About 20–25 percent of galaxies don’t fit well into the Hubble scheme – they are far too luminous.Such galaxies are called active galaxies. They differ from normal galaxies in both the luminosity and type of radiation they emit.

Active Galactic Nuclei


The radiation from these galaxies is called nonstellar radiation.

Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor. These galaxies are called starburst galaxies, and we will discuss them later.The galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center.



Active galaxies are classified into three types: Seyfert galaxies, radio galaxies, and quasars.

Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more luminous.



a) supermassive black holes at their cores.

b) dark matter.c) self-sustaining star formation.d) spiral density waves.e) hypernova explosions.

Question 8

Seyfert and radio galaxies could be powered by


a) supermassive black holes at their cores.

b) dark matter.c) self-sustaining star formation.d) spiral density waves.e) hypernova explosions.

Question 8

Seyfert and radio galaxies could be powered by

The Circinus Galaxy, a Seyfert galaxy about 4

Mpc away


The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact.



They may have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy.

Radio galaxies emit very strongly in the radio portion of the spectrum.



Radio galaxies may also be core dominated.



Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles.



Many active galaxies have jets, and most show signs of interactions with other galaxies.



Quasars – quasi-stellar objects – are starlike in appearance, but have very unusual spectral lines.



Eventually it was realized that quasar spectra were normal, but enormously redshifted.



a) they generate energy partly through H to He fusion like stars.

b) they show spectra similar to extremely bright O stars.

c) their luminosity varies like eclipsing binary stars.

d) in short exposure photographs, their images appear stellar.

e) they are dense concentrations of millions of stars.

Question 9

Quasars are “quasi-stellar” because


a) they generate energy partly through H to He fusion like stars.

b) they show spectra similar to extremely bright O stars.

c) their luminosity varies like eclipsing binary stars.

d) in short exposure photographs, their images appear stellar.

e) they are dense concentrations of millions of stars.

Question 9

Quasars are “quasi-stellar” because

Although short-exposure images can appear starlike, many

quasars show jets or other signs of intense activity.


Solving the spectral problem introduces a new problem – quasars must be among the most luminous objects in the galaxy, to be visible over such enormous distances.



Active galactic nuclei have some or all of the following properties:• High luminosity• Nonstellar energy emission• Variable energy output, indicating small

nucleus• Jets and other signs of explosive activity• Broad emission lines, indicating rapid rotation

The Central Engine of an Active Galaxy


This is the leading theory for the energy source in an active galactic nucleus: a black hole, surrounded by an accretion disk. The strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis.



In an active galaxy, the central black hole may be billions of solar masses. The accretion disk is whole clouds of interstellar gas and dust; they may radiate away as much as 10–20 percent of their mass before disappearing.




The jets emerging from an active galaxy can be quite spectacular.


Measurements of the core of the galaxy M87 indicate that it is rotating very rapidly.



One might expect the radiation to be mostly X- and gamma-rays, but apparently it is often “reprocessed” in the dense clouds around the black hole and re-emitted at longer wavelengths.



a) sudden bursts of star formation.b) supernova chain reactions in the core.c) the collapse of the core into a larger

black hole.d) close encounters with a nearby galaxy.e) dark matter becoming visible and

emitting light.

Question 10

In active galaxies, the central engine can be “fed” by


a) sudden bursts of star formation.b) supernova chain reactions in the core.c) the collapse of the core into a larger

black hole.d) close encounters with a nearby galaxy.e) dark matter becoming visible and

emitting light.

Question 10

In active galaxies, the central engine can be “fed” by

Collisions or tidal interaction between galaxies can provide new fuel to power the supermassive black hole engines of

active galaxies.


Particles will emit synchrotron radiation as they spiral along the magnetic field lines; this radiation is decidedly nonstellar.



a) the universe is static.b) the universe is collapsing.c) the universe is expanding.d) the Milky Way is the center of the

universe.e) There is no accepted interpretation.

Question 11

Hubble’s discovery of galaxy redshifts means


a) the universe is static.b) the universe is collapsing.c) the universe is expanding.d) the Milky Way is the center of the

universe.e) There is no accepted interpretation.

Question 11

Hubble’s discovery of galaxy redshifts means


Possible Final Questions to Ponder

1. 1. A solar eclipse can only happen during a new moon.2. Polaris will not always be the pole star due to

precession shifting the celestial pole.3. A tropical day is not the same length as a sidereal year.4. The larger the parallax shift, the closer you are to an

object.5. A fatal flaw with Ptolemy’s model is its inability to

predict phases of Mercury and Venus.6. The force of gravity varies with the product of masses

and inverse square of the separation distance.

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Chapter 15 Normal and Active Galaxies