Download Chapter 15 Normal and Active Galaxies

Chapter 15
Normal and
Active Galaxies
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Chapter 15
Normal and Active Galaxies
Copyright © 2010 Pearson Education, Inc.
Units of Chapter 15
Hubble’s Galaxy Classification
The Distribution of Galaxies in Space
Hubble’s Law
Active Galactic Nuclei
The Central Engine of an Active Galaxy
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Question 1
The Magellanic
Clouds are
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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.
Hubble’s Galaxy Classification
Spiral galaxies are classified according to the size
of their central bulge.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Question 2
What property is
shared by spiral
galaxies?
Copyright © 2010 Pearson Education, Inc.
a) ongoing star formation
b) a disk, bulge, and halo
c) globular clusters in the halo
d) open clusters in the disk
e) all of the above
Question 2
What property is
shared by spiral
galaxies?
Copyright © 2010 Pearson Education, Inc.
a) ongoing star formation
b) a disk, bulge, and halo
c) globular clusters in the halo
d) open clusters in the disk
e) all of the above
Hubble’s Galaxy Classification
Similar to the spiral galaxies are the barred
spirals.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Galaxy Classification
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.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Galaxy Classification
Ellipticals are classified according to their
shape, from E0 (almost spherical) to E7 (the
most elongated).
Copyright © 2010 Pearson Education, Inc.
Question 3
Based on their
shapes and stars,
elliptical galaxies
are most like the
Milky Way’s
Copyright © 2010 Pearson Education, Inc.
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.
Like the stars and globular
clusters in our halo, elliptical
galaxies contain little or no gas
and dust to make new stars.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Galaxy Classification
S0 (lenticular) and SB0 galaxies have a disk and
bulge, but no spiral arms and no interstellar gas.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Galaxy Classification
The irregular galaxies have a wide variety of shapes.
These galaxies appear to be undergoing interactions
with other galaxies.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Galaxy Classification
A summary of galaxy properties by type
Copyright © 2010 Pearson Education, Inc.
Hubble’s Galaxy Classification
Hubble’s “tuning fork” is a convenient way to
remember the galaxy classifications, although it
has no deeper meaning.
Copyright © 2010 Pearson Education, Inc.
The Distribution of Galaxies in Space
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 supernovae
Tully-Fisher
Copyright © 2010 Pearson Education, Inc.
Type I Supernovae
Type I supernovae all have about the same luminosity, as the
process by which they happen doesn’t allow for much variation.
Copyright © 2010 Pearson Education, Inc.
Apparent Vs. Absolute
Copyright © 2010 Pearson Education, Inc.
The Distribution of Galaxies in Space
The rotation of a galaxy results in Doppler
broadening of its spectral lines.
Copyright © 2010 Pearson Education, Inc.
Tully-Fisher
Tully-Fisher relation correlates a galaxy’s rotation speed
(which can be measured using the Doppler effect) to its
luminosity.
Copyright © 2010 Pearson Education, Inc.
The Distribution of Galaxies in Space
With these
additions, the
cosmic distance
ladder has been
extended to about
1 Gpc.
Copyright © 2010 Pearson Education, Inc.
The Distribution of Galaxies in Space
Here is the
distribution of
galaxies within
about 1 Mpc of
the Milky Way.
Copyright © 2010 Pearson Education, Inc.
The Distribution of Galaxies in Space
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.
Copyright © 2010 Pearson Education, Inc.
The Distribution of Galaxies in Space
A nearby galaxy cluster
is the Virgo Cluster; it is
much larger than the
Local Group, containing
about 3500 galaxies.
Copyright © 2010 Pearson Education, Inc.
Question 4
Hubble took
spectra of
galaxies in the
1930s. What did
he find?
Copyright © 2010 Pearson Education, Inc.
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?
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.
Redshifts of
galaxies indicate
they are moving
away from us.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Law
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Question 5
Hubble’s law is
based on
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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 6
Hubble’s constant
measures
Copyright © 2010 Pearson Education, Inc.
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
Velocity = H0 x Distance
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.
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.
Copyright © 2010 Pearson Education, Inc.
Question 7
To calibrate
Hubble’s constant,
astronomers must
determine
Copyright © 2010 Pearson Education, Inc.
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.
Distances to galaxies are determined
using a variety of “standard candles,”
including Cepheid variables,
supernova explosions, model galaxies,
and model clusters.
Copyright © 2010 Pearson Education, Inc.
Hubble’s Law
This puts the
final step on
our distance
ladder.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
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.
Copyright © 2010 Pearson Education, Inc.
Question 8
Seyfert and radio
galaxies could be
powered by
Copyright © 2010 Pearson Education, Inc.
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.
The Circinus
Galaxy, a Seyfert
galaxy about 4
Mpc away
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
The rapid variations in the
luminosity of Seyfert galaxies
indicate that the core must be
extremely compact.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
Radio galaxies emit very
strongly in the radio
portion of the spectrum.
They may have
enormous lobes,
invisible to optical
telescopes,
perpendicular to the
plane of the galaxy.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
Radio galaxies may also be core dominated.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
Core-dominated and radio-lobe galaxies are probably
the same phenomenon viewed from different
angles.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
Many active galaxies have jets, and most show
signs of interactions with other galaxies.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
Quasars – quasi-stellar
objects – are starlike
in appearance, but
have very unusual
spectral lines.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
Eventually it was realized that quasar spectra
were normal, but enormously redshifted.
Copyright © 2010 Pearson Education, Inc.
Question 9
Quasars are
“quasi-stellar”
because
Copyright © 2010 Pearson Education, Inc.
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.
Although short-exposure images
can appear starlike, many
quasars show jets or other signs
of intense activity.
Copyright © 2010 Pearson Education, Inc.
Active Galactic Nuclei
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.
Copyright © 2010 Pearson Education, Inc.
The Central Engine of an Active
Galaxy
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
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
The Central Engine of an Active
Galaxy
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.
Copyright © 2010 Pearson Education, Inc.
The Central Engine of an Active
Galaxy
The jets emerging from an active galaxy can be
quite spectacular.
Copyright © 2010 Pearson Education, Inc.
The Central Engine of an Active
Galaxy
Measurements of the core of the galaxy M87
indicate that it
is rotating very
rapidly.
Copyright © 2010 Pearson Education, Inc.
The Central Engine of an Active
Galaxy
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.
Copyright © 2010 Pearson Education, Inc.
Question 10
In active galaxies,
the central engine
can be “fed” by
Copyright © 2010 Pearson Education, Inc.
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.
Collisions or tidal interaction between
galaxies can provide new fuel to power
the supermassive black hole engines of
active galaxies.
Copyright © 2010 Pearson Education, Inc.
The Central Engine of an Active
Galaxy
Particles will emit synchrotron radiation as they
spiral along the magnetic field lines; this
radiation is decidedly nonstellar.
Copyright © 2010 Pearson Education, Inc.
Question 11
Hubble’s
discovery of
galaxy redshifts
means
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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.
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.
Copyright © 2010 Pearson Education, Inc.