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Astronomy: Horizons
10th edition
Michael Seeds
• Less than a century ago, astronomers
did not understand that there were
– Nineteenth-century telescopes revealed faint nebulae
scattered among the stars, some of which were spiral.
– Astronomers argued about the nature of these
– It was not until the 1920s, though, that they
understood that some were other galaxies much like
our own.
• It was not until recent decades that
astronomical telescopes could reveal
the tremendous beauty and intricacy of
the galaxies.
• In this chapter, you will try to understand
how galaxies form and evolve.
– You will discover that the amount of gas and dust
in a galaxy is a critical clue.
• You will also discover that interactions
between galaxies can influence their
structure and evolution.
The Shapes of Galaxies
• One, many galaxies have no disk, no
spiral arms, and almost no gas and
– These elliptical galaxies range from huge giants to
small dwarfs.
The Shapes of Galaxies
• Disk-shaped galaxies usually have spiral
arms and contain gas and dust.
– Many spiral galaxies have a barred structure.
– A small percentage of disk galaxies contain little gas
and dust.
The Shapes of Galaxies
• Finally, some galaxies are highly
irregular in shape and tend to be rich in
gas and dust.
The Shapes of Galaxies
• You might also wonder what proportion
of the galaxies are elliptical, spiral, and
• That is a difficult question to answer.
– In some catalogs of galaxies, about 70 percent are
The Shapes of Galaxies
• Spiral galaxies contain hot, bright stars—
and are thus very luminous and easy to
• Most ellipticals are fainter and harder to
• Small galaxies, such as dwarf ellipticals
and dwarf irregulars, may be very
common—but they are hard to detect.
The Shapes of Galaxies
• From careful studies, astronomers can
conclude that ellipticals are more
common than spirals and that irregulars
make up only about 25 percent of all
– Among spiral galaxies, about two thirds are
barred spirals.
How Many Galaxies Are There?
• In 1995, astronomers picked a seemingly
empty spot on the sky near the Big Dipper
and used the Hubble Space Telescope to
record a time exposure that lasted an
astonishing 10 days.
– This became known as a Hubble deep field.
– It was deep in that it recorded very faint objects.
– The image revealed that the ‘empty spot’ on the sky
was filled with galaxies.
How Many Galaxies Are There?
• There is no reason to believe that the
two regions of the sky chosen for study
are unusual.
– So, it seems likely that the entire sky is carpeted with
• At least 100 billion would be visible with
today’s telescopes.
– Surely, there are other galaxies too distant or too
faint to see.
Building Scientific Arguments
• Different kinds of galaxies have different
colors—depending mostly on how much
gas and dust they contain.
• If a galaxy contains large amounts of gas
and dust, it probably contains lots of young
– A few of those young stars will be massive, hot,
luminous O and B stars.
– They will produce most of the light and give the
galaxy a distinct blue tint.
Building Scientific Arguments
• In contrast, a galaxy that contains
little gas and dust will probably
contain few young stars.
– It will lack O and B stars.
– The most luminous stars in such a galaxy will be
red giants.
– They will give the galaxy a red tint.
Building Scientific Arguments
• As the light from a galaxy is a blend of
the light from billions of stars, the colors
are only tints.
• Nevertheless, the most luminous stars
in a galaxy determine the color.
– You can conclude that elliptical galaxies tend to
be red and the disks of spiral galaxies tend to be
Building Scientific Arguments
• Now, create a new scientific
argument and analyze a different
kind of observation.
– Why are most galaxies in catalogs spiral in
spite of the fact that the most common kind of
galaxy is elliptical?
Measuring the Properties of Galaxies
• Beyond the edge of Milky Way,
astronomers find many billions of galaxies.
• Great clusters—some containing
thousands of galaxies—fill space as far as
telescopes can see.
• The distances to galaxies are so large that
it is not convenient to express them in
light-years, parsecs, or even kiloparsecs.
• Instead, astronomers use the unit
megaparsec (Mpc), or 1 million pc.
– One Mpc equals 3.26 million ly, or approximately 2 x
1019 miles.
• Most distance indicators are objects
whose brightness is known.
• Astronomers often refer to them as
standard candles.
– If you can find a standard candle in a galaxy, you
can judge its distance.
• As their period is related to their
luminosity, Cepheid variable stars are
reliable distance indicators.
– If you know the period of
the star’s variation, you can
use the period–luminosity
relation to learn its absolute
– By comparing its absolute and
apparent magnitudes, you can
find its distance.
• Astronomers can use globular
clusters in a different way.
– Studies of nearby globular clusters with known
distances show that they are about 25 pc in diameter.
– If astronomers can detect globular clusters in a
distant galaxy, they can assume the clusters are
about 25 pc in diameter and use the small-angle
formula to find the distance to the galaxy.
• When a supernova explodes in a
distant galaxy, astronomers rush to
observe it.
– Studies show that type Ia supernovae—caused by
the collapse of a white dwarf—all reach about the
same absolute magnitude at maximum.
– By searching for Cepheids and other distance
indicators in nearby galaxies where type Ia
supernovae have occurred, astronomers have been
able to calibrate these supernovae.
• For example, a type Ia supernova was
seen in 2002 exploding in the galaxy
displayed in the image.
– Astronomers were able
to find Cepheid variables
in the galaxy and so
could find its distance.
– Then, they could
calculate the absolute
magnitude of the
supernova at its
• When type Ia supernovae are seen in more
distant galaxies, astronomers can measure
the apparent magnitude at maximum and
compare that with the known absolute
magnitude of these supernovae to find the
distance to the galaxy.
• Astronomers like to refer to distance
indicators such as Cepheids as standard
• However, an astronomer commented that
type Ia supernovae are more like standard
– When they explode, they reach the same maximum
luminosity and can be calibrated as distance
• At the greatest distances, astronomers
must calibrate the total luminosity of
the galaxies themselves.
– For example, studies of nearby galaxies show that
an average galaxy like Milky Way has a luminosity
about 16 billion times the sun’s.
– If astronomers see a similar galaxy far away, they
can measure its apparent magnitude and calculate
its distance.
• The most distant visible galaxies are
roughly 10 billion ly (3,000 Mpc)
• At such distances, you see an effect
akin to time travel.
• When you look at a galaxy only a few
million light-years away, you do not see
it as it is now but as it was millions of
years ago—when its light began the
journey toward Earth.
• When you look at a more distant
galaxy, you look back into the past by
an amount called the look-back time.
– This is the time in years equal to the distance to
the galaxy in light-years.
• You may have experienced look-back
time if you have ever made a longdistance phone call carried by satellite
or watched a TV newscaster interview
someone on the other side of the world
via satellite.
• A half-second delay occurs as a radio
signal carries a question 23,000 miles out
to a satellite, then back to Earth, and then
carries the answer out to the satellite and
again back to Earth.
– That half-second look-back delay can make people
hesitate on long-distance phone calls and produces a
seemingly awkward delay in intercontinental TV
• The Andromeda Galaxy has a look-back
time of about 2 million years—a mere eye
blink in the lifetime of a galaxy.
• When you look at more distant galaxies,
though, the look-back time becomes an
appreciable part of the age of the
• When you look at the most distant visible
galaxies, you are looking back over 10
billion years to a time when the universe
may have been significantly different.
– This effect will be important as you think about the
origin and evolution of galaxies and the universe as a
The Hubble Law
• Although astronomers find it difficult to
measure the distance to a galaxy, they
often estimate such distances using a
simple relationship.
– Early in the 20th century, astronomers noticed that the
lines in galaxy spectra were shifted slightly toward
longer wavelengths—redshifts.
– Interpreted as a consequence of the Doppler shift,
these redshifts implied that the galaxies had large
radial velocities and were receding from Earth.
The Hubble Law
• In 1929, the American astronomer Edwin
Hubble published a graph that plotted the
apparent velocities of recession versus
distance for a number of galaxies.
– The points in the graph fell along a straight line.
The Hubble Law
• This relation between apparent
velocity of recession and distance is
known as the Hubble law.
– The slope of the line is known as the Hubble
The Hubble Law
• The Hubble law is important in
astronomy for two reasons.
– It is taken as evidence that the universe is
– Astronomers use it to estimate the distance to
Diameter and Luminosity
• Elliptical galaxies cover a wide range of
diameters and luminosities.
– The largest, called giant ellipticals, are five times the
size of Milky Way.
– Many elliptical galaxies, though, are very small, dwarf
ellipticals—only 1 percent the diameter of our galaxy.
Diameter and Luminosity
• Clearly, the diameter and luminosity of
a galaxy do not determine its type.
– Some small galaxies are irregular and some are
– Some large galaxies are spiral and some are
• Other factors must influence the origin
and evolution of galaxies.
• This section will examine two
fundamental ways to find the masses
of galaxies.
– One method involves the rotation of galaxies.
– The other involves their motions.
• To begin, you can eliminate a common
• Some people think astronomers can
see galaxies rotating.
– Some galaxies definitely look like spinning pinwheels.
– You know, though, that the orbital period of the sun
around Milky Way is about 240 million years.
– So, galaxies rotate very slowly.
– No change is visible in a human lifetime.
• Nevertheless, the rotation of galaxies
can give you a clue to their masses.
– You know the stars in the outer parts of the galaxy
are in orbit.
– So, you can use Kepler’s third law to find the mass
from the size of the stellar orbits and their orbital
• You can’t observe the orbital period, P,
directly because humans don’t live long
enough to see a galaxy rotate.
• However, you can find the orbital velocity
of the stars by measuring their Doppler
– Then, you can find the orbital period by dividing the
circumference of the orbit by the velocity.
• As the galaxy rotates, one side moves
away from Earth and one side moves
toward Earth.
– So, the emission lines would
be redshifted on one side
of the galaxy and blueshifted
on the other side.
• You could measure those changes in
wavelength, use the Doppler formula to
find the velocities, and plot a diagram
showing the velocity of rotation at different
distances from the center of the galaxy.
– This diagram
is called
a rotation
• A related way of measuring a galaxy’s
mass is the velocity dispersion
– It is really a version of the cluster method.
– Instead of observing the motions of galaxies in a
cluster, astronomers observe the motions of
matter within a galaxy.
• In the spectra of some galaxies,
broad spectral lines indicate that
stars and gas are moving at high
– If astronomers assume the galaxy is bound by its
own gravity, they can ask how massive it must be
to hold this moving matter within the galaxy.
– This method, like the one before, assumes that
the system is not coming apart.
• The masses of galaxies cover a
wide range.
– The smallest contain about 10-6 as much mass
as the Milky Way.
– The largest contain as much as 50 times more
than the Milky Way.
Supermassive Black Holes in Galaxies
• Rotation curves show the motions of the
outer parts of a galaxy.
• It is also possible, though, to detect the
Doppler shifts of stars orbiting close to the
centers of galaxies.
• Although these motions are not usually
shown on rotation curves, they reveal
something astonishing.
– Stars near the centers of most galaxies are
orbiting very rapidly.
Supermassive Black Holes in Galaxies
• To hold stars in such small, short-period
orbits, the centers of galaxies must contain
masses of a million to a few billion solar
• Yet, no object is visible.
– The evidence seems to require that the nuclei of
galaxies contain supermassive black holes.
– You have learned that Milky Way contains a
supermassive black hole at its center.
– Evidently, that is typical of galaxies.
Supermassive Black Holes in Galaxies
• Such supermassive black holes cannot be
the remains of a dead star.
– That would produce a black hole of only a few
solar masses.
• Measurements show that the masses of
these supermassive black holes are
typically 0.5 percent the mass of the
nuclear bulges.
Supermassive Black Holes in Galaxies
• A galaxy with a large nuclear bulge has a
supermassive black hole whose mass is
greater than that in a galaxy with a small
nuclear bulge.
– This implies that the supermassive black holes
formed long ago as the galaxies began forming.
– Matter has continued to drain into the black holes.
– However, they do not appear to have grown
dramatically since they formed.
Supermassive Black Holes in Galaxies
• A billion-solar-mass black hole
sounds like a lot of mass.
• It is, however, roughly 1 percent of
the mass of a galaxy.
– The 2.6-million-solar-mass black hole at the center of
Milky Way contains only a thousandth of a percent of
the mass of the galaxy.
Dark Matter in Galaxies
• Given the size and luminosity of a
galaxy, astronomers can make a rough
guess as to the amount of matter it
should contain.
– They know how much light stars produce and how
much matter there is between the stars.
– So, it is quite possible to estimate very roughly the
mass of a galaxy from its luminosity.
Dark Matter in Galaxies
• When astronomers measure the masses
of galaxies, however, they find that the
measured masses are much larger than
expected from the luminosities of the
• This seems to be true of most galaxies.
– Measured masses of galaxies amount to 10 to 100
times more mass than you would expect from the
appearance of galaxies.
Dark Matter in Galaxies
• X-ray observations provide more
evidence of dark matter.
– X-ray images of galaxy clusters show that many of
them are filled with very hot, low-density gas.
Dark Matter in Galaxies
• The amount of gas present is much too
small to account for the dark matter.
– Rather, the gas is important because it is very hot and
its rapidly moving atoms have not leaked away.
– Evidently, the gas is held in the cluster by a very strong
gravitational field.
Dark Matter in Galaxies
• To provide enough gravity to hold such
hot gas, the cluster must contain much
more matter than is visible as galaxies.
– For instance, the detectable galaxies in the Coma
cluster amounts to only a small fraction of the total
mass of the cluster.
Gravitational Lensing and Dark Matter
• Albert Einstein described gravity as a
curvature of space.
– The presence of mass actually distorts space-time
around it.
– That is what you feel as gravity.
– Einstein predicted that a light beam traveling through
a gravitational field would be deflected by the
curvature of space-time, much as a golf ball is
deflected as it rolls over a curved putting green.
– That effect has been observed and is a strong
confirmation of Einstein’s theories.
Gravitational Lensing and Dark Matter
• Gravitational lensing occurs when light
from a distant object passes a nearby
massive object and is deflected by its
gravitational field.
– The gravitational field of the nearby object is
actually a region of curved space-time.
– It acts as a lens to deflect the passing light.
Gravitational Lensing and Dark Matter
• Astronomers have used gravitational
lensing to detect dark matter.
– When light from very distant galaxies passes through
a cluster of galaxies on its way to Earth, it can be
deflected by the strong gravitational field.
Gravitational Lensing and Dark Matter
• That distorts the images of the distant
galaxies into curving arcs.
– The amount of the distortion depends on the mass of
the cluster of galaxies.
Gravitational Lensing and Dark Matter
• Observations of gravitational lensing made
with very large telescopes reveal that
clusters of galaxies contain far more matter
than what we is seen.
• That is, they contain large amounts of dark
– This confirmation of the existence of dark matter is
independent of orbital motion and gives astronomers
much greater confidence that dark matter is real.
Gravitational Lensing and Dark Matter
• Dark matter is difficult to detect,
and it is even harder to explain.
– Some astronomers have suggested that dark
matter consists of low-luminosity white dwarfs
and brown dwarfs scattered through the halos of
Gravitational Lensing and Dark Matter
• Both observation and theory support the
idea that galaxies have massive extended
• Also, searches for white dwarfs and
brown dwarfs in the halo of Milky Way
have been successful.
– Nevertheless, the searches have not turned up
enough of these low-luminosity objects to make up
all the dark matter.
Gravitational Lensing and Dark Matter
• The dark matter can’t be hidden in
vast numbers of black holes and
neutron stars.
– Astronomers don’t see the X rays these objects
would emit.
– There is 10 to 100 times more dark matter than
visible matter.
– That many black holes would produce X rays that
would be easy to detect.
Gravitational Lensing and Dark Matter
• Until recently, neutrinos were thought to
be massless.
• Studies now suggest they have a very
small mass.
• Thus, they may represent part of the dark
– However, their masses are too low to make up all the
dark matter.
– There must be some other undiscovered form of
matter in the universe that is detectable only by its
gravitational field.
Gravitational Lensing and Dark Matter
• Dark matter is not a small issue.
– Observations show that 85 percent of the matter in
the universe is dark matter.
– The universe you see—the kind of matter that you
and the stars are made of—has been compared to
the foam on an invisible ocean.
• Dark matter remains one of the
fundamental unresolved problems of
modern astronomy.
The Evolution of Galaxies
• Why did some galaxies become
spiral, some elliptical, and some
– Clues to that mystery lie in the clustering of
Clusters of Galaxies
• The distribution of galaxies is not
entirely random.
– Galaxies tend to occur in clusters ranging from a few
to thousands.
– Deep photos made with the largest telescopes reveal
clusters of galaxies scattered out to the limits of
• This clustering of the galaxies can help
you understand their evolution.
Clusters of Galaxies
• For this discussion, you can sort clusters
into two groups—rich and poor.
• Rich galaxy clusters contain over a
thousand galaxies, mostly elliptical,
scattered through
a spherical volume
about 3 Mpc (107 ly)
in diameter.
Clusters of Galaxies
• A rich galaxy cluster is very crowded—with
the galaxies more concentrated toward the
• It often contains one or more giant elliptical
galaxies at the center.
Clusters of Galaxies
• The Coma cluster (located in the
constellation Coma Berenices) is a rich
– It lies over 100 Mpc from Earth and contains at least
1,000 galaxies—mostly E and S0 galaxies.
– Its galaxies are highly crowded around a central giant
elliptical galaxy and a large S0.
Clusters of Galaxies
• One of the nearest clusters, the Virgo
cluster, contains over 2,500 galaxies
and is—by the discussed definition—a
rich cluster.
– It does contain a giant elliptical galaxy, M87, near its
– However, it is not very crowded and contains mostly
spiral galaxies.
Clusters of Galaxies
• In contrast, poor galaxy clusters
contain fewer than 1,000 galaxies, are
irregularly shaped, and are less
crowded toward the center.
Clusters of Galaxies
• The Local Group, which contains Milky
Way, is a good example of a poor cluster.
– It contains a few dozen members scattered irregularly
through a volume slightly over 1 Mpc in diameter.
– Of the brighter galaxies, 14 are elliptical, 3 are spiral,
and 4 are irregular.
Clusters of Galaxies
• The total number of galaxies in the Local
Group is uncertain because some lie in
the plane of Milky Way and are difficult
to detect.
– For example, the
Sagittarius Dwarf,
a small dwarf galaxy,
has been found on
the far side of Milky
Way—where it is
almost totally hidden
behind the star clouds
of Sagittarius.
Clusters of Galaxies
• Even closer to the center of Milky
Way is the Canis Major Dwarf Galaxy.
– The galaxy was found by mapping the distribution of
red supergiants detected by the 2MASS infrared
– Other small galaxies
in the Local Group
have been found
hidden behind the
stars, gas, and dust
of Milky Way.
Clusters of Galaxies
• Classifying clusters as either rich or
poor reveals a fascinating and
suggestive clue to the evolution of
– In general, rich clusters tend to contain 80 to 90
percent E and S0 galaxies and few spirals.
– Poor clusters contain a larger percentage of spirals.
– Among isolated galaxies that are not in clusters, 80
to 90 percent are spirals.
Clusters of Galaxies
• This suggests that a galaxy’s
environment is important in
determining its structure.
– This has led astronomers to suspect that the
secrets of galaxy evolution lie in collisions
between galaxies.
Colliding Galaxies
• There are several important
points to note about interacting
Colliding Galaxies
• One, interacting galaxies
can distort each other
with tides—producing
tidal tails and shells of
– They may even trigger the
formation of spiral arms.
– Large galaxies can even absorb
smaller galaxies.
Colliding Galaxies
• Also, the interactions can trigger
star formation.
Colliding Galaxies
• Evidence left inside galaxies in the form
of motions and multiple nuclei reveals
that they have suffered past interactions
and mergers.
Colliding Galaxies
• Finally, the beautiful ring galaxies are
bull’s-eyes left behind by high-speed
Colliding Galaxies
• Evidence of galaxy mergers is all
– Milky Way is a cannibal galaxy—snacking on the
Magellanic Clouds as they orbit around it.
– Its tides are pulling the Sagittarius Dwarf Galaxy
– The Canis Major Dwarf galaxy has been almost
completely digested as tides pulled stars away to form
great streamers wrapped around Milky Way.
– Almost certainly, our galaxy has dined on other small
The Origin and Evolution of Galaxies
• You can also argue that spiral and
irregular galaxies cannot evolve into
elliptical galaxies.
– Spiral and irregular galaxies contain both young and
old stars.
– The old stars mean that spiral and irregular galaxies
can’t be young.
The Origin and Evolution of Galaxies
• The ellipticals appear to be the product of
galaxy mergers—which triggered star
formation and used up all the gas and dust.
– Astronomers see star formation being stimulated to high
levels in many colliding galaxies.
The Origin and Evolution of Galaxies
• The Antennae contain
over 15 billion solar
masses of hydrogen gas
and will become a
starburst galaxy as the
merger triggers rapid star
The Origin and Evolution of Galaxies
• Supernovae in a starburst galaxy may
eventually blow away any remaining
gas and dust that doesn’t get used up
making stars.
– A few collisions and mergers could leave a galaxy
with no gas and dust from which to make new stars.
– Astronomers now suspect that most ellipticals are
formed by the merger of at least two or three galaxies.
The Origin and Evolution of Galaxies
• In contrast, spirals seem never to
have suffered major collisions.
– Their thin disks are delicate and would be
destroyed by tidal forces in a collision with a
massive galaxy.
• Also, they retain plenty of gas and
dust and continue making stars.
The Origin and Evolution of Galaxies
• Milky Way has, evidently, never merged
with another large galaxy.
• It has, however, clearly cannibalized
smaller galaxies.
– Astronomers have found streams of stars in the halo
of the galaxy that are too metal rich for their location.
– Another stream contains globular clusters with similar
– These streams are evidently the remains of smaller
galaxies that were absorbed.
The Origin and Evolution of Galaxies
• Barred spiral galaxies may be the
product of tidal interactions.
– Mathematical models show that the bars are not
stable and eventually dissipate.
– It may take tidal interactions with other galaxies to
regenerate the bars.
– As well over half of all spiral galaxies have bars, you
can suspect that these tidal interactions are common.
Building Scientific Arguments
• A growing body of evidence suggests that
elliptical galaxies have been subject to
collisions in their past and that spiral
galaxies have not.
– During collisions, a galaxy can be driven to use up its
gas and dust in a burst of star formation.
– The resulting supernova explosions can help drive
gas and dust out of the galaxy.
– This explains why elliptical galaxies now contain little
star-making material.
Building Scientific Arguments
• The beautiful disk typical of spiral galaxies
is very orderly—with all the stars following
similar orbits.
• However, when galaxies collide, the stellar
orbits get scrambled—and an orderly disk
galaxy could be converted into a chaotic
swarm of stars typical of elliptical galaxies.
– It seems likely that elliptical galaxies have had much
more complex histories than spiral galaxies have had.