CHAPTER 15 THE MILKY WAY GALAXY
15-1 THE NATURE OF THE MILKY WAY GALAXY
How do astronomers know we live in a galaxy?
The hazy band of the Milky Way is our wheel-shaped galaxy seen from within, but its size
and shape are not obvious. William and Caroline Herschel counted stars at many locations
over the sky to show that our star system seemed to be shaped like a grindstone with the sun
near the center.
Later astronomers studied the distributions of stars, but, because gas and dust in space blocked
their view of distant stars, they concluded the star system was only about 10 kiloparsecs in
diameter with the sun at the center.
In the early 20th century, Harlow Shapley calibrated Cepheid variable stars to find the
distance to globular clusters and demonstrated that our galaxy is much larger than what we
can see and that the sun is not at the center.
Modern observations suggest that our galaxy contains a disk component about 75,000 ly in
diameter and that the sun is two-thirds of the way from the center to the visible edge. The
nuclear bulge around the center and an extensive halo containing old stars and little gas and
dust make up the spherical component.
The mass of the galaxy can be found from its rotation curve. Kepler’s third lawreveals that
the galaxy contains over 100 billion solar masses. If stars orbited in Keplerian motion, more
distant stars would orbit more slowly. They do not, and that shows that the halo may contain
much more mass than is visible. Because the mass in this galactic corona is not emitting
detectable electromagnetic radiation, astronomers call it dark matter.
15-2 THE ORIGIN OF THE MILKY WAY
How did our galaxy form and evolve? The oldest star clusters reveal that the disk of our galaxy is younger than the halo, and the
oldest globular clusters appear to be about 13 billion years old. So our galaxy must have
formed about 13 billion years ago.
Stellar populations are an important clue to the formation of our galaxy. The first stars to form,
termed population II stars, were poor in elements heavier than helium—elements that
astronomers call metals. As generations of stars manufactured metals in a process called
nucleosynthesis and spread them back into the interstellar medium, the metal abundance of
more recent generations increased. Population I stars, including the sun, are richer in metals.
Galactic fountains produced by expanding supernova remnants may help spread metals
throughout the disk.
Because the halo is made up of population II stars and the disk is made up of population I stars,
astronomers conclude that the halo formed first and the disk later. A theory that the galaxy
formed from a single, roughly spherical cloud of gas and gradually flatted into a disk has been
amended to include mergers with other galaxies and infalling gas contributing to the disk.
15-3 SPIRAL ARMS
What are the spiral arms?
You can trace the spiral arms through the sun’s neighborhood by using spiral tracers such as O
and B stars; but, to extend the map over the 15-1 ❙The Nature of the Milky Way entire
galaxy, astronomers must use radio telescopes to see through the gas and dust.
The most massive stars live such short lives they don’t have time to move from their place of
birth. Because they are found scattered along the spiral arms, astronomers conclude that the
spiral arms are sites of star formation.
The spiral density wave theory suggests that the spiral arms are regions of compression that
move around the disk. When an orbiting gas cloud overtakes the compression wave, the gas
cloud is compressed and forms stars. A density wave produces a two-armed spiral galaxy. Another process, self-sustaining star formation, may act to modify the arms with branches and
spurs as the birth of massive stars triggers the formation of more stars by compressing
neighboring gas clouds. This may account for the wooly appearance of flocculent galaxies.
15-4 THE NUCLEUS
What lies at the very center?
The nucleus of the galaxy is invisible at visual wavelengths, but radio, infrared, and X-ray
radiation can penetrate the gas and dust. These wavelengths reveal crowded central stars and
warmed dust.
The very center of the Milky Way Galaxy is marked by a radio source, Sagittarius A*. The
core must be less than an astronomical unit in diameter, but the motions of stars around the
center show that it must contain roughly 2.6 million solar masses. A supermassive black hole
is the only object that could contain so much mass in such a small space. CHAPTER 16 GALAXIES
16-1 THE FAMILY OF GALAXIES
What do galaxies look like?
Through 19th-century telescopes, galaxies looked like hazy spiral nebulae. Some astronomers
said they were other star systems sometimes called island universes, but others said they were
clouds of gas inside the Milky Way system. The controversy culminated in the Shapley-Curtis
Debate in 1920.
A few years later, with the construction of larger telescopes, astronomers could identify stars,
including Cepheid variable stars, in the spiral nebulae. That showed that the spiral nebulae
were galaxies.
Astronomers divide galaxies into three classes—elliptical, spiral, and irregular—with
subclasses specifying the galaxy’s shape.
Elliptical galaxies contain little gas and dust and cannot make new stars. Consequently, they
lack hot, blue stars and have a reddish tint.
Spiral galaxies contain more gas and dust in their disks and support active star formation,
especially along the spiral arms. Some of the newborn stars are massive, hot, and blue, and
that gives the spiral arms a blue tint. About two-thirds of spirals are barred spiral galaxies.
The halo and nuclear bulge of a spiral galaxy usually lack gas and dust and contain little star
formation. The halos and nuclear bulges have a reddish tint because they lack hot, blue stars.
Irregular galaxies have no obvious shape but contain gas and dust and support star formation.
16-2 MEASURING THE PROPERTIES OF GALAXIES
How do astronomers find the distances to galaxies?
Galaxies are so distant astronomers measure their distances in megaparsecs—millions of
parsecs. Astronomers find the distance to galaxies using distance indicators, sometimes called standard
candles, objects of known luminosity. The most accurate distance indicators are the Cepheid
variable stars. Globular clusters and type Ia supernovae explosions have also been calibrated
as distance indicators.
By calibrating additional distance indicators using galaxies of known distance, astronomers
have built a distance scale. The Cepheid variable stars are the most dependable.
When astronomers look at a distant galaxy, they see it as it was when it emitted the light now
reaching Earth. The look-back time to distant galaxies can be a significant fraction of the age
of the universe.
According to the Hubble law, the apparent velocity of recession of a galaxy equals its distance
times the Hubble constant. Astronomers can estimate the distance to a galaxy by observing its
redshift, calculating its apparent velocity of recession, and then dividing by the Hubble
constant.
How do galaxies differ in size, luminosity, and mass?
Once the distance to a galaxy is known, its diameter can be found from the small-angle
formula and its luminosity from the magnitude-distance relation.
Astronomers measure the masses of galaxies in two basic ways. The rotation curve of a galaxy
show the orbital motion of its stars, and astronomers can use the rotation curve method to find
the galaxy’s mass.
The cluster method uses the velocities of the galaxies in a cluster to find the total mass of the
cluster. The velocity dispersion method uses the velocities of the stars in a galaxy to find the
total mass of the galaxy.
Galaxies come in a wide range of sizes and masses. Some dwarf ellipticals and dwarf irregular
galaxies are only a few percent the size and luminosity of our galaxy, but some giant elliptical
galaxies are five times larger than the Milky Way Galaxy. Do other galaxies contain supermassive black holes and dark matter, as does our own galaxy?
Stars near the centers of galaxies are following small orbits at high velocities, which suggests
the presence of supermassive black holes in the centers of most galaxies.
The mass of a galaxy’s supermassive black hole is proportional to the mass of its nuclear
bulge. That shows that the supermassive black holes must have formed when the galaxy
formed.
Observations of individual galaxies show that galaxies contain 10 to 100 times more dark
matter than visible matter.
The hot gas held inside some clusters of galaxies and the gravitational lensing caused by the
mass of galaxy clusters reveal that the clusters must be much more massive than can be
accounted for by the visible matter—further evidence of dark matter.
16-3 THE EVOLUTION OF GALAXIES
Why are there different kinds of galaxies?
Rich clusters of galaxies contain thousands of galaxies with fewer spirals and more ellipticals.
Poor clusters of galaxies contain few galaxies with a larger proportion of spirals. This is
evidence that galaxies evolve by collisions and mergers.
When galaxies collide, tides twist and distort their shapes and can produce tidal tails.
Large galaxies can absorb smaller galaxies in what is called galactic cannibalism. You can see
clear evidence that our own Milky Way Galaxy is devouring some of the small galaxies that
orbit nearby and that our galaxy has consumed other small galaxies in the past.
Shells of stars, counterrotating parts of galaxies, streams of stars in the halos of galaxies, and
multiple nuclei are evidence that galaxies can merge.
Ring galaxies are produced by high-speed collisions in which a small galaxy plunges through
a larger galaxy perpendicular to its disk. The compression of gas clouds can trigger bursts of star formation, producing starburst galaxies. The rapid star formation can produce lots of dust, which is warmed by the stars to emit infrared radiation, making the galaxy an ultraluminous infrared galaxy.
The merger of two larger galaxies can scramble star orbits and drive bursts of star formation to use up gas and dust. Most larger ellipticals have evidently been produced by past mergers.
Spiral galaxies have thin, delicate disks and appear not to have suffered mergers with large galaxies.
A galaxy moving through the gas in a cluster of galaxies can be stripped of its own gas and dust and may become an S0 galaxy.
Rare isolated galaxies tend to be spirals and lack a bar or a strong two-armed spiral pattern, which suggests that gentle interactions with neighbors are needed to stimulate the formation of bars and spiral arms.
At great distance and great look-back times, the largest telescopes reveal that galaxies were smaller, more irregular, and closer together. There were more spirals and fewer ellipticals long ago.
At the largest distances, astronomers find small irregular clouds of stars that may be the objects that fell together to begin forming galaxies when the universe was very young. CHAPTER 17 GALAXIES WITH ACTIVE NUCLEI
17-1 ACTIVE GALAXIES
What evidence shows that some galactic nuclei are active?
Radio galaxies were first noticed because they emit energy at radio wavelengths, but later
studies showed that they emitted a wide range of wavelengths, so they are now called active
galaxies. The activity is in their cores, which are called active galactic nuclei.
Some galaxies have peculiar properties. Seyfert galaxies, for example, are spirals with small,
highly luminous cores.
Spectra of the nuclei of Seyfert galaxies show that they contain highly excited gas moving at
very high velocities.
Double-lobed radio sources emit radio energy from areas on either side of active galaxies.
These lobes appear to be inflated by relativistic jets ejected from the nuclei of the galaxies in
what is called the double-exhaust model.
Hot spots in radio lobes show where the jets push against the gas of the intergalactic medium
and inflate the lobes.
Some giant elliptical galaxies have small, energetic cores, with, in some cases, jets of matter
rushing outward.
What is the energy source of this activity?
Orbital motion around the nuclei of active galaxies reveals that the central objects have
masses ranging from a few million to a few billion solar masses. These are presumably
supermassive black holes.
Matter flowing through hot accretion disks into supermassive black holes can release
tremendous energy and eject jets in opposite directions. The creation of jets is not well
understood, but jets are also observed coming from accretion disks around protostars, around
neutron stars, and around stellar mass black holes. Active galaxies moving through the intergalactic medium leave behind trails of hot gas from
their jets. In other cases, the motions of the nucleus can produce twisted jets.
Supermassive black holes cannot have been formed by dying stars but must have formed as
the nuclear bulges of the galaxies began to form.
What triggers the nucleus of a galaxy into activity?
Most galaxies appear to contain supermassive black holes at their centers, but they are
dormant because large amounts of matter are not flowing inward. Only when a supermassive
black hole is fed does it erupt.
Interactions between galaxies can throw matter into the center, feed the black hole, and trigger
eruptions. This explains why active galaxies are often distorted or have nearby companions.
According to the unified model, what an observer sees depends on the tilt of the accretion disk.
If you see into the core, you see broad spectral lines and rapid fluctuations. If you see the disk
edge on, you see only narrow spectral lines produced by slower moving gas above and below
the disk.
If the jet from the black hole points directly at Earth, you see a BL Lac object, also known as a
17-2 QUASARS
What are the most distant active galaxies?
The quasars appear to be the cores of very distant, highly luminous active galaxies.
Einstein’s relativistic Doppler formula refers to objects moving through space, so it cannot be
used to analyze the redshifts of galaxies and quasars because those redshifts are not produced by the Doppler effect. Nevertheless, astronomers know that quasars are very far away because
they have very high redshifts.
To be visible at such great distances, quasars must be ultraluminous.
Because quasars can change their brightness quickly, you can conclude they must be
small—only a few times larger than our solar system.
Observations of the spectra of hazy objects near quasars and the spectra of quasar fuzz show
that quasars are the active cores of very distant galaxies.
Gravitational lensing by very distant galaxies can form multiple images of quasars, and that is
further evidence that the quasars must be very distant.
Superluminal expansion refers to blobs of material that appear to be rushing away from some
quasars faster than the speed of light. This is an illusion caused when a relativistic jet points
nearly at Earth, so it does not contradict the laws of physics or the modern understanding of
quasars.
What can active galaxies reveal about the history of the universe?
Because quasars lie at great distances, they appear as they were over 10 billion years ago
when the universe was young and just forming galaxies.
The best images show that the host galaxies of quasars are distorted, and that suggests that
they have erupted because they have been involved in mergers or collisions. Such interactions
were more common in the distant past before the universe had expanded very much and
galaxies were closer together.
It is also possible that at least some quasars are erupting while matter falls together to create a
supermassive black hole as a galaxy begins to form. That is, some quasars may be caused by
the formation of the first galaxies when the universe was young.
Quasars are most common with redshifts of about 2, which shows that there was an age when
galaxy formation, interaction, and mergers were more common. The so-called dead quasars today are the dormant black holes at the centers of galaxies where little matter is flowing into the black hole.