1. Discovering the Essential Universe
Neil F. Comins
Discovering the Essential Universe
Fourth Edition
CHAPTER 11
The Galaxies
The center of the Milky Way Galaxy, as seen by the Chandra X-ray telescope.
The false colors reveal extremely hot gases that surround black holes, neutron
stars, and white dwarves. The image is approximately 300 by 800 light-years.
The supermassive black hole at the center of our Galaxy is enshrouded by the hot,
white gas cloud in the center of the image. (NASA/UMass/D. Wang et al.).

2. Schematic diagram of the Milky Way Galaxy, seen from above.
FIGURE 15-1 Schematic Diagrams of the Milky Way
(a) This edge-on view shows the Milky Way’s disk, containing most of
its stars, gas, and dust, and its halo, containing many old stars.
Individual stars in the halo are too dim to show, so the bright
regions in the halo represent clusters of stars. (b) Our Galaxy
has at least four major arms and several shorter arm segments,
all spiraling out from the ends of a bar of stars and gas
that passes through the Galaxy’s center. The bar’s existence
was confirmed by the
Schematic diagram of the Milky Way Galaxy, seen from above.

3. The Structure of Our Galaxy
Our Galaxy has a disk about 100,000 light-years diameter and about 2000 light-years thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars.
Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds.

4. Wide-angle view of our galaxy: the Milky Way.
FIGURE 15-5 Our Galaxy
This wide-angle photograph
spans half the Milky Way. The Northern Cross is at the left, and
the Southern Cross is at the right. The center of the Galaxy is
in the constellation Sagittarius, in the middle of this photograph.
The dark lines and blotches are caused by hundreds of
interstellar clouds of gas and dust that obscure the light from
background stars, rather than by a lack of stars. (Dirk Hoppe)
Wide-angle view of our galaxy: the Milky Way.

5. The Structure of Our Galaxy
A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic nuclear bulge.
Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming.
The Sun is located about 26,000 light-years from the galactic nucleus, between two major spiral arms. The Sun moves in its orbit at a speed of about 828,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy.

6. We are about 26,000 light years from the center.
FIGURE 15-9 A Map of the Galaxy
(a) This map, based on radio telescope surveys of 21-cm
radiation, shows the distribution of hydrogen gas
in a face-on view of the Galaxy. This view just hints at spiral
structure. The galactic nucleus is marked with a dot surrounded
by a circle. Details in the large, blank, wedge-shaped
region toward the upper left of the map are unknown,
because gas in this part of the sky is moving perpendicular to
our line of sight and thus does not exhibit a detectable
Doppler shift. Inset: This drawing, based on visible-light data,
shows that our solar system lies between two arms of the Milky
Way Galaxy. (b) This drawing labels the major spiral arms in
the Milky Way. (a: Courtesy of G. Westerhout; inset: National
Geographic; b: NASA/JPL-Caltech/R. Hart [SSC])
We are about 26,000 light years from the center.
The overall diameter is about 100,000 light years.

7. Two views of a barred spiral galaxy, M83.
FIGURE Two Views of a Barred Spiral Galaxy
The galaxy M83 is in the southern constellation of Centaurus, about
12 million light-years from Earth. (a) At visible wavelengths, spiral
arms are clearly illuminated by young stars and glowing H II
regions. (b) A radio view at 21-cm wavelength shows the emission
from neutral hydrogen gas. Note that the spiral arms are
more clearly demarcated by visible stars and H II regions than
by 21-cm radio emission. (a: S. Van Dyk/IPAC; b: VLA, NRAO)
Two views of a barred spiral galaxy, M83.

8. Mysteries at the Galactic Fringe
From studies of the rotation of the Galaxy, astronomers estimate that its total mass is about 1 x 1012 Msun. Much of this mass is still undetectable.

9. But there is a larger amount of matter that we can’t see,
the Dark Matter, which extends
out much further than the
visible Milky Way.
FIGURE Our Galaxy
As seen from the side, three
major visible components of our Galaxy are a thin disk, a
nuclear bulge, and a halo. As noted earlier, there is also a
central bar. The visible Galaxy’s diameter is about 100,000
light-years, and the Sun is about 26,000 light-years from the
galactic center. The disk contains gas and dust along with
Population I (young, metal-rich) stars. The halo is composed
almost exclusively of Population II (old, metal-poor) stars. Inset:
The visible matter in our Galaxy fills only a small volume compared
to the distribution of dark matter, whose composition is
presently unknown. Its presence is felt by its gravitational
effect on visible matter.

10. An Infrared picture shows the disk and central bulge.
FIGURE Infrared View of the Milky Way
Taken by the COBE satellite in 1997, this infrared image shows the disk and nuclear bulge of our Galaxy. Most of the sources scattered above and below the disk are nearby stars. Stars appear
white, whereas interstellar dust appears orange. Note that the dust that obscures
light from more distant stars in Figure 15-5 is quite bright in this infrared image. (The
COBE Project, DIRBE, NASA)
An Infrared picture shows the disk and central bulge.
(taken by the COBE satellite in 1997)

11. Views toward the galactic nucleus on a more detailed scale.
FIGURE The Galactic Center
(a) This wide-angle view at infrared wavelengths shows a
50° segment of the Milky Way centered on the
nucleus of the Galaxy. Black represents the dimmest regions of
infrared emission, with blue the next strongest, followed by
yellow and red; white represents the strongest emission. The
prominent band diagonally across this photograph is a layer
of dust in the plane of the Galaxy. Numerous knots and blobs
along the plane of the Galaxy are interstellar clouds of gas
and dust heated by nearby stars. (b) This close-up infrared
view of the galactic center covers the area outlined by the
white rectangle in (a). (c) This infrared image shows about
300 of the brightest stars less than 1 ly from Sagittarius A*,
which is at the center of the picture. The distribution of stars
and their observed motions around the galactic center imply
a very high density (about a million solar masses per cubic
light-year) of less luminous stars. (a, b: NASA; c: R. Schödel et al.
MPE/ESO)
Views toward the galactic nucleus on a more detailed scale.

12. The Structure of Our Galaxy
The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained.
A supermassive black hole of about 4 x 106 Msun exists in the galactic nucleus.
The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called nuclear bulge, through which a bar of stars and gas extend. The entire Galaxy is surrounded by a halo of matter that includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter.

13. Radio mapping of the motion of stars in the center of the Milky Way.
The supermassive
black hole has the
mass of 4 million
Suns.
FIGURE Two Views of the Galactic Nucleus
(a) A radio image taken at the VLA of the galactic
nucleus and environs. This image covers an area of
the sky eight times wider than the Moon. SNR means supernova
remnant. The numbers following each SNR are its right
ascension and declination. The Sgr (Sagittarius) features are
radio-bright objects. (b) The colored dots superimposed on this
infrared image show the motion of six stars in the vicinity of the
unseen massive object (denoted by the star) at the position of
the radio source Sagittarius A*, part of Sgr A in (a). The orbits
were measured over an 11-year period. This plot indicates that
the stars are held in orbit by a M black hole.
(a: Naval Research Laboratory produced by N. E. Kassim, D. S. Briggs,
T. J. W. Lazio, T. N. LaRosa, J. Imamura & S. D. Hyman. Originally from
the NRAO Very Large Array. Courtesy of A. Pedlar, K. Anantharamiah,
M. Gross & R. Ekers; b: Keck/UCLA Galactic Center Group)

14. Orbits of stars around the Milky Way.
FIGURE Orbits of Stars in Our Galaxy
This disk galaxy, M58, looks very similar to what the Milky Way
Galaxy would look like from far away. The colored arrows
show typical orbits of stars in the nuclear bulge (blue), disk
(red), and halo (yellow). Interstellar clouds, clusters, and other
objects in the various components have similar orbits.
(NOAO/AURA/NSF)
Orbits of stars around the Milky Way.

15. Travelling at 500,000 miles per hour, the Solar System takes
FIGURE The Galaxy’s Rotation Curve
The blue curve shows the orbital speeds of stars and gas in the Galaxy, and
the dashed red curve shows Keplerian orbits due to the gravitational
force from known objects. Because the data (blue
curve) do not show any such decline, there is, apparently, an
abundance of dark matter that extends to great distances from
the galactic center. This additional mass gives the outer stars
higher speeds than they would have otherwise.
Travelling at 500,000 miles per hour, the Solar System takes
230 million years to go one trip around the Milky Way.

16. Spiral galaxy classification
FIGURE 16-1 Spiral Galaxies (Nearly Face-on Views)
Edwin Hubble classified spiral galaxies
according to the tightness of their spiral arms and
the sizes of their nuclear bulges. Sa galaxies have the largest
nuclear bulges and the most tightly wound spiral arms,
whereas Sc galaxies have the smallest nuclear bulges and the
least tightly wound arms. The images are different colors
because they were taken through filters that pass different colors.
(a: NASA/Hubble Space Institute; b: Robert Gendler; c: Anglo-
Australian Observatory)

17. The size of the nucleus varies in spiral galaxies
FIGURE 16-3 Spiral Galaxies Seen Nearly Edge-on from the Milky Way
(a) Because of its large nuclear bulge, this galaxy
(called the Sombrero Galaxy) is classified as an Sa. If we
could see it face-on, the spiral arms would be tightly wound
around a voluminous bulge. (b) Note the smaller nuclear bulge
in this Sb galaxy. (c) At visible wavelengths, interstellar dust
obscures the relatively insignificant nuclear bulge of this Sc
galaxy. (a: European Southern Observatory; b: © Malin/IAC/RGO;
c: Brand Ehrhorn/Adam Block/NOAO/AURA/NSF)
Spiral type a, type b, and type c are classified by the
tightness of the spiral arms and the size of the bulge.

19. Types of Galaxies
The Hubble classification system groups galaxies into four major types: spiral, barred spiral, elliptical, and irregular.
The arms of spiral and barred spiral galaxies are sites of active star formation.
According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused by the births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches the star-forming regions into elongated arches of stars and nebulae that we see as spiral arms.

20. Types of Galaxies
According to the spiral density wave theory, spiral arms of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB associations, which highlight the arms.
Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies.

21. FIGURE 16-4 Variety in Spiral Arms
The differences in spiral
galaxies suggest that at least two mechanisms create spiral
arms. (a) This flocculent spiral galaxy has fuzzy, poorly defined
spiral arms. (b) This grand-design spiral galaxy has well-defined
spiral arms. (a: NASA; b: Gemini Observatory/AURA)

22. FIGURE 16-8 Dynamics of a Grand-Design Spiral Galaxy
This figure summarizes the activities
taking place in a grand-design spiral galaxy. (Image of spiral galaxy M101: NASA and ESA;
image of globular cluster M3: S. Kafka and K. Honeycutt, Indiana University/WIYN/NOAO/NSF)

23. Barred spiral galaxies
FIGURE 16-9 Barred Spiral Galaxies
As with spiral galaxies,
Edwin Hubble classified barred spirals according to the
tightness of their spiral arms (which correlates with the sizes of
their nuclear bulges). SBa galaxies have the most tightly
wound spirals and largest nuclear bulges, SBb have moderately
tight spirals and medium-sized nuclear bulges, and SBc
galaxies have the least tightly wound spirals and the smallest
nuclear bulges. (a: Johan H. Knapen and Nik Szymanek, University
of Hertfordshire; b: ESO, European Southern Observatory; c: Jean-
Charles Cuillandre/CFHT/Photo Researchers, Inc.)
Barred spiral galaxies are classified by the tightness
of the spiral arms, like ordinary spiral galaxies.

25. Elliptical galaxies have no arms and
FIGURE Giant Elliptical Galaxies
The Virgo cluster is a rich, sprawling collection of
more than 2000 galaxies about 50 million
light-years from Earth. Only the center of this huge cluster
appears in this photograph. The two largest galaxies in the
cluster are the giant elliptical galaxies M84 and M86. (Royal
Observatory, Edinburgh)
Elliptical galaxies have no arms and
can be much larger than the Milky Way.

26. Various ellipticals, with different elongations.
FIGURE Elliptical Galaxies Hubble classified elliptical
galaxies according to how round or elongated they
appear. An E0 galaxy is round; a very elongated elliptical
galaxy is an E7. Three examples are shown. (a: J. D. Wray,
McDonald Observatory; b, c: 1999 Princeton University Press/Zsolt
Frei and James E. Gunn)
Various ellipticals, with different elongations.

27. Irregular galaxies. FIGURE 16-13 Irregular Galaxies
(a) At a distance of only 179,000 light-years, the Large
Magellanic Cloud (LMC), an Irr I irregular galaxy,
is the third closest known companion of our Milky Way
Galaxy. (The Milky Way’s closest known companion, the
Canis Major Dwarf, is shown in Figure ) About
62,000 light-years across, the LMC spans 22° across the sky,
about 44 times the angular size of the full Moon. Note the
huge H II region (called the Tarantula Nebula or 30 Doradus).
Its diameter of 800 light-years and mass of 5 million solar
masses make it the largest known H II region. (b) The small
irregular (Irr II) galaxy NGC 4485 interacts with the highly
distorted Sc galaxy NGC 4490, also called the Cocoon
Galaxy. This pair is located in the constellation Canes
Venatici. (a: Anglo-
Irregular galaxies.

28. Hubble’s “tuning fork” model summarizes the types.
FIGURE Hubble’s Tuning Fork Diagram
Hubble summarized
his classification scheme for galaxies with this tuning
fork diagram. Elliptical galaxies are classified by how oval they
appear, whereas spirals and barred spirals are classified by the
sizes of their central bulges and the correlated winding of their
spiral arms. An S0 or SB0 galaxy, also called lenticular galaxy,
is an intermediate type between ellipticals and spirals. It has a
disk but no spiral arms.
Hubble’s “tuning fork” model summarizes the types.

29. Clusters and Superclusters
Galaxies group into clusters rather than being randomly scattered through the universe.
A rich cluster contains at least a thousand galaxies; a poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical.
Our Galaxy is a member of a poor, irregular cluster, called the Local Group.
Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain more spiral and irregular galaxies. Giant elliptical galaxies are often found near the centers of rich clusters.

30. Fornax cluster of galaxies
FIGURE A Cluster of Galaxies
This group of galaxies,
called the Fornax cluster, is about 60 million light-years from
Earth. Both elliptical and spiral galaxies are easily identified. The
barred spiral galaxy at the lower left is NGC 1365, the largest
and most impressive member of the cluster. For a closer view of
NGC 1365, see Figure (Anglo-Australian Observatory/Royal
Observatory, Edinburgh)
Fornax cluster of galaxies

31. Superclusters in our neighborhood.
FIGURE Superclusters in Our Neighborhood
This diagram shows the distances and relative positions of superclusters
within 950 million light-years of Earth. Note also the labeling
of some of the voids, which are large, relatively empty
regions between superclusters. (Kirk Korista)
Superclusters in our neighborhood.

32. 2dF Galaxy Survey shows 62,559 galaxies in two wedges
FIGURE Structure in the Universe
This map shows the distribution of 62,559 galaxies in two wedges extending in
opposite directions from Earth out to distances of 2 billion light-years.
For an explanation of right ascension (r.a.), see Section
1-3. Note the prominent voids surrounded by thin areas full of
galaxies. (Courtesy of the 2dF Galaxy Redshift Survey Team)
2dF Galaxy Survey shows 62,559 galaxies in two wedges
going away from our location. Note the huge voids.

33. A sponge with voids. FIGURE 16-18 Foamy Structure of the Universe
A sponge that recreates the distribution of bright clusters of galaxies
throughout the universe. The empty spaces in the foam are
analogous to the voids found throughout the universe. The
spongy regions are analogous to the locations of most of the
galaxies. (Image Source/Super Stock)
A sponge with voids.

34. The Local Group FIGURE 16-19 The Local Group Our Galaxy belongs to a
poor, irregular cluster that consists of about 40 galaxies, called
the Local Group. This map shows the distribution of about
three-quarters of the galaxies. The Andromeda Galaxy (M31)
is the largest and most massive galaxy in the Local Group. The
second largest is the Milky Way itself. M31 and the Milky Way
are each surrounded by a dozen satellite galaxies. The recently
discovered Canis Major Dwarf Galaxy is the Milky Way’s
nearest known neighbor.
The Local Group

35. The Coma cluster (in Coma Berenices) has large quantities of
FIGURE The Coma Cluster
(a) This rich, regular cluster
that contains thousands of galaxies is about 300 million light-years
from Earth. (b) Regular clusters are composed mostly of
elliptical and lenticular galaxies and are sources of X rays. This
Chandra image of Coma’s central region is 1.5 million light-years
across. The gas cloud emitting most of these X rays is 100
million K. (a: O. Lopez-Cruz and I. Shelton, NOAO/AURA/NSF; b:
NASA/CXC/SAO/A. Vikhlinin, et al.)
The Coma cluster (in Coma Berenices) has large quantities of
Hot gas between the galaxies, showing up in X-ray pictures.

36. Clusters and Superclusters
No cluster of galaxies has an observable mass large enough to account for the observed motions of its galaxies; a large amount of unobserved mass must be present between the galaxies.
Hot intergalactic gases emit X rays in rich clusters.
When two galaxies collide, their stars initially pass each other, but their interstellar gas and dust collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation. The gravitational effects of a galactic collision can cast stars out of their galaxies into intergalactic space.
Galactic mergers occur; a large galaxy in a rich cluster may grow steadily through galactic cannibalism, sometimes producing a giant elliptical galaxy.

37. The Hercules Cluster, 700 million light years away.
FIGURE The Hercules Cluster
This irregular cluster, which is about 700 million light-years
from Earth, contains a high proportion of spiral
galaxies, often associated in pairs and small groups. (NOAO)
The Hercules Cluster, 700 million light years away.

38. FIGURE 16-26 Interacting and Colliding Galaxies
(a) Pairs of colliding galaxies often exhibit long “antennae” of stars
ejected by the collision. This particular system is known as
NGC 4676 or “the Mice” (because of its tails of stars and
gas). It is 300 million light-years from Earth in the constellation
Coma Berenices. The collision has stimulated a firestorm of
new star formation, as can be seen in the bright blue regions.
Mass can also be seen flowing between the two galaxies,
which will eventually merge. (b) These two galaxies, NGC
2207 (right) and IC 2163, are orbiting and tidally distorting
each other. Their most recent close encounter occurred 40 million
years ago when the two were perpendicular to each other
and about 1 galactic diameter apart. Computer simulations
indicate that they should eventually coalesce. (a: NASA, H.
Ford/JHU, G. Illingworth/UCSC/Lick, M. Clampin/STScI, G.
Hartig/STScI, The ACS Science Team, and ESA; b: NASA)

40. FIGURE 16-27 Merging Galaxies
This contorted object, NGC 6240, in
the constellation Ophiuchus is the
result of two spiral galaxies in the
process of merging. The widespread
blue area reveals that the collision
between the two galaxies has triggered
an immense burst of star formation.
Inset: The Chandra X- ray telescope
has revealed that at the heart
of this system are two supermassive
black holes. Within a few hundred
million years, these black holes are
expected to merge into one more
massive black hole. (R. P. van der
Marel/J. Gersgen/STScI/NASA; inset:S.
Komossa/ G. Hasinger/MPE, et al.,
CXC/NASA)

41. Computer simulations of the merger of two galaxies.
FIGURE Simulated Galactic Cannibalism
This computer simulation shows a small galaxy (yellow
stars) being devoured by a larger, disk-shaped
galaxy (blue stars, white gas). Note how spiral arms are generated
in the disk galaxy by its interaction with the satellite
galaxy. (Lars Hernquist, Institute for Advanced Study with simulations
performed at the Pittsburgh Supercomputing Center)
Computer simulations of the merger of two galaxies.

42. Remote galaxies show rotation curves similar to the Milky Way.
FIGURE The Rotation Curves of Four Spiral Galaxies
This graph shows how the orbital speed of material in the disks
of four spiral galaxies varies with the distance from the center
of each galaxy. If most of each galaxy’s mass were concentrated
near the center of the galaxy, these curves would fall off at
large distances. But these and many other galaxies have flat
rotation curves that do not fall off. This indicates the presence
of extended halos of dark matter. See Figure to compare
these to the Milky Way’s rotation curve.
Remote galaxies show rotation curves similar to the Milky Way.

43. Dark matter can cause gravitational lensing.
FIGURE Gravitational Lensing of Extremely Distant Galaxies
(a) Schematic of how a gravitational lens works. Light
from the distant object changes direction due to the gravitational
attraction of the intervening galaxy and underlying dark matter.
The more distant galaxy appears in two places to the
observer on the right. (b) Three examples of gravitational lensing:
(1) The bluer arc is a galaxy that has been lensed by the
redder elliptical galaxy. (2) A pair of bluish images of the same
object lensed symmetrically by the brighter, redder galaxy
between them. (3) The lensed object appears as a blue arc
under the gravitational influence of the group of four galaxies.
(c) Superimposed in blue on this image of the galaxy cluster CL
is the location of dark matter that is gravitationally
lensing the galaxies behind it. (b: NASA, ESA, A. Bloton [Harvard-
Smithsonian CfA] and the SLACS Team; c: NASA, ESA, and M.J. Jee
[Johns Hopkins University])
Dark matter can cause gravitational lensing.

44. Examples of gravitational lensing .
FIGURE Gravitational Lensing of Extremely Distant Galaxies
(a) Schematic of how a gravitational lens works. Light
from the distant object changes direction due to the gravitational
attraction of the intervening galaxy and underlying dark matter.
The more distant galaxy appears in two places to the
observer on the right. (b) Three examples of gravitational lensing:
(1) The bluer arc is a galaxy that has been lensed by the
redder elliptical galaxy. (2) A pair of bluish images of the same
object lensed symmetrically by the brighter, redder galaxy
between them. (3) The lensed object appears as a blue arc
under the gravitational influence of the group of four galaxies.
(c) Superimposed in blue on this image of the galaxy cluster CL
is the location of dark matter that is gravitationally
lensing the galaxies behind it. (b: NASA, ESA, A. Bloton [Harvard-
Smithsonian CfA] and the SLACS Team; c: NASA, ESA, and M.J. Jee
[Johns Hopkins University])
Examples of gravitational lensing .

45. Gravitational lensing in a distant cluster.
FIGURE Gravitational Lensing of Extremely Distant Galaxies
(a) Schematic of how a gravitational lens works. Light
from the distant object changes direction due to the gravitational
attraction of the intervening galaxy and underlying dark matter.
The more distant galaxy appears in two places to the
observer on the right. (b) Three examples of gravitational lensing:
(1) The bluer arc is a galaxy that has been lensed by the
redder elliptical galaxy. (2) A pair of bluish images of the same
object lensed symmetrically by the brighter, redder galaxy
between them. (3) The lensed object appears as a blue arc
under the gravitational influence of the group of four galaxies.
(c) Superimposed in blue on this image of the galaxy cluster CL
is the location of dark matter that is gravitationally
lensing the galaxies behind it. (b: NASA, ESA, A. Bloton [Harvard-
Smithsonian CfA] and the SLACS Team; c: NASA, ESA, and M.J. Jee
[Johns Hopkins University])
Gravitational lensing in a distant cluster.

46. Superclusters in Motion
A simple linear relationship exists between the distance from Earth to galaxies in other superclusters and the redshifts of those galaxies (a measure of the speed at which they are receding from us). This relationship is the Hubble law: recessional velocity = Ho x distance, where Ho is the Hubble constant.
Astronomers use standard candles—Cepheid variables, the brightest supergiants, globular clusters, H II regions, supernovae in a galaxy, and the Tully-Fisher relation—to calculate intergalactic distances. Because of difficulties in measuring the distances to remote galaxies, the value of the Hubble constant, Ho, is not known with complete certainty.

47. FIGURE 16-31 Five Galaxies and Their Spectra
The photographs of these five elliptical galaxies were all taken at the
same magnification. They are labeled according to the constellation
in which each galaxy is located. The spectrum of
each galaxy is the hazy band between the comparison spectra
at the top and bottom of each plate. In all five cases, the
so-called H and K lines of calcium are seen. The recessional
velocity (calculated from the Doppler shifts of the H and K
lines) appears below each spectrum. Note that the fainter—
and thus more distant—a galaxy is, the greater is its redshift.
(Carnegie Observatories)

48. FIGURE The Hubble Law
The distances and recessional velocities of distant galaxies are
plotted on this graph. The straight line is the “best
fit” for the data. This linear relationship between distance and
speed is called the Hubble law. For historical reasons, distances
between galaxies, clusters of galaxies, and superclusters
of galaxies are usually given in megaparsecs, Mpc, rather
than millions of light-years.

49. Supernovae can be used to estimate distance.
Two Supernovae in NGC 664 In 1997 the rare
occurrence of two supernovae in the same galaxy
at the same time was observed in the spiral
galaxy NGC 664, located about 300 Mly (90 Mpc) from
Earth. Supernovae observed in remote galaxies are important
standard candles used by astronomers to determine the distances
to these faraway objects. The two supernovae overlap
each other, as shown. The upper, yellow-orange supernova
was observed to occur 2 months before the hotter, blue one,
which was observed to occur less than 2 weeks before this
image was made and had not yet achieved maximum brightness.
(Perry Berlind and Peter Garnavich, Harvard Smithsonian
Center for Astrophysics)
Supernovae can be used to estimate distance.

50. Methods used to determine distances.
FIGURE Techniques for Measuring Cosmological Distances
Astronomers use different methods to determine different
distances in the universe. All of the methods shown here
are discussed in the text.
Methods used to determine distances.

51. Jets (or lobes) can be seen around some galaxies.
FIGURE 17-1 Cygnus A (3C 405)
Radio image produced from observations made at the
Very Large Array. Most of the radio emissions from Cygnus A come from the radio lobes
located on either side of the peculiar galaxy seen in the inset, a Hubble Space Telescope image.
Each of the two radio lobes extend about 160,000 light-years from the optical galaxy and contain
a brilliant, condensed region of radio emission. Inset: At the heart of this system of gas lies
a strange-looking galaxy that has a redshift that corresponds to a recessional speed of 5% of
the speed of light. According to the Hubble law, Cygnus A is therefore 635 million light-years
from Earth. Because Cygnus A is one of the brightest radio sources in the sky, this remote
galaxy’s energy output must be enormous. (R. A. Perley, J. W. Dreher, J. J. Cowan, NRAO; inset:
William C. Keel, Robert Fosbury)
Jets (or lobes) can be seen around some galaxies.

52. Quasars and Other Active Galaxies
An active galaxy is an extremely luminous galaxy that has one or more unusual features: an unusually bright, star like nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects.
A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from Earth, according to the Hubble law.
To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the diameter of our solar system.

53. FIGURE 17-3 Quasar 3C 273
This combined X-ray and
infrared view shows the star like object associated with the
radio source 3C 273 and the luminous jet it has created. The
jet is also visible in the radio and visible parts of the spectrum.
By 1963, astronomers determined that the redshift of this
quasar is so great that, according to the Hubble law, it is
nearly 2 billion light-years from Earth. (S. Jester, D. E. Harris,
H. L. Marshall, K. Meisenheimer, H. J. Roser, and R. Perley)

54. Quasars and Other Active Galaxies
An active spiral galaxy with a bright, star like nucleus and strong emission lines in its spectrum is categorized as a Seyfert galaxy.
An active elliptical galaxy is called radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream out in opposite directions.
BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively rapid variations in luminosity.
Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.

55. FIGURE 17-9 Peculiar Galaxy NGC 5128
(Centaurus A) This extraordinary radio galaxy is
located in the constellation Centaurus, 11 million
light-years from Earth. At visible wavelengths a dust lane
crosses the face of the galaxy. Superimposed on this visible
image is a false-color radio image (green) showing that vast
quantities of radio radiation pour from matter ejected from the
galaxy perpendicular to the dust lane, along with radio
emission (rose-colored) along the dust lane, and X-ray
emission detected by NASA’s Chandra X-ray Observatory
(blue). The X rays may be from material ejected by the black
hole or from the collision of Centaurus A with a smaller
galaxy. Inset: This X-ray image from the Einstein Observatory
shows that NGC 5128 has a bright X-ray nucleus. An X-ray
jet protrudes from the nucleus along a direction perpendicular
to the galaxy’s dust lane. (X ray: NASA/CXC/M. Karovska et al.;
radio 21-cm: NRAO/VLA/J. Van Gorkom/Schminovich, et al.; radio
continuum: NRAO/VLA/J. Conden, et al.; optical: Digitized Sky
Survey U.K. Schmidt Image/STScI; inset: X ray—NASA/CXC/Bristol
U./M. Hardcastle; radio—NRAO/VLA/Bristol U./M. Hardcastle)

56. FIGURE 17-10 Head-Tail Source NGC 1265
This active elliptical galaxy is moving at a high speed through the intergalactic
medium. Because of this motion, the two tail jets trail
the galaxy at its head, giving this radio source a distinctly
windswept appearance. (NRAO)

57. FIGURE 17-11 Binary Head-Tail Source
This combined radio and X-ray image of 3C 75 shows the head-tail sources
emanating from supermassive black holes in a pair of galaxies
that are in the process of merging. The black holes are separated
by 25,000 light-years and are 300 million light-years away
from Earth. (X-ray: NASA/CXC/D. Hudson, T. Reiprich, et al. [AIFA])

58. How do we explain such huge jets?
FIGURE Giant Elliptical Galaxy M87
M87 is located near the center of the sprawling,
rich Virgo cluster, which is about 50 million light-years
from Earth. Embedded in this radio image of gas is the
galaxy M87 from which it has been ejected. Images at different
radio and visible wavelengths reveal a variety of details
about the structure of the jets of gas. M87’s extraordinarily
bright nucleus and the gas jets result from a 3 billion-solar-mass
black hole, whose gravity causes huge amounts of gas and an
enormous number of stars to crowd around it. (NASA and the
Hubble Heritage Team [STScI/AURA]; Frazer Owen [NRAO], John
Biretta [STScI] and colleagues)

59. Supermassive Central Engines
Many galaxies contain huge concentrations of matter at their centers.
The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources are probably matter ejected from the accretion disks that surround supermassive black holes at the centers of galaxies.
Some matter that spirals in toward a supermassive black hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space.

60. Jets form around supermassive black holes.
FIGURE Supermassive Black Holes as Engines for Galactic Activity
(a) In the accretion disk around a supermassive black
hole, in-swirling gas heats and expands. Pulled inward, compressed,
and heated further, some of it is eventually expelled
perpendicular to the disk in two jets. (b) The giant elliptical
galaxy NGC 4261 is a double-radio source located in the
Virgo cluster, about 100 million light-years from Earth. An
optical photograph of the galaxy (white) is combined with a
radio image (orange and yellow) to show both the visible
galaxy, which does not emit much radio energy, and its jets,
which do. Inset: This Hubble Space Telescope image of the
nucleus of NGC 4261 shows a disk of gas and dust about
800 ly (250 pc) in diameter, orbiting a supermassive black
hole. (b: NASA; inset: ESA)
Jets form around supermassive black holes.

61. Magnetic fields help confine the jet.
FIGURE Focusing Jets by Magnetic Fields
The hot, ionized accretion disk (red-yellow) around
the black hole rotates and creates a magnetic field
that is twisted into spring-shaped spirals above and below the
disk. Some of the accretion disk’s gas falling toward the black
hole is overheated and squirted at high speeds into the two
tubes created by the magnetic fields. The fields keep the gas
traveling directly outward from above and below the disk, thus
creating the two jets.
Magnetic fields help confine the jet.

62. FIGURE 17-19 Orientation of the Central Engine and Its Jets
BL Lacertae objects, quasars,
double-radio sources, and active galaxies
appear to be the same type of object viewed from different
directions. If one of the jets is aimed almost directly at Earth,
we see a BL Lac object. If the jet is somewhat tilted to our line
of sight, we see a quasar. Tilted farther and we see an active
galaxy. If the jets are nearly perpendicular to our line of
sight, we see a double-radio source. The central region of the
system is shown in Figures 17-16a and