1. Galaxies

2. 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.

3. 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.

4. 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 hydrogen in the clouds. Using radio telescopes to study this radio wave emission, we can map out the structure of the galaxy.

5. The Structure of Our Galaxy
The Milky Way galaxy is a disk with at least four bright arms of stars, gas, and dust which spiral 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. 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.

7. 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. This is a trillion times the mass of the Sun.
Some of the mass is in the stars which are visible, because they are hot and emit visible light.
There are about 200 billion stars, and most of them are less massive than the Sun, so they are only a fraction of the total mass. (1 trillion = 1000 billion)
Much of the remaining mass is still undetectable by any method. Thus it is called dark matter.

8. This is a sketch of the Milky Way from the side.
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.

9. 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.
The lower part of the figure
shows the extent of the
dark matter compared to
the disk of the 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. 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. (Four million times the Sun’s mass)
The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the nuclear bulge, through which a bar of stars and gas extends. 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.

11. 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.

12. The X-ray pictures of the center of the Milky Way
show lots of white dwarfs, neutron stars and ….

13. We believe that there is a supermassive black hole in
the center of the Milky Way galaxy, with a mass of
about 4 million solar masses, with an accretion disk.
Stars have been
seen in orbit around
the supermassive
black hole in the
center of the Milky
Way, and their orbits
have been studied
to deduce the mass
of the black hole.

14. 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.
(show ESOcast2)
link
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)

15. Spiral galaxy classification Compare the number of spirals and the thickness of the bulge
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)

16. 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.

17. The Sombrero galaxy - M104 – an Sa galaxy

18. 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.

19. 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.

20. 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)

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. M74 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)

23. M83 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)

24. 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)

25. Barred spiral galaxies
Barred spiral galaxies are
classified by the tightness
of the spiral arms, like
ordinary 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.)

26. NGC 1300

27. 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.

28. Various elliptical galaxies, 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 elliptical galaxies,
with different elongations.

29. A cluster of ellipticals – Abell 2218
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)
A cluster of ellipticals – Abell 2218

30. An edge-on Lenticular Galaxy - NGC 5866
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)

31. 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.

32. A nearby irregular galaxy, site of SN1987A.
FIGURE 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-
A nearby irregular galaxy, site of SN1987A.

33. 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.

34. 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.

35. 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

36. 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.

37. 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.

38. The Virgo supercluster is shown in an interactive map http://www
The Virgo supercluster is shown in an interactive map This is only part of a larger structure called Laniakea, discovered in This larger structure may contain over 100,000 galaxies and is about 500 million light years across. Here is a short (4 min) video describing this research: A shorter preview video with different graphics is at The observable universe is much larger than the size of this supercluster. Perhaps 50 thousand superclusters like this could fit in the part of the universe that we can see.

39. 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.