1. Chapter 16: Other Galaxies
Edwin Hubble is one of Time magazine’s most important people of the 20th Century:
.  Hubble was also on the cover of the February 9, 1948 issue of Time. 
A web site devoted to Hubble’s life and work is at .
SEDS maintains an excellent database of Messier objects:  .

2. WHAT DO YOU THINK? Do all galaxies have spiral arms?
Are galaxies isolated objects?
Are all other galaxies moving away from the Milky Way?

3. You will discover… How galaxies are categorized by their shapes.
That galaxies contain huge amounts of dark matter.
That galaxies are found in clusters.
How some galaxies merge while others devour their neighbors.
That the universe is expanding.

4. Types of Galaxies
Spiral
Barred Spiral
Elliptical
Irregular

5. Hubble’s Diagram of Galaxy 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, while 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 a
lenticular galaxy, is an intermediate type between
ellipticals and spirals. It has a disk but no spiral arms.

6. 1. Spiral Galaxies Type Sa Type Sb Type Sc
FIGURE 16-1 Spiral Galaxies (Nearly Face-on Views) Edwin
Hubble classified spiral galaxies according to the tightness of the
spiral arms and the size of the nuclear bulge. Sa galaxies have
the largest nuclear bulges and the most tightly wound spiral
arms, while Sc galaxies have the smallest nuclear bulges and the
least tightly wound arms. The images are different colors
because they were taken through filters passing different colors.
(left: NASA/Hubble Space Institute; middle: Robert Gendler; right:
Anglo-Australian Observatory)
Type Sa
Type Sb
Type Sc

7. Andromeda (M31) FIGURE 16-2 Andromeda (M32)
Andromeda is a beautiful spiral galaxy and is
the only galaxy visible to the naked eye from
the Earth’s northern hemisphere. Without a
telescope, it appears to be a fuzzy blob in the
constellation of Andromeda. Located only 0.9
Mpc (2.9 Mly) from us, Andromeda is
gravitationally bound to the Milky Way, and it
covers an area in the sky roughly 5 times as
large as the full Moon. Two other galaxies,
M32 and M110), are also labeled on this
photograph. The points of light peppering the
image are stars in our Galaxy. (Bill and Sally
Fletcher/Tom Stack and Associates)

8. Spiral Galaxies Seen Edge-on
FIGURE 16-3
Spiral Galaxies Seen Nearly Edge-on from the
Milky Way (a) Because of its large nuclear bulge, this 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: Anglo-Australian Observatory/David Malin Images; c: Dr. Rudy Schild,
Smithsonian Astrophysical Observatory)
M104: Sa galaxy
NGC 891: Sb galaxy
NGC 4631: Sc galaxy

9. Variety in Spiral Arms
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.
M33: A Spiral Galaxy with Flocculent Spiral Arms – created by explosions.
M74: a Grand Design Spiral Galaxy – created by waves, like ripples in water.

10. 2. Barred Spiral Galaxies
FIGURE 16-8 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, while SBc galaxies have the least
tightly wound spirals and the smallest nuclear bulges. (a: Johan H.
Knapen and N. K. Szymanek, University of Hertfordshire; b: ESO,
European Southern Observatory; c: Jean-Charles Cuillandre/CFHT/
Photo Researchers, Inc.)
M58: SBa galaxy
M83: SBb galaxy
NGC 1365: SBc galaxy

11. 3. Elliptical Galaxies M105: E0 galaxy M49: E4 galaxy
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 here. (a: J. D. Wray, McDonald
Observatory; b & c: © 1999 Princeton University Press/Zsolt Frei and
James E. Gunn)
M105: E0 galaxy
M49: E4 galaxy
NGC 4526: E7 galaxy

12. 4. Irregular Galaxies Large Magellanic Cloud, Irr 1 galaxy
FIGURE Irregular Galaxies (a) At a distance
of only 179,000 ly, 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 15-
16.) About 62,000 ly across, the LMC spans 22% across the sky,
about 44 times the angular size of the full Moon. Note the huge
a Large Magellanic Cloud, an Irr 1 galaxy b NGC 4485 (Irr 2) and NGC 4490 (Sc) galaxies
H II region (called the Tarantula Nebula or 30 Doradus) toward
the left side of this image. Its diameter of 800 ly and mass of 5
million Suns makes it the largest known H II region. (b) The
small irregular (Irr II) galaxy NGC 4485 (bottom galaxy) 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-Australian Observatory; b: Hoher Observatory)
Large Magellanic Cloud, Irr 1 galaxy
NGC 4485 (Irr 2) and NGC 4490 (Sc) galaxies

13. Galaxies are Clumped in Space
Galaxies occur in groups called Clusters. The galaxies in a Cluster are bound by gravity to each other, and can collide.
Our Galaxy belongs to a Cluster called “The Local Group.”
Clusters of galaxies occur in bigger groups called Superclusters. Most Superclusters are not gravitationally bound units like Clusters are.
All the Clusters are moving away from each other as the Universe expands.

14. A Cluster of Galaxies The 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, photos by David
Malin, © 1984 Royal Observatory, Edinburgh)
The Fornax Cluster of Galaxies

15. The Local Group – Our Cluster
FIGURE The Local Group Our Galaxy belongs to a
poor, irregular cluster consisting of about 40 galaxies called the
Local Group. This map shows the distribution of about threequarters
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.
Our Galaxy belongs to a poor, irregular (wimpy) cluster – consisting of about 40 galaxies (including M31 Andromeda) – called the Local Group.

16. Colliding Galaxies
Galaxies that belong to the same Cluster can collide.
Sometimes these galaxies pass through each other, and sometimes they combine.
The individual stars in the colliding galaxies do not hit each other, but the huge clouds of gas and dust do collide.

17. Colliding Galaxies
FIGURE 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 ly 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. (a: NASA, H. Ford/JHU, G. Illingworth/
UCSC/Lick, M. Clampin/STScI, G. Hartig/STScI,
The ACS Science Team, and ESA; b: NASA)
Pairs of colliding galaxies often exhibit long “antennae” of stars ejected by the collision.

18. Colliding Galaxies “Mano a Mano”
FIGURE Interacting and Colliding
Galaxies. (b) These two galaxies, NGC 2207
(right) and IC 2163, are orbiting and tidally
distorting each other. Their most recent close
encounter occurred 40 Myr ago when the two
were perpendicular to each other and about
one 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)
NGC 2207 (right) and IC 2163 are orbiting and gravitationally distorting each other.

19. Our Local Supercluster and Other Superclusters
Our Local Group (cluster) is at the center of the diagram. Our Local Supercluster extends out to the Virgo Cluster.
FIGURE Clusters of Galaxies in Our Neighborhood
This is a drawing of a sphere of space 250 Mpc (800 Mly) across
centered on the Earth in the Local Cluster. The spherical dots
represent the locations of the nearby clusters of galaxies,
while the flat circles represent the projection of the cluster
locations onto the plane of the Milky Way. To better see the
three-dimensionality of this figure, yellow arcs are drawn from
each cluster down to the green projection of the Milky Way’s
plane extended out through the universe. The Great Attractor is
an as-yet-unseen mass toward which the Local Group and other
nearby clusters of galaxies are flowing.

20. Beyond Superclusters – Filamentary Structure in the Universe
FIGURE Structure in the
Universe (a) This infrared map called 2MASS,
for 2-Micron All Sky Survey, shows the light
from 1.6 million galaxies. The entire sky is
projected onto an oval; the blue band running
vertically across the center of the image is
light from the plane of the Milky Way. Note
the filamentary structure with regions almost
devoid of galaxies, surrounded by thin regions
full of them.
2-Micron All Sky Survey (Infrared) – 1.6 million galaxies shown.

21. Beyond Superclusters – Filamentary Structure in the Universe
FIGURE Structure in the
Universe (b) This map shows the
distribution of 62,559 galaxies in two wedges
extending out in opposite directions from the
Earth. For an explanation of right ascension,
see Section 1-3. Note the prominent voids
surrounded by thin areas full of galaxies.
(a: 2MASS; IPAC/Caltech; and the University of
Massachusetts; b: Courtesy of the 2dF Galaxy
Redshift Survey Team/Anglo-Australian
Observatory)
Distribution map of 62,559 galaxies in two wedges extending out in opposite directions from the Earth (done with galaxy redshifts).

22. The Universe is Expanding
The Redshift of Superclusters shows us that the Universe is expanding. This Redshift is called the “Cosmological Redshift,” because it is caused by the expansion of space.
The farther away a galaxy is from us, the faster it moves away from us: GALAXY SPEED = GALAXY DISTANCE x HUBBLE’S CONSTANT (H0).
This is called “Hubble’s Law,” after Edwin Hubble, who discovered it.

23. Hubble’s Law
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.
The farther away a galaxy is from us, the faster it moves away from us: GALAXY SPEED = GALAXY DISTANCE x HUBBLE’S CONSTANT (H0).

24. The Hubble Telescope looks back to when the Universe was very young.

25. HST – Galaxies 12.5 Billion LY Away
FIGURE Distant Galaxies (a) The young cluster of
galaxies MS , shown on the left, contains many orbiting
pairs of galaxies, as well as remnants of recent galaxy collisions.
Several of these systems are shown at the right. This cluster is
located 8 billion light-years away from Earth. (b) This image of
more than 300 spiral, elliptical, and irregular galaxies contains
several that are an estimated 12 billion light-years from Earth.
Two of the most distant galaxies are shown in the images on the
right, colored in red at the centers of the pictures. (a, b: P. Van
Dokkum, Uner of Granengen, ESA and NASA)

26. HST – Galaxies >13 Billion LY Away
FIGURE Distant Galaxies (a) The young cluster of
galaxies MS , shown on the left, contains many orbiting
pairs of galaxies, as well as remnants of recent galaxy collisions.
Several of these systems are shown at the right. This cluster is
located 8 billion light-years away from Earth. (b) This image of
more than 300 spiral, elliptical, and irregular galaxies contains
several that are an estimated 12 billion light-years from Earth.
Two of the most distant galaxies are shown in the images on the
right, colored in red at the centers of the pictures. (a, b: P. Van
Dokkum, Uner of Granengen, ESA and NASA)

27. WHAT DID YOU THINK? Do all galaxies have spiral arms?
No. Galaxies may be either spiral, barred spiral, elliptical, or irregular. Only spirals and barred spirals have arms.
Are galaxies isolated objects?
No. Galaxies are grouped in clusters, and clusters are grouped in superclusters.
Are all other galaxies moving away from the Milky Way?
All galaxies except those in our Local Group (cluster) are receding from us. Some local galaxies are actually moving toward us.