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Galaxies
Learning Outcome (Student will…):
•compare characteristics & classification of various
galaxies
In this unit, you will learn
The basic types of galaxies
 Shape & parts of the Milky Way Galaxy
 How galaxies are grouped into clusters
and larger structures
 What happens when galaxies collide
 What observations indicate the presence
of dark matter in other galaxies
 How galaxies formed and evolved
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Hubble’s Tuning Fork Diagram
Spiral Galaxies
Spiral Galaxies
•Spiral galaxies have ‘arms’
emerging from a bright central
nucleus
•Spirals are classified according
to how tightly or loosely wound
the arms are
•brightness of the central
nucleus is correlated to the
tightness of the arm
Barred Spiral Galaxies
Barred Spiral Galaxies
•The spiral galaxies M 91 (left) and M 109 (right) have bars across their nuclei
from which spiral arms unwind.
•In nearly all spirals (barred or not) the galaxies rotate such that the spiral
arms trail behind in the rotation
•Bars form when stellar orbits in a spiral galaxy become unstable and deviate
from a circular path; they are a sign of mature spiral galaxies
•The Milky Way is thought to be a barred spiral galaxy.
Elliptical Galaxies
Elliptical Galaxies
•Elliptical galaxies lack spiral arms and dust
•They contain older stars than spirals.
•The elliptical galaxies M 32 (below) and M 110 (right) show
varying degrees of ellipticity.
Lenticular Galaxies
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have a disk but no discernible spiral arms
Irregular Galaxies
Irregular Galaxies
•Irregular galaxies lack any specific form and contain stars, gas and dust
generally associated with a youth.
•The irregular galaxy at right is the Large Magellanic Cloud, a satellite of the
Milky Way located about 180,000 light years from the sun.
•The LMC is about 60,000 light years across. The bright reddish feature in the
upper right is the “Tarantula Nebula” a region of star formation in the LMC.
Regions of the Milky Way Galaxy
radius of disk = 50,000 l.y. (20 kpc)
thickness of disk = 1,000 l.y. (300 pc)
number of stars = 200 billion
Sun is in
disk, 8kpc
out from
center
The Galactic Disk:
• Most stars are here. Nearly all the
interstellar gas.
• Old Stars (1010yrs) to Younger Stars
(106yrs)
• Star Formation is occurring now.
• Composition: Old Metal Poor to Young
Metal Rich stars.
• Motions – coplanar, direct, elliptical orbits.
• Spiral Arms  density waves
The Galactic Bulge & Halo
The Galactic Halo
• Thin scattering of stars & clusters
• Stars (OLD), globular clusters (OLD), no
interstellar material.
• Metal Poor material (mostly H, He, very little
else)
• Random eccentric orbits
The Galactic Bulge
• Like the halo, only more crowded
Active Galaxies
•The galaxy NGC 7742 is an
otherwise normal spiral
galaxy except for its
extraordinarily bright
nucleus that outshines the
rest of the galaxy
•Such galaxies, i.e. spirals
with extremely bright nuclei,
form a class of active
galaxies known as Seyfert
galaxies.
Hubble Space Telescope Image
Active Galaxies
•The elliptical galaxy M87, shown
below in a wide-field ground-based
image, has a very bright, point-like
nucleus from which a jet of material
emanates.
•The jet is seen in great detail from
an HST image at right.
Hubble Space Telescope Image
Active Galaxies
NGC 4139
•This image shows the
spiral galaxy NGC 4319
and the quasar Markarian
205.
•The distance to NGC is
80 million light years,
which Mkn 205 is 14 times
farther away at a distance
of 1 billion light year.
•The very distant quasar
is nearly as bright as the
much closer galaxy. The
extraordinary brightness
of quasars, which is a
blending of the term
quasi-stellar radio source,
indicates that some
incredibly powerful
mechanism must be
producing enormous
amounts of energy from a
small volume of space.
Mkn 205
Hubble Space Telescope Image
Clusters of Galaxies
•Rather than occurring individually in space, galaxies are grouped in clusters ranging in
size from a few dozens to thousands of galaxies.
•The Coma Cluster, shown at right, is 300 million light years from the Milky Way and
contains more than 1,000 (and possibly as many as 10,000) galaxies.
•The Milky Way is a member of a small cluster called the Local Group which contains
about 40 galaxies. The largest member of the Local Group is M 31, with the Milky Way
coming in second in size.
The Local
Group
Galaxies in Collision & Mergers
•In this close encounter between two spiral galaxies, their arms are dramatically warped
and massive star formation is triggered when the hydrogen gas clouds in the two collide.
•It is believed the Milky Way may have “cannibalized” small galaxies in the past through
collision.
http://burro.cwru.edu/JavaLab/GalCrashWeb/
Key Ideas
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The Hubble Classification: Galaxies can be grouped
into four major categories: spirals, barred spirals,
ellipticals, and irregulars.
The disks of spiral and barred spiral galaxies are sites of
active star formation.
Elliptical galaxies are nearly devoid of interstellar gas
and dust, and so star formation is severely inhibited.
Lenticular galaxies are intermediate between spiral and
elliptical galaxies.
Irregular galaxies have ill-defined, asymmetrical shapes.
They are often found associated with other galaxies.
Key Ideas
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Clusters and Superclusters: Galaxies are grouped into
clusters rather than being scattered randomly throughout
the universe.
Our Galaxy is a member of a poor, irregular cluster
called the Local Group.
Key Ideas
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Galactic Collisions and Mergers: When two galaxies
collide, their stars pass each other, but their interstellar
media collide violently, either stripping the gas and dust
from the galaxies or triggering prolific star formation.
The gravitational effects during a galactic collision can
throw stars out of their galaxies into intergalactic space.
Galactic mergers may occur; a large galaxy in a rich
cluster may tend to grow steadily through galactic
cannibalism, perhaps producing in the process a giant
elliptical galaxy.
Key Ideas
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The Dark-Matter Problem: The luminous mass of a
cluster of galaxies is not large enough to account for the
observed motions of the galaxies; a large amount of
unobserved mass must also be present. This situation is
called the dark-matter problem.
Gravitational lensing of remote galaxies by a foreground
cluster enables astronomers to glean information about
the distribution of dark matter in the foreground cluster.
Key Ideas
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Formation and Evolution of Galaxies: Observations
indicate that galaxies arose from mergers of several
smaller gas clouds.
Whether a protogalaxy evolves into a spiral galaxy or an
elliptical galaxy depends on its initial rate of star
formation.