Chapter 21
Galaxies
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The components of spiral galaxies are the same as in
our own Galaxy: disk, core, halo, bulge, spiral arms.
Type Sa tends to have the most tightly bound spiral
arms, with Types Sb and Sc progressively less tight,
although the correlation is not perfect.
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Similar to the spiral galaxies are the barred spirals.
Historically they were recognized later but were
found to be equally numerous. Like the spirals the
are designated by how tightly wound they are.
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Elliptical galaxies have no spiral arms and no
disk. They come in many sizes, from giant
ellipticals of trillions of stars, down to dwarf
ellipticals of less than a million stars.
Ellipticals also contain very little, if any, cool
gas and dust, and show no evidence of
ongoing star formation.
Many do, however, have large clouds of high
temperature gas, extending far beyond the
visible boundaries of the galaxy.
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Ellipticals are classified according to their
shape from E0 (almost spherical) to E7 (the
most elongated).
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S0 (Lenticular) and SB0 galaxies have a
flattened disk and bulge, but no spiral
arms and, like the Ellipticals, have little
interstellar gas and dust
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The irregular galaxies have a wide variety of
shapes, but no fixed shapes. The Small and
Large Magellanic Clouds are close neighbors
to our own Milky Way
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Here are three other irregular galaxies:
NGC 4485 and NGC 4490 on the left, and
M82 on the right
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Here is a summary of galaxy properties
by type
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When Edwin Hubble announced that many of the
strange nebulae in the sky were really Galaxies, he was
tasked with sorting and classifying them. Hubble’s
“tuning fork” is a convenient way to remember the
galaxy classifications, although it has no deeper meaning
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Cepheid variables allow measurement of distance to
galaxies to about 25 Mpc away. However, some galaxies
have no Cepheids, and most are farther away than 25
Mpc. New distance measures are needed.
• Tully–Fisher relation correlates a galaxy’s rotation
speed (which can be measured using the Doppler
effect) to its luminosity.
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• Type I supernovae all have about the same luminosity,
since the process by which they happen doesn’t allow
for much variation, so the inverse square can be applied
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With these additions
then, the cosmic
distance ladder has
been extended to
about 1 Gpc
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The Local Group
Here is the distribution of
galaxies within
about 1 Mpc of
the Milky Way.
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There are three spirals in this group – the Milky
Way, Andromeda, and M33. These and their
satellites – about 45 galaxies in all – form the Local
Group.
Such a group of galaxies, held together by its own
gravity, is called a galaxy cluster
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A nearby galaxy cluster is the Virgo cluster; it is
much larger than the Local Group, containing
about 3500 galaxies.
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All galaxies (with a few
nearby exceptions)
show a Doppler Red
Shift that suggest they
are moving away from
us.
This chart shows red
shift correlated with
their recession velocity
and distance:
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These plots show the relation between
distance and recession velocity for the five
galaxies in the previous figure, and then for a
larger sample
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The relationship (slope of the line) is characterized by
Hubble’s constant H0
The currently most accepted value for Hubble’s
constant
Measuring distances using Hubble’s law actually is
better the farther away the object is; random
motions are overwhelmed by the recessional
velocity.
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The Doppler Effect relates the change in
wavelength to the recession velocity
Δλ / λ0 = Vr / C
The Hubble Law relates recession velocity
with distance
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This puts the final
step on our distance
ladder. However the
Hubble Law does
need some final
tweaking when the
recession velocities
approach reletavistic
values
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About 20–25% of galaxies
don’t fit well into the
Hubble scheme – they are
far too luminous. Such
galaxies are called Active
Galaxies. They differ from
normal Galaxies in both
the luminosity and type of
radiation they emit
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The radiation from active galaxies is called
non-stellar radiation.
Many luminous galaxies are experiencing an
outburst of star formation, probably due to
interactions with a neighbor. These galaxies
are called starburst galaxies
The galaxies we will discuss now are those
whose activity is due to events occurring in
and around the galactic center.
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This active
galaxy has starformation rings
surrounding a
very luminous
core called an
Active Galactic
Core
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Active galaxies are
classified into three
types: Seyfert galaxies,
radio galaxies, and
quasars. Seyfert galaxies,
such as this, resemble
normal spiral galaxies,
but their cores are
thousands of times more
luminous
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The rapid
variations in the
luminosity of
Seyfert galaxies
indicate that the
core must be
extremely
compact
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Radio galaxies emit
very strongly in the
radio and X-ray
portions of the
spectrum. Many
have enormous
lobes, invisible to
optical telescopes,
perpendicular to the
plane of the galaxy.
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Core-dominated
and radio-lobe
galaxies are
probably the
same
phenomenon
viewed from
different angles.
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Quasars – quasi-stellar objects – are star-like in
appearance, but have very unusual spectral
lines. Eventually it was realized that quasar
spectra were normal, but enormously
redshifted:
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Solving the spectral
problem introduces a
new problem –
quasars must be
among the most
luminous objects in
the universe, to be
visible over such
enormous distances.
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Active galactic nuclei have some or all of
the following properties:
• high luminosity
• non-stellar energy emission
• variable energy output, indicating small
nucleus
• jets and other signs of explosive activity
• broad emission lines, indicating rapid
rotation
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Homing in on the Galactic Nucleus
•
•
•
•
The stellar Bar
Sagittarius A
The central star cluster
A massive Black Hole?
– Sag A*
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This is the leading theory
for the energy source in
an active galactic nucleus:
a black hole, surrounded
by an accretion disk. The
strong magnetic field
lines around the black
hole channel particles
into jets perpendicular to
the magnetic axis.
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Model Black Hole
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One might expect the
radiation from such a
powerful source to be mostly
X rays and gamma rays and
we often see quasars that
way, but apparently it is often
“reprocessed” in the dense
clouds around the black hole
and we see it reemitted at
longer wavelengths
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Particles in the jet will emit synchrotron
radiation as they spiral along the magnetic
field lines; this radiation is decidedly nonstellar.
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In an active galaxy, the central black
hole may be billions of solar masses.
The accretion disk is whole clouds of
interstellar gas and dust; they may
radiate away as much as 10–20% of
their mass before disappearing.
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End of Chapter 21
End of Unit 5
End of Astro 1040 and Phys
1040
Thank you for letting me be
your instructor