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Galaxies
Chapter 23: Our Galaxy
A galaxy is a large collection of stellar and
interstellar matter
Isolated in space
All held together by gravity
The galaxy we live in the Milky Way
Galaxy
PHY 150
Dr. Akins
Shape
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Views of the MW
The Galaxy or MW, for short
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Shape, cont
In the late 1700’s
astronomers attempted to
determine the shape of
the galaxy
Herschels counted stars
Found approximately
equal numbers of stars in
all directions
Concluded we are at the
center of the galaxy
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Stars, gas, dust, neutron stars, black holes,
etc
Today astronomers
think the MW is
similar to the
Andromeda Galaxy
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Our View of the MW
Top is an artist’s
conception of the MW
Bottom is an optical
view
We see a faint band of
light across the sky
Remember, we are in
the galaxy
We can’t get a view of
the entire galaxy
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Variable Stars
Pulsating variable stars became important
to studying the Galaxy
Instability Strip
Became a new yardstick
RR Lyrae and Cepheid
Both have characteristic light curves
Pulsations are due to internal instabilities
Period is the time it takes to complete a
cycle of bright to faint to bright
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The pulsating stage
occurs when the star
has moved off the main
sequence
The star is internally
unstable
Cepheid variables are
high mass stars
RR Lyrae are low mass
stars that hit the bottom
edge of the strip
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Cepheids
RR Lyrae
Pulsate in distinctive
sawtooth pattern
Large variation in
periods
From 1 to 100 days
In the upper instability
strip of the HR
diagram
All have very similar
light curves
Periods range from
0.5 to 1 day
Lie at the bottom of
the instability strip
Ionized gas in their
atmosphere causes
the pulsations
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Measuring Distance with Variable
Stars
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Once identified as a Cepheid or RR Lyrae
variable, you can tell its luminosity
You can observe its apparent brightness
Form its luminosity, you can determine the
distance
Same rule we used for stars in previous
chapters
All RR Lyrae have
approximately the same
luminosity
Cepheids have
luminosities related to
their period
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Period Luminosity Relationship
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~100 x Sun
Long periods → high
luminosities
Short periods → low
luminosities
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Globular Clusters
Globular Cluster Distribution
Many RR Lyrae stars are found in globular
clusters
Using the variable stars, Shapley found
Globular clusters are at great distances from the Sun
The clusters’ three-dimensional distribution in space
is roughly spherical, with a 30 kpc diameter
Many thousands of parsecs
Not centered on the Sun
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Shapley’s Conclusions
Center of the Galaxy
Distribution of clusters maps out the extent
of the Galaxy
About 8 kpc from the Sun
In Sagittarius
Called the Galactic Center
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MW in Non-Visible Wavelengths
Distances in 1000’s of
pc
Gas and dust block
our view of the center
Also way astronomers
originally thought the
galaxy was much
smaller
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MW In Other Wavelengths
Visible light suffers interstellar extinction
Globular cluster
distribution centered
around Sagittarius
The center of the distribution was the
center of the Galaxy
Now we call it the halo
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Galactic nucleus is completely obscured from
our view
The longer the wavelength of the radiation
used to “see” into the galaxy, the farther
we can see
Longer wavelengths, less interstellar
extinction
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A Picture of the MW
Components of the Galaxy
Another view of the
structure of the galaxy
Two main
components
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Disk
Sphere
Each have different
characteristics
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Spherical Component
Halo
Scattered in a roughly spherical shape
around the center
Includes
Consists of thinly scattered stars and globular
clusters
Roughly spherical
Contains only old stars (Pop II)
No gas and dust
No current star formation
Stars have random orbits in three dimensions
Reddish in color
Halo
Nuclear bulge
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Contains both young and old stars
More old stars farther from the center
Football shaped
Little gas and dust, especially in outer regions
Possibly some ongoing star formation in inner
regions
Ring of gas and dust near center
Yellow-white/reddish-yellow
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None for last 10 billion years
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Disk Component
Dense cloud of stars surrounding the center of
the galaxy
Slightly flattened
Or at least very little
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Nuclear Bulge
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Contains all matter confined to the plane of
rotation
General characteristics
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Highly flattened
Contains both young and old stars
Contains gas and dust
Site of on-going star formation
Gas and stars move in circular orbits in the galactic
plane
Overall white, blue in the spiral arms
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Disk Component, cont
Diameter ~25 kpc
Part of the disk component
Long curves containing
Also seen as 75,000 ly
Spiral Arms
A bit smaller than the 25 kpc
Bright stars
Star clusters
Gas
Dust
Uncertainty
No sharp boundaries
Star density decreases
Different types of stars are located in different parts
Gas and dust block view
Affects both thickness and diameter estimates
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Star Orbits, Summary
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Orbit Comparison
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Stellar Populations, Summary
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21 cm Radiation
Neutral hydrogen emits 21 cm wavelength radiation
when its electron flips
Different spin relationships have different energies
When the electron moves to the lower energy state, the
photon given off has λ = 21 cm
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Radio wavelength
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21 cm Map
Spiral Arms
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The 21-cm radiation
also allows
astronomers to map
the spiral arms
Analysis of the
Doppler shifts of the
signals
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Spiral Arms, Map
Spiral Arms
Looking at various objects at various
wavelengths leads to
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Spiral Arms, final
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At least four major spiral arms
Several short arm segments
Bar instead of circular central bulge
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Artist’s Conception of the MW
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M83
Mass
Measured by studying the motion of gas
clouds and stars in the Galactic disk
Total mass (in solar masses) =
orbital size (AU)3 / orbital period (years)2
Kepler’s
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Third Law as modified by Newton
Using the information for our Sun, the
mass of the Galaxy is ~1011 solar masses
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But this is only the mass that is within the orbit
of the Sun
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Uncertainties in Mass
Measurement
Mass and Sun’s Orbit
Knowing actual distance from center to
Sun
Gives a lower limit since only the mass
inside the Sun’s orbit, but how to increase
to account for the rest?
Complex rotation of the galaxy
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Differential rotation
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Differential Rotation
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Mass, cont
The orbital speed of any object depends
on the mass inside its orbit
Therefore, we need to measure the orbital
speed of objects as far out to the edge of
the Galaxy as possible
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15 kpc is as far as we have measured so far
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Rotation Curve
Dark Matter
Note the rotation curve does not drop as it would
in Keplerian motion
If all the mass of the Galaxy was within the
visible part, the rotation curve should follow the
dashed line
However, the curve actually rises out to the ~50
kpc
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This is the distance that can be reliably measured
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Dark Matter, cont
Dark Halo
Mass calculates indicate there is ~9 x 1011
solar masses of mass contained within the
50 kpc
The region of a galaxy beyond the visible halo where
dark matter is believed to reside
Composition?
MACHOs
WIMPs
Other
Something else
This is three times more than was in the
visible part of the Galaxy
Dark matter is the general term given to
this mass we cannot see
Brown and black dwarfs, white dwarfs, ordinary matter that’s too
dim or small to see
Particles have not been confirmed experimentally
Neutrinos (with mass)
Or not really there, Newton’s Laws and General Relativity have to
be modified
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Not generally accepted, but being discussed
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Dark Halo
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“Seeing” Dark Matter
Can only be determined by its gravitational
effect on other matter
Rotation curves in galaxies
Gravitational Lensing
Temperature distributions
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Gravitational Lensing
Bullet Cluster
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Galactic Center
Colliding galaxy
clusters
Best evidence of dark
matter
The stars, gas, and
dark matter all
behave differently
This allows the
various types of
matter to be
distinguished
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View of the Center
There should be an area densely
populated with stars at the center of the
Galaxy
The interstellar medium blocks our view of
the center
The center of the Galaxy is in the direction
of Sagittarius
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More Views
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Near the Center
At the Center
Measurements indicate a large mass and
a small size
A supermassive black hole fits these
requirements
The object at the center of the Galaxy is
call Sgr A*
Bright radio sources
Filaments suggest a strong magnetic field
in the vicinity
Also many bright X-ray sources in the
region
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Evidence of Sgr A*
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Sag A*
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The Milky Way Galaxy
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Violent
High energy output
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The Milky Way Galaxy
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