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Charles Hakes
Fort Lewis College
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Chapter 14
Variable Stars
The Milky Way
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Fort Lewis College
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Outline
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Variable Stars
The Milky Way
Dark Matter
Review Questions
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Fort Lewis College
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For waves, their velocity is the product of:
A) frequency times wavelength
B) period times energy
C) frequency times period
D) amplitude times wavelength
E) amplitude times frequency
Charles Hakes
Fort Lewis College
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For waves, their velocity is the product of:
A) frequency times wavelength
B) period times energy
C) frequency times period
D) amplitude times wavelength
E) amplitude times frequency
Charles Hakes
Fort Lewis College
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Which of these does not exist?
A) a six solar mass black hole
B) a million solar mass black hole
C) a 3.8 solar mass neutron star
D) a .06 solar mass brown dwarf
E) a 1.3 solar mass white dwarf
Charles Hakes
Fort Lewis College
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Which of these does not exist?
A) a six solar mass black hole
B) a million solar mass black hole
C) a 3.8 solar mass neutron star
D) a .06 solar mass brown dwarf
E) a 1.3 solar mass white dwarf
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Fort Lewis College
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Variable Star Observations
• Some stars appear to change in brightness
in a very regular way.
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Fort Lewis College
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Figure 14.4c
Variable Stars - Cepheid example
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Fort Lewis College
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Figure 14.4ab
Variable Stars
• RR Lyrae light
curve
• Cepheid light
curve
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Fort Lewis College
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Variable Star Observations
• RR Lyrae and Cepheid variables are types
of variable stars. The brightness varies in
a very regular way
• The stars can be recognized and identified
by observing the light variations
• http://www.astro.princeton.edu/~jhartman/M3_movies.html
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Fort Lewis College
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Figure 14.5
Variable Stars on
the H–R Diagram
• Once identified,
the luminosity
(absolute
magnitude) is
known.
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Fort Lewis College
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Figure 14.6
Period–Luminosity Plot
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Fort Lewis College
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Variable Star Observations
• Globular clusters contain many RR Lyrae
variables, so their distances can be
determined.
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Fort Lewis College
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Figure 14.8
Globular Cluster
Distribution
• The center of the
Milky Way can be
estimated by
observing the center
of all globular
clusters.
• Diameter of this halo
is ~30 kpc (kiloparsecs)
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Fort Lewis College
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Figure 14.9
Stellar Populations in our Galaxy
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Fort Lewis College
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Variable Star Observations
• Cepheid variables are so bright that they
can be seen in neighboring galaxies.
• We can therefore determine the distances
to those galaxies.
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Fort Lewis College
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Figure 14.7
Variable Stars on Distance Ladder
• Greater distances
can be determined
than typically
available through
spectroscopic
parallax, because
these variables
are so bright.
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Fort Lewis College
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You observe two Cepheid variable stars. Star A has a
period of 10 days. Star B has a period of 30 days.
Which is more luminous?
A) A
B) B
C) they are the same
D) not enough information
Charles Hakes
Fort Lewis College
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You observe two Cepheid variable stars. Star A has a
period of 10 days. Star B has a period of 30 days.
Which is more luminous?
A) A
B) B
C) they are the same
D) not enough information
Charles Hakes
Fort Lewis College
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You observe Cepheid stars in two different galaxies (A
and B). They have the same apparent brightness.
Star in galaxy A has a period of 10 days. Star in galaxy
B has a period of 30 days. Which galaxy is closer?
A) A
B) B
C) they are the same distance
D) not enough information
Charles Hakes
Fort Lewis College
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You observe Cepheid stars in two different galaxies (A
and B). They have the same apparent brightness.
Star in galaxy A has a period of 10 days. Star in galaxy
B has a period of 30 days. Which galaxy is closer?
A) A
B) B
C) they are the same distance
D) not enough information
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Fort Lewis College
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Mapping the Milky Way
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Fort Lewis College
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Chapter 14
Spiral Galaxy - NGC 4603
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Fort Lewis College
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Chapter 14
Spiral Galaxy - 7331
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Fort Lewis College
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Figure 14.1
Galactic Plane
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Fort Lewis College
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Figure 14.2a Spiral Galaxies - The Andromeda galaxy is the
Milky Way’s big sister. Distance ~800kpc
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Fort Lewis College
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Figure 14.2bc
Spiral Galaxies
• A view of spiral
galaxies from faceon and edge-on.
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Fort Lewis College
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Figure 14.3
Herschel’s Galaxy Model
• early attempt to map the galaxy by simply
counting stars in a given direction.
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Fort Lewis College
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Mapping the Milky Way
• Radio observations can determine
much of the structure and rotation
rates.
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Fort Lewis College
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Figure 14.10
Observations of the Galactic Disk
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Fort Lewis College
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Mapping the Milky Way
• Radio observations can determine
much of the structure and rotation
rates.
• Orderly rotation in the plane.
• Random orbits in the halo.
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Fort Lewis College
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Figure 14.12
Stellar Orbits in Our Galaxy
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Fort Lewis College
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Figure 14.13
Milky Way
Formation
• Recall solar
system formation.
• Halo objects
formed before the
gas and dust fell to
a plane.
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Fort Lewis College
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Mass of the Milky Way
• Recall Newton’s modification to
Kepler’s third law:
total mass (solar) =
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Fort Lewis College
orbit size (A.U.)3
orbit period (years)2
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Figure 14.18
Galaxy Rotation Curve
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Fort Lewis College
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Mass of the Milky Way
• There is apparently more mass than
can be seen.
• Unseen mass out to ~50 kpc.
• Recall radius of observable Milky Way
is ~15 kpc.
• Dark Matter
• Can detect gravitational effects
• Cannot detect any other way.
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Fort Lewis College
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Dark Matter
• Is not atomic or molecular clouds - we would
detect those using spectroscopy.
• Could be brown dwarfs or white dwarfs - very
difficult to see.
• MACHOs - MAssive Compact Halo Objects
• Could be exotic subatomic particles
• WIMPs - Weakly Interacting Massive Particles
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Fort Lewis College
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Fort Lewis College
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Figure 14.19
Gravitational
Lensing
Review Questions
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Fort Lewis College
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A star has an apparent magnitude of +1.0 and an
absolute magnitude of +1.0. If the distance between
Earth and the star increases, the apparent magnitude
would _____, and the absolute magnitude would _____.
A) increase; decrease
B) decrease; increase
C) increase; not change
D) decrease; not change
E) not change; increase
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Fort Lewis College
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A star has an apparent magnitude of +1.0 and an
absolute magnitude of +1.0. If the distance between
Earth and the star increases, the apparent magnitude
would _____, and the absolute magnitude would _____.
A) increase; decrease
B) decrease; increase
C) increase; not change
D) decrease; not change
E) not change; increase
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Fort Lewis College
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Using spectroscopic parallax, you find a star’s distance to
be 76 parsecs. You now find out that the star isn’t a main
sequence star, but is a red giant. Your distance estimate is
A) too large
B) too small
C) fine - no significant change in estimate is needed.
Charles Hakes
Fort Lewis College
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Using spectroscopic parallax, you find a star’s distance to
be 76 parsecs. You now find out that the star isn’t a main
sequence star, but is a red giant. Your distance estimate is
A) too large
B) too small
C) fine - no significant change in estimate is needed.
Charles Hakes
Fort Lewis College
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A star has apparent magnitude of +8.0 before it goes
nova and increases its luminosity by 10,000 times. Its
apparent magnitude after it goes nova is.
A) +8.0
B) +18.0
C) -8.0
D) -2.0
E) +3.0
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Fort Lewis College
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A star has apparent magnitude of +8.0 before it goes
nova and increases its luminosity by 10,000 times. Its
apparent magnitude after it goes nova is.
A) +8.0
B) +18.0
C) -8.0
D) -2.0
E) +3.0
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Fort Lewis College
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Which is correct?
1 - The new moon rises at noon.
2 - The first quarter moon rises at noon.
3 - The full moon rises at noon.
4 - The third quarter moon rises at noon.
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Fort Lewis College
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Which is correct?
1 - The new moon rises at noon.
2 - The first quarter moon rises at noon.
3 - The full moon rises at noon.
4 - The third quarter moon rises at noon.
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Fort Lewis College
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In Paris, France (50 degrees north latitude),
what is the longest day of the year?
1:
2:
3:
4:
March 21
June 21
September 21
December 21
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Fort Lewis College
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In Paris, France (50 degrees north latitude),
what is the longest day of the year?
1:
2:
3:
4:
March 21
June 21
September 21
December 21
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Paris, France?
1:
2:
3:
4:
Due east
North of east
South of east
Can’t tell with information given
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Paris, France?
1:
2:
3:
4:
Due east
North of east
South of east
Can’t tell with information given
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Fort Lewis College
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Review
• What was the most important thing you learned?
• Toast could become a black hole…
• With fusion, iron is a dead end.
• Elements heavier than iron require energy input for
creation.
• The sun was created from “enriched” interstellar dust
• neutron stars are about 20 km in diameter, and rotate
very quickly.
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Fort Lewis College
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Review
• What questions do you still have about
today’s topic?
• How do you detect a black hole?
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Fort Lewis College
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Review
• What was the most important thing you learned?
• If the surface gravity is so high that light can’t escape,
then it is a black hole.
• Any object has a radius that if it is compressed below that
radius, light cannot escape.
• Schwarzchild radius is 3km x M(solar)
• Center of the milky way could be a very massive black
hole
• We can see black holes when things accelerate and fall
into them.
• When charged particles start orbiting a black hole, they
radiate in x-rays.
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Fort Lewis College
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Review
• What was the most important thing you
didn’t learn?
• If the Earth fell into a black hole it would still
orbit the same
• ?
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Fort Lewis College
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Review
• What questions do you still have about
today’s topics?
• What is solar mass?
• Something with the same mass as the Sun.
• Is it just a myth that black holes suck stuff in?
• Gravitational force is the same, if distance and mass
is the same.
• If the sun were replaced by a black hole, the planets
would not fall in.
• How many supernovas explode each year?
• A supernova happens once per century – we are due.
• Can we send something into a black hole?
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Fort Lewis College
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Three Minute Paper
• Write 1-3 sentences.
• What was the most important thing
you learned today?
• What questions do you still have
about today’s topics?
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Fort Lewis College
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