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Galaxies
Island Universes
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Copyright – FORS1 VLTI, European Southern Observatory
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Galactic Distances
• How do we know the distance to objects in
space?
• Stellar parallax:
– Parallax of nearby stars relative to background
stars.
– Good out to ~500 pc.
• What about farther than that?
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Standard Candles
• “Standard Candles”
• If we know how bright something looks,
• And we know how bright it should be
(luminosity),
• Result  Distance
• We do this everyday
with size.
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m versus M
• If you know how luminous a star REALLY is and
how bright it looks from Earth, you can
determine how far away it must be to look that
faint.
• m – M give you distance.
 distance
m  M  5log 10 
 10pc



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Example
Distance = 1000 pc
• Deneb is A2Ia star
m = 1.25
A2  Blue star
Ia  Supergiant
M = -8.8
 distance
m  M  5log 10 
 10pc
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


Standard Candles
• Other “Standard Candles”
• Variable stars.
• Stars that change in luminosity.
– RR Lyra stars
– Cepheid variables
• Supernovae
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Variable Stars
• For RR Lyrae stars:
– Average luminosity
is a standard candle
– Always ~ 100 x Sun
• For Cepheid
variables:
– Pulsation period is
proportional to
average luminosity
– Observe the period
 find the
luminosity
• Good to 15 Mpc!
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Variables in Clusters
M3
Copyright – K. Stanek (Harvard)
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Nearby Galaxies
•
•
•
•
•
Cepheids
Period
Luminosity  Mv
Know mv
Get Distance
 distance
m  M  5log 10 
 10pc



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How it works
•
•
•
•
•
•
•
•
For Cepheid in M100
P = 20 days.
From P-L: L = 10000 x Sun
Msun = 5, so MCep = -5
m = 20
m – M = 25
So 25/5 = 5 = log(d/10pc)
How log works:
– What is 100 = 10x?
– Same as saying 2 = log(100)
 distance
m  M  5log 10 
 10pc



• So 5 = log(d/10pc)
• d/10pc = 100000
• D = 1,000,000 pc
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Supernovae
• A special type of Supernova (Type 1a) seems
to be a good standard candle.
• If all Type 1a supernovae have same
maximum luminosity then look to see the
maximum apparent brightness from Earth
and get distance.
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M95 supernova – Copyright Adam Block, Mt. Lemmon SkyCenter
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The Local Group
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Groups
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The Virgo Cluster
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Clusters
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Concept Test
• A standard candle can be any object (or class
of object) that:
a. Always has the same luminosity.
b. Has some means of knowing its luminosity
without first needing to know its distance.
c. Can vary in brightness (as long as it always has
the same average luminosity).
d. Has a known absolute magnitude.
e. Always gives off the same amount of energy,
regardless of distance from us.
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Distant Galaxies
• Can’t see individual stars.
• Supernovae rare.
• Can use nearby galaxies to get distances to
further galaxies.
• Distance ladder:
–
–
–
–
–
Parallax  nearby stars
Nearby stars  H-R diagram
H-R diagram  distant stars (variables)
Variable stars  nearby galaxies
Nearby galaxies  distant galaxies?
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21cm Radiation
• Neutral hydrogen (HI) gives off light, l = 21cm.
Milky Way HI emission – Copyright J. Dickey
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Extragalactic HI
• Observe HI in other galaxies.
• Measure wavelength of 21 cm radiation.
• Doppler Shift: Get velocity away from us.
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Hubble’s Law
Ho = 71 km/s/Mpc
velocity  distance
v  HoD
• Measure the velocity
of every galaxy.
• Nearly all are
redshifted.
• Use Cepheids to
measure distances to
nearby galaxies.
• Result: The faster it’s
moving, the farther
away it is.
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Map the Universe
• v = Ho D
• If you know Ho:
71 km/s/Mpc
• Measure v
• Get D
• Find:
Voids
Walls
Clusters
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140 Mpc
70 Mpc
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Concept Test
• Imagine that Cepheid variables were more
luminous than previously thought. As a
result, Hubble’s constant would be:
a. Smaller than previously thought.
b. Larger than previously thought.
c. Unchanged since we aren’t changing either the
velocity or position of the galaxy.
d. None of the above.
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Limits to Hubble’s Law
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•
•
•
Negative velocity?
Galaxy pairs?
Clusters?
Orbits?
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Homework #17
• For Monday Read: Bennett Ch 13
• Do Chapter 13 Quiz
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