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Class 21 : Other galaxies
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The distance to other galaxies
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The velocities of galaxies
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Cepheid variable stars.
Other methods.
Doppler shifts.
Hubble’s law.
The morphology of galaxies
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Hubble Types.
I : Standard Candles
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How to measure distances in astronomy:
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Find an object whose luminosity you know.
Measure the energy (flux) that you observe.
Use the inverse square law to figure the
distance to the object…
Such an object (i.e., one with a known
luminosity) is called a “standard candle.”
Type Ia supernova in NGC4526 (HST pic)
II : Cepheid variables as
distance indicators
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Very important type of
star for measuring
distance.
Luminous variable stars.
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“Breathe” in and out.
Periodicities range from
3-30 days.
Period and luminosity
closely related!
Good standard candles.
Have to calibrate the
luminosity relation of
Cepheids using parallax to
nearby Cepheids.
From web site of
Davison Soper
(Univ. of Oregon)
Edwin Hubble
Hubble found Cepheids in M31
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Edwin Hubble
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Found a Cepheid in the
Andromeda nebula (M31).
M31 is 750 kpc (about 2
million light years) away.
Firm evidence for the
“island universes”
hypothesis… M31 is a
whole other galaxy.
The Andromeda galaxy (M31)
M100
Cepheid in the Virgo galaxy cluster with
Hubble Space Telescope (16 Mpc away…)
The Virgo cluster
Beyond the Cepheids…
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Hubble could only find Cepheids in the closest
few galaxies (1-2 Mpc).
Even Hubble Space Telescope cannot find
Cepheids beyond the Virgo cluster (16 Mpc).
Beyond 1-2Mpc, Hubble used…
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Brightest star method – identify the brightest
“star” in the galaxy and assume that it is the same
as the brightest star in nearby galaxies. BUT,
brightest object may not be a star at all!
Overall galactic apparent brightness method –
for distant galaxies, simply use overall brightness of
galaxy to gauge distance. Very error prone!
III : The Doppler effect & the
velocity of galaxies
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Think about sound waves…
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Let  = frequency (number of waves passing certain
fixed point in one second).
Let  = wavelength (distance between two “crests”
of the wave).
Let c = speed of the wave.
c = 
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Suppose source is moving towards you with
speed v…
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Waves get squeezed in direction of motion (i.e.,
wavelength decreases).
So, frequency must go up.
Same thing happens for light!
v
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Technique for measuring a galaxy’s velocity:
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Measure the spectrum of light from the galaxy.
Look for characteristic “fingerprint” in the spectrum
whose wavelengths are known from lab experiments.
Measure actual wavelength and interpret shift as due
to Doppler effect.
Example of a real galaxy spectrum…
Blueshifts and redshifts
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If galaxy is moving towards us, wavelengths are
shortened  spectrum blueshifted.
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If galaxy is receding from us, wavelenths are lengthened
 spectrum redshifted.
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Slipher measured velocities of nearby galaxies – by
1922, he found that 36 out of 41 were moving away
from us!
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[The first hint of Hubble’s remarkable result…]
IV: Hubble’s Law
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Hubble measured distance and plotted it
against velocity…
Latest data (from SNe Type Ia)
Everything’s rushing away!
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Hubble found that all distant galaxies are
rushing away from us!
Found that speed of recession is proportional
to distance of galaxy (Hubble’s law):
v = H0d
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H0 is called Hubble’s constant.
Modern measurements : H0 = 65 km/s/Mpc.
Can be used to determine distance to galaxies
that are very far away… (just need spectrum.)
V : Morphology of galaxies
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Spiral galaxies…
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Disk of younger stars (+ spiral structure).
 Color of disk can be dominated by massive/hot
stars…makes disk look blue!
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Bulge of older stars.
 All massive/hot stars have died.
 Only cooler stars still live.
 So bulge looks red.
M51
Credit : Tony and Daphne Hallas
M81
NGC891
NGC4622
NGC4414
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Elliptical galaxies…
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Featureless, often elongated ball of stars.
Very similar to bulge component of spirals.
M87
M87
… finally, irregular galaxies
LMC
The Hubble sequence…
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Why is there a variety of morphologies?
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Nature vs. nurture …
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Are ellipticals & spirals “born” different?
Do spirals turn into ellipticals or vice versa?
Current thinking…
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Galaxy collisions are basic driving force for
galaxy structure…