The Milky Way and Beyond: Archives: Ch 21,22, 48 and 49, Secrets: Ch 38,43,53

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The Milky Way and Beyond Archives Ch 21,22, 48
and 49, Secrets Ch 38,43,53

• What is the Milky Way? What is its extent?
• What were the spiral nebulae?

Messiers 100 Objects not single stars or
planets nebulae (Ptolemy knew 5 cloudy
stars)
planetary nebulae
gaseous nebulae
spiral nebulae
star clusters
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Early Speculations on the Milky Way

• 1750 Thomas Wright Milky Way "an optical
  effect due to our immersion in what locally
  approximates to a flat layer of stars.
  speculates there may be others. Milky Way
  results from looking along vs. across a flattened
  disk
• 1755 Immanuel Kant suggested many of the
  nebulous objects (spiral nebulae) seen in
  telescopes were island universes i.e., other
  star-filled systems like the Milky Way, and he
  concluded they must be rotating to avoid
  collapsing

from Wrights Original Theory of the Universe
If spiral nebulae were systems of stars equal but
beyond the Milky Way then Lightunresolved
stars Alternatively they could be swirling solar
systems just forming inside our galaxy. In which
case Lightgas. Or unresolved star clusters in
MW. (Huggins showed in 1864 from spectra some
nebulae were just clouds of gas in the Milky Way.)
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Big Telescopes and Spiral Nebulae
1.3 meter diameter (50), 12 meter length built
by William Herschel 1789 with money from King
George for discovering Uranus. Hershel found
1000 nebulae
Photo of remains, 1839 (first glass plate photo)
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Spiral Nebulae Before Photography
Engravings from William Herschel, 1800
Spiral nebulae drawn by Lord Rosse, 1848
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Why Do you think Early Telescopes were limited?

1. weight of lens warped its shape
2. hard to grind shape accurately
3. bad locations for telescopes
4. lens materials warped by temperature changes
5. all of the above
6. none of the above

answer, e
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Spiral Nebulae Island Universes or Solar Systems?
Were the spiral nebulae twins of the MW or the
Solar System?
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As Wright suggested, one could easily imagine the
band of the Milky Way resulting from our being
embedded in one of the Spiral nebulae
Could MW be like this?
You are here
Herschel tried counting stars, distribution
suggests we are not at center of Milky Way
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Spiral Nebulae and Astrophotographyprovides
deeper images, but no easy resolution!
1889 Huggins, on Andromeda shows a planetary
system at a somewhat advanced stage of evolution
already several planets have been thrown off, and
the central gaseous mass has already condensed to
a moderate size--I.e., a proto-solar
system Agnes Clerke, astro-historian in 1900
No competent thinker with all of the evidence
could maintain any single nebula to be a starry
system of equal rank as the Milky Way
What do you see? Solar system at a stage we
never saw? or Milky Way from a perspective we
dont have?
1888 astro-photograph by I. Roberts
--by 1900, 120,000 spiral nebulae known (Keeler
at Lick). Enormous debate! The only sure way to
resolve the debate was to measure their
distances! But how?
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The Local Solar Neighborhood
Parallax limited range to a few parsecs and a
few hundred stars (photographic plates had
enabled even this) How to survey the
extent of the MW and location of spiral nebulae?
You are here
Problem of an ant who never left an ant hill
trying to understand the extent of world.
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How Do We Measure Vast Distances?
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Leavitt at HCO Discovers a Cosmic Yardstick

• computers--hired female assistants

Henrietta Swan Leavitt, 1868-1921
HCO Southern Station (Peru), photographic plates
of Magellanic clouds (which appeared to be
composed of stars)
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Giant Variable Stars in the Small Magellanic Cloud
Large
Small
1907 Leavitt noticed that the brightness of some
of the brightest stars in the SMC changed
cyclically over days, weeks. These variables,
called Cepheids (after ? Cephei), had another
property worthy of notice. The slower they
varied (i.e., longer Period), the brighter (on
average) they appeared. Since all must be at
about same distance (see right panel), it must be
that their luminosity depends on their period.
Huge discovery!! Cepheid Variables could be used
as a cosmic yardstick!
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Cepheid Variables
Longer Period
brighter
Shorter Period
luminosity
fainter
Harlow Shapley (Mt. Wilson) explained in 1914
that Cepheids were actually pulsating stars,
ballooning in and out. (Something gone awry
with delicate balance between gravity and
thermal pressure.) Spectra showed oscillating
doppler shift. Bigger/brighter ones oscillate
more slowly.
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Question
If the Ceheid brightness varies as middle panel,
radial velocity would look like
velocity
Blueshift Redshift
a)
Blueshift Redshift
velocity
b)
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Question If a Cepheid with the Same period as
another appears on average 4 times brighter, it
must be

• 1/4 as far b) 4 times farther
• c) 1/2 as far

answer c, b L/(4?d2) so b2/b14(d1/d2)2 so if
d11, d21/2
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Cepheids are Standard Candles
Faintness tells how far away a standard candle is
Same luminosity, different brightness so
different distance
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Apparent brightness, b
Recall Apparent brightness, b, is a stars
intrinsic luminosity L (energy/time, e.g. watts)
divided by the area it shines upon. ergs/(cm2
sec). (Also called flux, f)
b L/(4?d2) d distance to star
This is the inverse-square law of light.
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Inverse Square Law Cepheids
d(L/4?b)1/2
Infer Measure with telescope
Period tells this

• Distance brightness Luminosity
• 1 L/(4?12) L
• 2 L/(4?22) L
• 3 L/(4?32) L

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Period-Luminosity Relation
Leavitt found that a Cepheid period predicted its
luminosity
Modern Version
long Period
short Period
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• Analogy--if you saw a Christmas tree lit up from
  afar you could
• calibrate the relative luminosity of different
  lights---if you
• found that slower blinking lights were brighter,
  you would have
• a rule you might apply to gauge the relative
  distance of other Christmas trees

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Cepheids are Supergiants!
Hertzsprung and then Shapley calculated the true
luminosity of a dozen of the closest Cepheids in
the Milky Way (using a type of parallax).
Results Cepheids are bright Supergiants!! About
103 Lsun for short periods (Plt4 days) to 104
Lsun for long periods (Pgt40 days). A very
bright lighthouse!
Long P
Short P
Barnards star (0.0004 Lsun)
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Aside My Current Research, extending range of
parallax by Perpendicular Spatial Scanning
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Star Scans, X vs Y
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Astrometric Precision
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How Far Can we See A Cepheid?
Barnards Star, 0.0004 Lsun, D1.8 parsecs
Visible with Binoculars (roof telescope), (m9.6
mag),

• The Sun is 2500 times more luminous than
  Barnards star
• a Cepheid can be 10,000 more luminous than Sun
  so
• Lceph/LBarnard 2.5x107

So if a distant Cepheid looked the same as
Barnards star, I.e., bceph/ bbarnard1 then
DcephDbarnard(Lceph/Lbarnard)1/2
1.8pc500010 kiloparsec (104 pc). Thats how
far the Cepheid is.
1900s telescopes (bigger, 60 located on
mountaintops, photography), could see 10,000
times fainter than example, or 100x in distance
Cepheids could measure distances of 1
Megaparsec (106 parsecs)! A million times
further than parallax a trillion times distance
to Sun or 3.2 million light years!. Something
brighter than Cepheid could be seen farther
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Cepheids Calibrate Globular Clusters
If Cepheid variable, 104 Lsun
Then Globular Cluster106 Lsun
1914 Harlow Shapley began to observe Cepheids in
GC, found they were 100x more luminous so he
could see them 10 times farther. Assuming
they were all the same he could measure
their distance and their location in space.
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One Light Calibrates Another
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Shapley Locates Globulars throughout the Milky Way
Globular Clusters are easy to see, 100s.
Most of the brightest Non-star sources in the
sky are globulars.
Harlow Shapley 1885-1972
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Are we at center of our galaxy?

• The gas and dust in the disk of the galaxy absorb
  and scatter light. So, cant see stars very far
  along plane of Milky Way at visible wavelengths.

GCs
Limit of view
Limit of view
J. Kapteyn (1910) and others erroneously believed
we were near the center, based on counting stars
in different directions and that the Milky Way
was only about 10,000 pc in size. But is this
right? View limited! Other estimates of the size
of Milky Way Hertzsprung 1906 2000 pc Oliver
Walkey 1700 pc Eddington 1914 2000-3000 pc
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Analogy Which way is downtown? How Big
is the City?
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Map of the Globular Clusters--were not at the
Center!
2-D, Galactic Longitude and radius
The system of globular clusters projected on the
plane of the Galaxy. The galactic longitude is
indicated for every 30. The local
neighborhood is completely within the smallest
circle, which has a radius of 1,000 parsecs. The
larger circles have radii increasing by intervals
of 10,000 parsecs. The dotted line indicates the
suggested major axis of the system, and the
cross the adopted center.

• The general direction of the galactic center is
  clearly toward
• the dense star clouds of Sagittarius and
  Scorpio
• the position of globular clusters suggested
  that the actual
• diameter of the galactic system is of the order
  of 100,000 parsecs

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2-D, Galactic Longitude and Latitude
Galactic plane

• Direction of center of distribution is obvious!
• (Looks a little like earlier stars counts but now
  we know its complete because you can see the
  globular clusters above the plane and its dust)

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We are not at the center!

• So Harlow Shapley 1917 saw that globular
  clusters are concentrated in one direction.
  Assuming they orbit the center of the galaxy
  (i.e., their center of massMW center)
• gt derived distance to the center!

globular clusters
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Shapleys Universe
Shapleys Universe

• What went wrong with
• previous estimates?
• DUST!

Kapteyns Universe
Sun
10,000 pc
Cant see more than 2000 pc due to the effect of
dust, on this scale the disk looks pretty much
the same in both directions
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In 1921, Universe Milky Way Galaxy
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Infrared Light from Stars cuts through dust,
reveals center
Visible
infrared

• Infrared from COBE (DIRBE) 1 3 mm composite
• infrared star counts would have got the answer
  right!

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Dust Clouds and Dust Grains
Visible light
Absorbed
Dust Grains size visible wavelength
Infrared light
Passes through
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Actually, Shapley overestimated size by a factor
of 2 (because some extinction of Globular
Clusters too) but basic picture right. And what
of the spiral nebulae? Now that Milky Way was
so big, could they lay beyond it? Or was the
Milky Way the whole Universe? In 1921 the stage
was set for the Great Debate (rivaled only by
the Scopes Monkey Trial)
Modern Picture
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Apparent Magnitude scale
apparent Magnitude scale of brightness invented
by Greeks for eye (each magnitude is 2.512 times
brighter/fainter than last magnitude--see Richter
scale) M-2.5 log b/bVega
2.5125100
Barnards
Each increase by 5 magnitudes equals a decrease
in brightness by a factor of 100 and an increase
in distance by a factor of 10
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Midterm, Last Year
A-ish
B-ish
C-ish
D-ish
median
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Midterm, This Year
median
B-ish
A-ish
C-ish
The Herd
D-ish
Stragglers
Yikes!
Vulture Bait
For a specific question about grading, see Dan,
TAs If less than 50, please see me this week