The Shape of the Universe by dfhdhdhdhjr


									The Shape of the Universe

           This logo denotes A102 appropriate
          The Progress of Distance
          Measurements (in Earth Radii)
           Ptolemy   Copernicus           Modern
Mercury    100       430 (.4 AU)          9,600 (.4 AU)
Venus      600       820 (.75 AU)         17,000 (.72 AU)
Earth      1,200     1,100 (1 AU)         23,500 (1 AU)
Mars       5,000     1,700 (1.5 AU)       35,700 (1.5 AU)
Jupiter    11,500    6,000 (5.5 AU)       122,000 (5.2 AU)
Saturn     17,000    10,500 (9.5 AU)      224,000 (9.5 AU)
Stars      20,000    too far to measure   6,300,000,000 to
                                          nearest star
       Starting backwards
To help with learning how our modern
ideas about the cosmos were formed, we’ll
see the ‘answer’ first and then see how
these ideas evolved
Therefore, before we delve into the
historical aspects, let’s study our galaxy:
the Milky Way.
           Quite visible

A long exposure
picture though a
telescope can get
you this, but the
Milky Way is
obvious if the sky is
dark enough.
If you could see it from afar…
Where the Sun is:
A spiral galaxy
100-500 billion stars
100,000 LY across the disk
Central bulge 15,000 LY thick
Halo out to 75 kpc ~ 470,000 LY across
Sun located 28,000 LY from nucleus
1 Rotation takes ~ 240-270Myr
Side View
View from above

            In the nucleus:
A supermassive black
hole, ~ 4 million solar
Likely that most
galaxies with bright
nucleii have a SMBH
Right: star movement
near the SMBH,
Consistently 0.5% of Galactic Mass
Galaxies were found to      A SMBH accounts for
rotate at speeds that       some of the speed, s, but
could no be accounted for   not all.
by stars alone. (See the
Doppler shift?)
Not seeing
Companions: LMC, GMC
We always tend to put ourselves in the
middle of things
Watch how this theme takes centuries to
          Nebulous Ideas

We’ve seen early ideas about the shape of the
Descartes’ Vortices circa 1670
The spill of stars is across the sky is quite
apparent to the naked eye, so the Milky Way
was well-known to the ancients
– Some Messier objects are even visible (just barely)
But with the introduction of telescopes many
other clumps of stars became obvious
– What were they?
– How far away?
– Are they all basically the same?
How would you differentiate these?
 Thomas Wright (circa 1750)
Thought of the
Universe as made up
of spheres, but not
with the Earth at the
He imagined we were
wedged between two
concentric spheres,
seeing the Milky Way
in the tangent plane
          Emmanuel Kant
Philosopher ~1755
Postulated that:
– The Galaxy (loosely, Greek for Milky Way)
  was a disk of stars that includes our Sun
– The Milky Way might not be unique; that is,
  many disk-like systems exist distributed
  throughout space
– These disk-like systems were 'island
  universes' and could be observed as faint
William Hershel, while “star gauging”, saw that
stars were not evenly distributed across the sky
and imagined the Galaxy to be this disk-shape,
– all stars have the same brightness
– the galaxy has a uniform density
– he can see to the edge
The dimensions of the galaxy were given in a
unit Herschel christened the siriometer, the
distance from the solar system to the star Sirius.
He estimated the galaxy to be 1,000 siriometers
in diameter and 100 siriometers thick. The actual
distance to Sirius was not known in Herschel’s
day, but the modern figure is 48 trillion miles or 8
light years.
Thus, according to Herschel, the Milky Way is
8,000 light years in diameter and 800 light years
thick, roughly 10% the value of modern
Herschel discovers that some double stars are
true star systems, with the stars revolving
around a common center of gravity
Thus, they must be at approximately the same
distance from us
Since most of these systems contain stars of
unequal brightness, not all stars have the same
luminosity—causes him to change his earlier
– Out with the lens or grindstone model
         Nebulous Nebulae
Herschel, as did Messier, catalogued many
nebulae, faint fuzzy patches, some of which
resolved into stars and some of which did not
Some like Kant thought that they might be
distant versions of the Milky Way
– Island universes, coined by Alexander von Humboldt
  in 1845)
– That designation figured large in the upcoming Great
Others like Pierre Laplace thought they might be
solar systems in formation
From all this incomplete data, three
      big questions emerge:
1. What is the shape of the Galaxy?
2. What is its size?
3. Are the other nebulae part of the Milky
   Way or separate from it?
  – Secondarily:
       What were these nebula?
       Did they move? How fast? Which way?
       How big?
     During the 19th Century
Parallax angles for other nearby stars were
found by Bessel and other observers.
– Alpha Centuri (the nearest at 1.3 parsecs or 4 light
– Vega
– Altair
From the now known distances, it was quickly
determined that 61 Cygni was significantly less
luminous than the Sun while Vega and Altair
were more luminous. Only Alpha Centuri had a
luminosity nearly the same as the sun.
          William Huggins
His application of
spectroscopy comes to
have a critical second use
towards the second half of
the 19th century:
spectroscopic parallax
The categorization of stars
turns out to reveal their
Annie Jump Cannon (1863-1941)

Attended Wellesley
– Studied physics and astronomy
  and learned to make
  spectroscopic measurements
1896: she became a member
of the group of women hired
by Harvard College
Observatory director Edward
Pickering to reduce data and
carry out astronomical
              Edward Pickering
 Directed the Harvard
 College Observatory for
 forty-two years
 Instituted the Henry
 Draper *Memorial
 Catalog in 1884 as a
 long-term project
  – Obtain optical spectra of as
    many stars as possible
  – Index and classify the stars
    by their spectra.

* Well-known amateur Astronomer
Pickering’s “Harem”
 AKA “computers”      AJC
     First System of Ordering
Pickering and
Fleming classified
stars in the 1890s
based on the
strength of H lines
 – A for strongest
   H lines
 – B for H plus He
 – C for more He,
     Cannon’s Improvement
By 1901she had
looked at the
spectra of ¼ million
stars (!) and
rearranged them
according to
redundancy and
adding subdivisions
Oh Be A Fine *Girl,
Kiss Me

*or Guy
         Cannon’s Canons
Published catalogs of variable stars (including
300 she discovered)
First recipient of an honorary doctorate from
– She earned a B.S. from Wellesley
First woman elected an officer of the American
Astronomical Society
Curator of Astronomical Photographs at Harvard
William C. Bond Astronomer at Harvard
      Cecilia Payne (1900-1979)
Studied with AJC
Her book, Stellar Atmospheres,
“undoubtedly the most brilliant
Ph.D. thesis ever written in
She used the new quantum
mechanical understanding of
atomic structure, elaborating on
Planck’s model, to show how and
why the spectral lines of the various
elements varied with respect to
spectral type
                 But the rub…
In the chapter entitled “The Relative Abundance of the
Elements”, CP could not account for the fact that, even though
H and He are most abundant in stars, they are rare on Earth
“If . . . the earth originated from the surface layers of the sun,
the percentage composition of the whole earth should resemble
the composition of the solar (and therefore of a typical stellar)
atmosphere. . . . Considering the possibility of atomic
segregation both in the earth and in the star, it appears likely
that the earth’s crust is representative of the stellar
This ultimately meant that the theory of solar system formation
of the time was incorrect
 Henry Norris Russell (1877-1922),
and Ejnar Hertzsprung (1873-1967)
 they derived
 (perfected) this
 diagram in the
 early 20th C.
 – See the
           Astrometrical CSI
Once a star's spectrum is
identified, the star can be
correctly placed on the H-
R diagram.
Knowing a star's proper
location on the H-R
diagram makes it
possible to determine its
intrinsic brightness
– Absolute magnitude
The inverse square law
then gives the distance,
out to 10,000 pc
Henrietta Leavitt and
Cepheid Variables
 Another of Pickering’s
 “a straight line can readily
 be drawn ... showing that
 there is a simple relation
 between the brightness of
 the variables and their
 Actually, all stars are
  – The Sun varies 0.07%
Cepheids vary
greatly, and their
output is closely
correlated with their
Using the inverse
square law,
Cepheids are also
good distance
– Standard Candles
The Inverse Square Law
With this technique she estimated the Small
Magellanic Cloud to be about 50,000 parsecs
away -- making it one of the most distant objects
known at the time (1912).
          Other observations
For some astronomers,
the spirals and nebulae
were new planetary
systems in formation
For others, they
exemplified what a
complex star system like
our own Milky Way might
look like if we could see it
from a great distance.

                               The Milky Way star system
                               Cornelius Easton (1900)
           The Stage is Set
Better stellar distancing and new observations
add fuel to what would become known as The
Great Debate:
– Is the Universe just the Milky Way? Or
– Is our galaxy just one of many island universes?
Two camps emerge, one out of the Mt. Wilson
(LA) observatory, the other out of the Lick
observatory (SJ)
Civil but intense
– And neither side had it exactly right!
Harlow Shapley (left) Mt. Wilson Observatory
     / Heber D. Curtis Lick Observatory
   HS                 Positions                        HC

The galaxy is approximately       The galaxy is approximately
300,000 light years in            30,000 light years in diameter
diameter and the sun is           and the sun is located near the
located far from the center.      center.
The spiral nebulae are            The spiral nebulae are “island
associated with the galaxy,       universes”, i.e., other galaxies
although outside the main         comparable in size to the Milky
body. The nature of the           Way.
spirals is not known, but is      The universe contains a large,
probably some combination         indeterminate, number of
of gas and faint stars.           galaxies spread out over a
The Milky Way and its “halo”      large, indeterminate volume of
of globular clusters and spiral   space.
nebulae is all there is to the
In 1915, Shapley used Cepheids contained in
globular clusters to estimate the distance to
each cluster
He calculated that M13 is about 30,000 pc away
    In 1916, Adriaan van Maanen (1884-1946)
    announced he had photographic proof of rotation in
    face-on spiral nebulae: T* = 85,000 years
    He calculated that if this spiral were millions of light
    years away and comparable to the size of the Milky
    Way, a point on the edge of this galaxy would be
    traveling at a speed greater than the speed of light


Actual T = ~200 million years
Shapley to van Maanen, 1916

“Congratulations on the nebulous
results! Between us we have put a crimp
in the island universes, it seems, -- you by
bringing the spirals in [i.e., closer] and I by
pushing the Galaxy out. We are indeed
clever, we are”
In 1885 there was a nova in the
Andromeda galaxy that almost reached
naked eye visibility. If M31 is really a
million light years away, to appear that
bright the peak luminosity of this single
star would have to equal the combined
luminosity of billions of suns, something
inconceivable (at the time)
Shapley’s three basic reasons for rejecting the theory
    that the spiral nebulae are “island universes”

The spiral nebulae are distributed uniformly about the
Milky Way with a very distinct “zone of avoidance”
containing no spiral nebulae running exactly along the
galactic equator.
 – Why would randomly distributed galaxies at vast distances
   always avoid only this specific region of the sky?
Spectroscopic studies showed conclusively that many
nebulae are gaseous
 – Even “stellar” nebulae have a gas component, suggesting that
   there’s a continuum from gas to stars
The enormous size of the Milky Way argues against the
island universe theory
Shapley’s Picture of the Universe
         On the other hand
1917: George Ritchey
photographed a nova
in the spiral nebula,
NGC 6946.
– NGC 6946 must be
  more than just a
  glowing cloud of gas
  and dust.
– This was seen in 2004
Must be a huge,
distant collection of
           Curtis argued:
A large number of novae have been
identified in spiral nebulae. This makes
sense if spiral nebulae are island
universes, but hard to explain if spirals are
part of the Milky Way. Why should this
region of the Milky Way have such a high
concentration of novae? Why not in the
galactic plane?
If the spirals are as close as Shapely claims
(20,000 light years for M31), then the large
number of novae observed there have very high
absolute magnitudes (very low absolute
luminosity), therefore much dimmer than novae
observed in the Milky Way. Why should these
objects and these objects alone have
extraordinarily dim novae?
On the other hand, if the spiral distance is more
like 5 times Shapley’s estimate, then the
absolute magnitudes correlate well with the
absolute magnitudes of local Milky Way novae.
If we assume the spirals are galaxies like the Milky Way,
we can use their observed properties as analogies for
our own galaxy. Many edge-on spirals, for example,
show dark bands of obscuring material along the major
axis of their disks. We see similar regions of obscuring
matter in the Milky Way. This offers an explanation for
why spirals are never seen in the Milky Way (the so-
called zone of avoidance) – they are blocked from view.
  Jacobus Cornelius Kapteyn
Worked on the photographic
plates of David Gill for 4
years to produce a catalog of
455,000 Southern stars
Plan of Selected Areas
– Hoped to organize Astronomers
  all over the world for a massive
  star catalog
Founded the productive
Dutch school of Astronomers
Found evidence that proper
motion was not random
– Two streams oppositely directed
– Led to discovery of galactic
Hale met Kapteyn at a conference
in 1904 and invited him to become
a research associate at the Mount
Wilson Observatory
Kapteyn launched a plan for a
major study of the distribution of
stars in the Galaxy, using counts
of stars in different directions
"First attempt at a theory of the
arrangement and motion of the
sidereal system" was published in       George Ellery Hale
His sidereal models worked for
high galactic latitudes but failed in
the disk because he didn’t
account for extinction
Kapteyn’s Island Universe
Harlow Shapley                         Shapley          Curtis

presented the case
                        Distance to    Cepheids in      how do we
for his model at a      globular       globular         know that
                        clusters?      clusters are     Cepheids in
meeting of the                         the same as      globular
                                       those nearby     clusters are
National Academy of                                     like those that
                                                        are nearby?
Sciences                What are the   rotation of      novae have
                        spirals?       spirals proves   been
Heber Curtis argued                    they are close   observed in
                                       they must be     some of these
for Kapteyn's "Island                  true nebulae     nebulae they
                                                        must be
Universe" model                                         collections of
                                                        stars like our
                                                        own system
            The tie-breaker
In 1923 Edwin Hubble
(1899-1953) used the
new 100-inch
telescope at the Mt.
Wilson observatory to
photograph M31.
For the first time ever,
individual stars could
be distinguished on
the photographic
Hubble scratched “N” on the plate, first believing
the Cepheid was a nova
His discovery of a Cepheid in M31 made it
possible to apply Henrietta Leavitt's period-
luminosity method and determine that M31 is
located at the remarkable distance of 1 million
light-years (300,000 parsecs)
For most Astronomers this settled the issue of
the spiral nebulae and the island universe
theory, and also gave a clue as to how really big
the cosmos is
     Who was right about what
HS                                                           HC
The galaxy is approximately       The galaxy is approximately
300,000 light years in            30,000 light years in diameter
diameter and the sun is           and the sun is located near the
located far from the center.      center.
The spiral nebulae are            The spiral nebulae are “island
associated with the galaxy,       universes”, i.e., other galaxies
although outside the main         comparable in size to the Milky
body. The nature of the           Way.
spirals is not known, but is      The universe contains a large,
probably some combination         indeterminate, number of
of gas and faint stars.           galaxies spread out over a
The Milky Way and its “halo”      large, indeterminate volume of
of globular clusters and spiral   space.
nebulae is all there is to the
But now new questions arose
Why are globular clusters so different from
other stars in a galaxy?
What is the significance of the different
shapes and structures of these distant star
Do galaxies evolve?
Why do distant galaxies appear to be
moving away from us?
Why do distant galaxies appear to
   be moving away from us?
A whole other can of worms
A whole other PPT!

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