The Scale of The Universe How Big Is It Really?

Background:

Brief History of the Size of the Universe

• Planets thought to define solar system size
• Stars held on spherical shell(s)
• Kepler estimates size of the solar system
• Thomas Wright (1750) makes spherical shell model of the Galaxy
• Kant postulates Galaxy, other galaxies
• Messier sketches some nebulae
• Kant postulates nebular hypothesis for formation of solar system
• Herschel defines Galaxy size with star counts as well as our apparent position in it.
• Parsons resolves spiral nebulae (sees them break up into individual stars)
• Kapteyn models the Galaxy based on star counts and star brightnesses.
• Sir James Jeans theorizes that globular clusters are the precursors of spiral nebulae.
• Slipher measures radial velocities of spiral nebulae.
• Energy sources of stars still unknown
• Age of the Earth estimated to be 2 billion years.

Shapley's View of our Galaxy and the Universe in 1920

• Finds Cepheid distances to globular clusters
• Postulates globulars centered on Galaxy center (more of these tracers are seen in one direction of the sky suggesting the sun is can not be in the center of the galaxy)
• Postulates Galaxy 10 times larger than Kapteyn's model
• Postulates spiral nebulae gas clouds repelled by Galaxy
• Believes Van Maanen result of spiral nebulae rotation

Curtis's View of our Galaxy and the Universe in 1920

• Galaxy size similar to Kapteyn's: small and flattened
• Spiral nebulae galaxies just like our Galaxy:
  

• Didn't believe Shapley's Cepheid variable distances

  

Main Questions of the Debate:

• What is the nature of the ``nebulae''?

• What is the size of our Galaxy?

• Is the Sun in the center of the Galaxy?

Some ``Nebulae''

Andromeda Spiral ``Nebula'' (Messier 31)

Just fuzzy blobs to astronomers at turn of century...

Who are these Guys?

Harlow Shapley

Heber Doust Curtis

Shapley believed:

• diameter of our Galaxy was 300,000 light-years

• Sun was not at Galaxy's center, but 60,000 light-years away

• Milky Way was so large, it was the entire universe

• spiral nebulae were gaseous clouds repelled by Milky Way's light pressure

Curtis believed:

• diameter of our Galaxy was 30,000 light-years (10x smaller!)

• Sun was very close or at center of Galaxy

• spiral nebulae were galaxies -- island universes in their own right

To understand why, first need to understand Cepheid variable stars...

The period over which a Cepheid variable star fluctuates in observed brightness is related to its intrinsic brightness (or luminosity). Therefore, by measuring the period, you can determine the intrinsic brightness. By comparing the intrinsic brightness to the observed brightness, you can derive the distance to the star. Shapley used Cepheids/RR Lyraes to determine distances to clusters. Curtis did not believe that Cepheid's had a strong period-luminosity relation.

Plot of Time vs. Brightness

Shapley's arguments:

Sun far from center of Galaxy because

• globular clusters more concentrated towards one half of sky

• variable star distances implied globular clusters in concentrated half were further away

Milky Way large because

• variable star distances implied Galaxy was larger than previously supposed

Spiral nebulae within our Galaxy because

• great extent of Galaxy precluded spirals from being other galaxies

• van Maanen's proper motions implied spiral rotation speeds close to c (distances to nebulae must be small for implied rotation to be in physically acceptable range)

Curtis's arguments:

Sun at center of relatively small Milky Way because

• star count analysis and distance estimates from spectral types and intrinsic brightnesses of stars suggested smaller, heliocentric Galaxy

• did not believe that Cepheid variables were good distance indicators

Spiral nebulae are galaxies beyond our own because

• novae in Milky Way similar to novae in spiral nebulae, implying very distant nebulae

• Slipher's spectroscopic measurements of high recessional speeds for nebulae implied they were not Galactic objects (which would be moving much more slowly relative to Sun)

• apparent sizes of spiral nebulae consistent with his estimate for Milky Way when nebulae placed at extragalactic distances

• photographs revealed absorbing material in nebulae consistent with that he thought present at Galaxy's edge

• did not believe van Maanen's measurements

Edwin Hubble to the rescue...

at the 100in on Mount Wilson

identification of Cepheids in Andromeda

• located Cepheids in nearest major spiral nebula

• derived distances larger than even Shapley's estimate of Milky Way's size

• Shapley admits he was wrong about nebulae (an issue that he did not consider at the heart of the 1920 debate)

So, who won the debate?

• each correct on major point, incorrect on major point

Correct

• Shapley: Sun located well off center of Galaxy

• Shapley: Galaxy much bigger than previously thought (although he overestimated the size)

• Shapley: Cepheid variables are good distance indicators

• Curtis: spiral nebulae are external galaxies

• Curtis: van Maanen's results were hooey

Both were incorrect that interstellar absorption of starlight by dust was unimportant.

The picture of our Galaxy today...

bulge, thin disk, and spherical halo globular clusters reside mostly in halo disk consists of mostly young stars, gas, and dust spiral arms of disk are where most new stars form bulge is mostly older stars Milky Way is over 100,000 light-years across Sun is located in disk, one-half to two-thirds of distance from center

 

Our galaxy itself contains a hundred billion stars;
it's a hundred thousand light years side to side.
It bulges in the middle - sixteen thousand light years thick -,
but out by us it's just three thousand light years wide.
With thirty thousand light years from galactic centrepoint
we go round every two hundred million years -
and our galaxy is only one of millions, of billions,
in this amazing and expanding universe!
                   Eric Idle, The Galaxy Song ("The Meaning of Life")