THE STRUCTURE OF OUR GALAXY
A Great Debate: Shapley vs. Curtis and the Scale of the Universe (1920)
- What is the nature of the ``nebulae''?
- What is the size of our Galaxy?
- Is the Sun in the center of the Galaxy?
Just fuzzy blobs to astronomers at turn of century...
Andromeda Spiral ``Nebula'' (Messier 31)
Who are these Guys?
Heber Doust Curtis
To read their obituaries, click
- 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
- 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 fluctates 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
To look at a real movie of a Cepheid varying, click here.
To learn more about Cepheid variables, click here.
Sun far from center of Galaxy because
Milky Way large because
- globular clusters more concentrated towards one half of sky
- variable star distances implied globular clusters in concentrated half
were further away
Spiral nebulae within our Galaxy because
- variable star distances implied Galaxy was larger than previously supposed
- 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
Sun at center of relatively small Milky Way because
Spiral nebulae are galaxies beyond our own 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
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
Both were incorrect that interstellar absorption of starlight by
dust was unimportant.
- 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
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
but out by us it's just three thousand light years
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")
Our Galaxy in visible light (an edge-on disk as seen from Earth)
Our Galaxy at other wavelengths...
To read some more about the Shapley-Curtis Debate, click on the following: