Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 24: "The Realm of the Nebulae"
Readings: Ch 26, section 26-1 & 26-2
- Nature of the "Spiral Nebulae"
- Island Universe Hypothesis (Kant)
- Nebular Hypothesis (Laplace)
- The Road to the Galaxies:
- Leavitt: Cepheid Period-Luminosity Relation
- Shapley-Curtis Debate (1920)
- Hubble: Cepheids in Andromeda
William Parsons, 3rd Earl of Rosse (c. 1845)
Built a 72-inch telescope known as the "Parsonstown Leviathan"
Discovered the "Spiral Nebulae"
- Appeared to be disks with a spiral pattern to them
- Some appeared edge-on disks bisected by dark bands
- His telescope could not resolve them into stars.
Island Universe Hypothesis
Kant's idea (1755) revived by Alexander von Humboldt (1845):
- The Spiral Nebulae are other Milky Ways (or galaxies) made
- Very distant and external to our Galaxy.
Big Picture: The Milky Way is one of many galaxies in a
vast Universe of Milky Ways
Revival of a Solar System model of Pierre Simone Laplace (1796):
- Spiral Nebulae are swirling gas clouds
- Nearby and internal to our Milky Way
- Might be forming solar systems
Big Picture: The Milky Way is the Universe.
The Great Debate
The problem hinges on the finding cosmic distances:
In the context of the two competing ideas,
- How big is the Milky Way?
- How distant are the Spiral Nebulae?
- Island Universe Hypothesis:
- Spiral Nebulae are much more distant than the "edge" of our
Galaxy, and so very large (as big as our Galaxy).
- Nebular Hypothesis:
- The Spiral Nebulae are nearby, thus inside our Galaxy and
and thus smaller than it.
Henrietta Swan Leavitt
One of Pickering's "computers" at Harvard.
Given the task of finding variable stars in photographs of the
By 1912 she found:
- Brighter variables had longer periods
- They fit onto a Period-Luminosity Relation
But, she had no luminosity calibration.
The Stepping Stone
Pickering kept Leavitt from following up her discovery of the
- Recognized Leavitt's variables as Cepheids.
- Did the necessary luminosity calibration.
Shapley (after 1915):
- Refined the calibration of the P-L Relation
- Used the RR Lyrae version to get distances to Globular Clusters
The Shapley-Curtis Debate (1920)
Debate on "The Scale of the Universe" sponsored by the
National Academy of the Sciences.
- Harlow Shapley: (Harvard)
- Defended his model for the Galaxy and the more "conventional"
- Heber Curtis: (Lick Observatory)
- Defended the Kapteyn Model and the alternative Island
The Battleground Questions:
The main issues debated were as a follows:
- What is the size of the Milky Way Galaxy?
- Kapteyn's star counts vs. Shapley's clusters.
- What is the distance to the Andromeda Nebula,
the largest of the Spiral Nebula?
- Tried to estimate using observations of "nova" outbursts
- What are the motions of the Spiral Nebulae?
- Proper Rotation vs. Radial Velocities
The Galaxy is 100 kpc across.
The 1885 "nova" in the Andromeda Nebula gave it a
luminosity distance of only 10 kpc:
- This is smaller than 100 kpc for the Galaxy, hence it is internal.
Van Manaan's "Proper Rotation" of M101:
- If very distant, it implies a rotation speed faster than the
speed of light!
Typical novae in Andromeda give it a distance of ~150 kpc:
- Size is ~10 kpc, like Kapteyn's Milky Way, hence it is external.
See dark obscuring bands in edge-on spirals
- Like what we see in the Milky Way
Spirals have large radial velocities:
- They would be able to escape the Milky Way
The outcome of the "Great Debate" was largely inconclusive:
- Shapley had better arguments, but in the end was wrong.
- Curtis was right, but his arguments were weak.
The issues preventing a conclusions were:
Until someone could measure the distance to Andromeda (or reproduce
van Manaan's observations), it was not possible to resolve the debate.
- Nobody had a good distance to the Andromeda Nebula.
- Nobody could reproduce van Manaan's proper rotation observations.
Hubble Ends the Debate
The resolution of the debate was provided by the work of
astronomer Edwin Powell Hubble in 1923:
Using Shapley's P-L relationship for Cepheids he got a luminosity
distance of 300 kpc, far bigger than the Milky Way according to
Shapley (or anybody else).
- He was using the new 100-inch telescope on Mt. Wilson in California.
- Photographic study found Cepheid Variables in the Andromeda Nebula
By 1925, Hubble had acquired more refined data:
This showed that the Andromeda Nebula was not in our Galaxy, but was in
fact about as big as the Milky Way!
- Got good observations of 10 Cepheid Variables in Andromeda
- Estimated a Distance of ~1000 kpc.
Hubble's observations conclusively ended the debate on the nature of the
The Realm of the Galaxies
Call the Milky Way "The Galaxy"
Spiral "Nebulae" now called Spiral Galaxies:
- Stellar systems like the Milky Way
- Typical sizes are 10s of kpc
- Typical distances are many Megaparsecs (Mpc)
The Universe suddenly became a much bigger place.
The title of this lecture, "Realm of the Nebulae", is borrowed from the
title of Edwin Hubble's classic 1936 book. Even today, many of the
fundamental questions raised in the book are still active areas of
extragalactic research. Aimed at an educated popular audience, it still
rewards reading today.
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Updated: 2006 February 5
Copyright © Richard W. Pogge, All Rights Reserved.