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Galaxy NGC4414 from HST Astronomy 162:
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

Key Ideas

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

Spiral Nebulae

William Parsons, 3rd Earl of Rosse (c. 1845)

Built a 72-inch telescope known as the "Parsonstown Leviathan"

Discovered the "Spiral Nebulae"


Island Universe Hypothesis

Kant's idea (1755) revived by Alexander von Humboldt (1845):

Big Picture: The Milky Way is one of many galaxies in a vast Universe of Milky Ways


Nebular Hypothesis

Revival of a Solar System model of Pierre Simone Laplace (1796):

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,
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 Magellanic Clouds.

By 1912 she found:

But, she had no luminosity calibration.


The Stepping Stone

Pickering kept Leavitt from following up her discovery of the P-L Relation.

Hertzsprung (1913):

Shapley (after 1915):


The Shapley-Curtis Debate (1920)

Debate on "The Scale of the Universe" sponsored by the National Academy of the Sciences.

The Debaters:

Harlow Shapley: (Harvard)
Defended his model for the Galaxy and the more "conventional" Nebular Hypothesis.

Heber Curtis: (Lick Observatory)
Defended the Kapteyn Model and the alternative Island Universe Hypothesis.

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

Shapley's Arguments:

The Galaxy is 100 kpc across.

The 1885 "nova" in the Andromeda Nebula gave it a luminosity distance of only 10 kpc:

Van Manaan's "Proper Rotation" of M101:

Curtis' Arguments:

Typical novae in Andromeda give it a distance of ~150 kpc:

See dark obscuring bands in edge-on spirals

Spirals have large radial velocities:


The Outcome

The outcome of the "Great Debate" was largely inconclusive:

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.

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).

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!

Hubble's observations conclusively ended the debate on the nature of the Spiral Nebulae.


The Realm of the Galaxies

Call the Milky Way "The Galaxy"

Spiral "Nebulae" now called Spiral Galaxies:

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.