Astronomy 162: Lecture 23

Astronomy 162: Introduction to Stars, Galaxies, and Cosmology

Todd Thompson
Department of Astronomy
The Ohio State University

Lecture 23: "Nebluae"

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"

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 of stars.
Very distant and external to our Galaxy.

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):

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:

How big is the Milky Way?
How distant are the Spiral Nebulae?

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:

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 P-L Relation.

Hertzsprung (1913):

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.

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:

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!

Curtis' Arguments:

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

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:

Nobody had a good distance to the Andromeda Nebula.
Nobody could reproduce van Manaan's proper rotation observations.

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:

He was using the new 100-inch telescope on Mt. Wilson in California.
Photographic study found Cepheid Variables in the Andromeda Nebula

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:

Got good observations of 10 Cepheid Variables in Andromeda
Estimated a Distance of ~1000 kpc.

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:

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.