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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 23: The Milky Way

Readings: Ch 25, Section 25-1

Key Ideas

The Milky Way is our Galaxy
Diffuse band of light crossing the sky
Galileo: Milky Way consists of many faint stars

The Nature of the Milky Way
Philosophical Speculations: Wright & Kant
Star Counts: Herschels & Kapteyn
Globular Cluster Distribution: Shapley

The Milky Way

Diffuse band of light crossing the night sky.

All human cultures have named it:

Our words "Galaxy" and "Milky Way" are derived from Greek and Latin:

"The Starry Messenger"

Galileo (1610):
"For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters."
This was the first observation that showed that the Milky Way was simply made of many many unresolved faint stars.

Philosophical Interlude

Thomas Wright (1750):


Wright Milky Way
[Section of Wright's original woodcut]
From our location near the Sun:

A Theory of the Heavens

Immanuel Kant (1755):


Later became known as the "Island Universe" Hypothesis (term coined by Alexander von Humboldt, Kosmos, 1845).

The Herschels' Star Gages

William & Caroline Herschel (1785): Contrary to some accounts (including your textbook!), Herschel did not need to assume that all stars are the same luminosity for his Star Gage method to work. He did make this assumption in other work, however, hence the confusion by some (copying the same mistake from each other?).


Herschel Star-Gages Map of 1785
The Milky Way Map of William Herschel. Click to see a larger version.
Figure 4 from On the Construction of the Heavens by William Herschel, published in Philosophical Transactions of the Royal Society of London, Vol. 75 (1785), pp. 213-266. Image scanned by the author.

The Kapteyn Universe

Jacobus Kapteyn (1901 thru 1922):


Kapteyn Milky Way
Sketch based on Kapteyn's paper.

Harlow Shapley (1915 thru 1921)

Astronomer at the Harvard College Observatory who was working on the nature of Globular Clusters of Stars.

Shapley noted two facts about Globular Clusters:


Shapley's Globular Cluster Distribution
Graph based on Shapley's original 1918 globular cluster data. The Sun is located at the center of the axes as the yellow sun symbol, and the center of the Milky Way inferred from the distribution of globular clusters is marked by the red X at about 15kpc from the Sun. The gray horizontal band is a region of obscuration noted by Shapley.

The Greater Milky Way

Shapley's Results (1921):

Right basic result, but too big:

The Problem of Absorption

Absorption of Starlight by Interstellar Dust:

Absorption by Interstellar Dust affects all attempts to map the Milky Way:

The Milky Way

Our present view of the Milky Way today is largely Shapley's model, but corrected for the effects of interstellar absorption:
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Updated: 2007 March 6
Copyright © Richard W. Pogge, All Rights Reserved.