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:

a Celestial River
a Celestial Road or Path

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

Greek: Galaxias kuklos = "Milky Band"
Latin: Via Lactea = "Road of Milk"

"The Starry Messenger"

Galileo (1610):

Galileo observed the Milky Way with his new telescope.
Published his findings in his pamphlet, Siderius Nuncius (The Starry Messenger)

"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):

Picture motivated by theological considerations
Wright made no new observations.

Model:

Milky Way is a thin spherical shell of stars.
The Sun is located inside the shell about midway between the inner and outer edges.

[Section of Wright's original woodcut]

From our location near the Sun:

Looking along the shell: See a broad band of stars ("Milky Way")
Look out along the thin part of the shell: See few stars

A Theory of the Heavens

Immanuel Kant (1755):

Misread a newspaper account of Wright's model.
Also made no observations of his own.

Model:

Lens-shaped disk of stars rotating about its center.
No special place for the Sun.
Other "nebulae" are distant, rotating milky ways like ours.

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

The Herschels' Star Gages

William & Caroline Herschel (1785):

Counted stars along 683 lines of sight using their 20-foot long, 19-inch diameter telescope.
Assumed that stars are uniformly distributed through space, and do not extend beyond the edges of the Milky Way.
Assumed that their telescope could resolve all stars within the boundaries of the Milky Way.

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?).

Model:

Flattened Milky Way ("grindstone")
Sun located very near the center.

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):

Used photographic star counts
Estimated distances statistically based on parallaxes & proper motions of nearby stars.
Neglected interstellar absorption of starlight (assumes that stars were faint only because they far away, not because interstellar absorption blocks some of the light).

Model:

Milky Way is a flattened disk ~15 kpc across & ~3 kpc thick
The Sun is located slightly off-center.

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:

They are uniformly distributed above & below the plane of the Milky Way.
They are concentrated on the sky toward the direction of the constellation of Sagittarius, where the Milky Way appears brightest.

Observations:

Estimated Globular Cluster distances from observations of their RR Lyrae stars and their magnitudes and diameters.
Used these distances to map the globular cluster distribution in space.

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):

Globular clusters form a subsystem centered on the Milky Way.
The Sun is ~15 kpc from the MW center.
MW is a flattened disk ~100 kpc across

Right basic result, but too big:

Shapley ignored interstellar absorption
Caused him to overestimate the distances.

The Problem of Absorption

Absorption of Starlight by Interstellar Dust:

Interstellar space is filled with gas and dust
This dust absorbs and scatters starlight passing through it,
This makes more distant objects look fainter than they otherwise would be if there were no interstellar dust.
If left unaccounted for, it can lead to serious overestimates of Luminosity Distances.

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

Shapley & Kapteyn thought it was smaller than it actually is, and so overestimated the size of the Milky Way.
Robert Trumpler (1930) showed that interstellar dust absorption was significant.

The Milky Way

Our present view of the Milky Way today is largely Shapley's model, but corrected for the effects of interstellar absorption:

A flattened disk of stars with a central bulge.

[Credit: Two Micron All Sky Survey Project]
The Sun is located 8 kpc away from the Galactic Center, which is in the direction of the constellation of Sagittarius
The disk is ~30 kpc in diameter and ~1 kpc thick at the location of the Sun.

[Credit: Spitzer Science Center Illustration]
The Galactic Center and much of the disk of our Galaxy is obscured by interstellar dust in the plane of the Galaxy.

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