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

Lecture 23: The Milky Way

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

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 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."

Philosophical Interlude

• 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]

• 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

• 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 Gauges

• Counted stars along 683 lines of sight using their 48-inch telescope.
• Assumed that all stars are the same luminosity, so the relative brightness give relative distance.
• Assumed that they could see all the way to the edges of the system.

• 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

• 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)

Shapley noted two facts about Globular Clusters:

• They were uniformly located above & below the Milky Way.
• They were concentrated on the sky toward Sagittarius.

Observations:

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

Sketch based on Shapley's original data, uncorrected for interstellar absorption. The Sun is located at the center of the axes (looking roughly side-on), and the center of the Milky Way inferred by Shapley is marked by the red X.

The Greater Milky Way

• Globular clusters form a subsystem centered on the Milky Way.
• The Sun is 16 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

• 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

• 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.