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

Lecture 25: A Tale of Two Galaxies:
The Milky Way & Andromeda

Readings: Ch 25, section 25-2 & 25-6, see also Ch 21, section 21-4

Key Ideas

Disk & Spheroid Structure

Population I Stars
Young, metal-rich disk & Open Cluster stars
Ordered, nearly circular orbits in the disk

Population II Stars
Old, metal-poor spheroid & Globular Cluster stars
Disordered, elliptical orbits in all directions

Chemical Evolution

Central Supermassive Black Holes

What are Galaxies?

A Galaxy is a large assembly of stars, gas, and dust held together by gravity.


Our own galaxy, the Milky Way, and its neighbor, the Andromeda Galaxy, have about 200 Billion stars each, or about 400 Billion stars total.

For reference, this is comparable to the total number of OREOtm cookies baked since 1912; 362 Billion and counting. Kind of gives new meaning to the phrase "astronomical numbers"...

Andromeda (M31)

Nearest bright galaxy to the Milky Way:

Shares many similarities to the Milky Way

Andromeda gives us an approximate outside view of our own Galaxy.

Disk & Spheroid

Spiral galaxies have a disk/spheroid structure:
Extended, thin disk of stars, gas, & dust
Crossed by spiral arms of blue stars & dust.

Thick, centrally concentrated spheroid of stars
Little or no gas or dust

Walter Baade (c. 1944)

Walter Baade was a German-American astronomer working at the Mt. Wilson Observatory in the 1940s and 50s. During WWII, as a German immigrant, he was prohibited from doing any war work, so he spent his time using the 100-inch Telescope at Mt. Wilson while the Los Angeles area was blacked out.

This let him take the deepest red- & blue-light photos ever taken of the Andromeda Galaxy to that time.

Baade made H-R diagrams of disk & spheroid stars:

Led to a revolution in our understanding of stellar populations.

Stellar Ages (Revisited)

Massive Stars live Short Lives:

Star Cluster H-R Diagrams:

"Old" = 10 Gyr or more.

Stellar Populations

Baade divided stars into two "Populations":

Population I:
Disk and Open Cluster stars

Population II:
Spheroid and Globular Cluster stars

Distinguished by: Location, Age, & Chemical Composition

Population I

The Disk & in Open Clusters

Mix of young and old stars

Metal rich (roughly solar)

Often very gas rich, especially for the young stars.

Population II

The Spheroid & in Globular Clusters

Oldest stars, >10 Gyr

Metal Poor (0.1-1% solar)

Gas poor, no star formation

Stellar Orbits

The two stellar populations are also distinguished by how they orbit around the centers of their galaxies:

Pop I Disk Stars:

Pop II Spheroid Stars:

Contrast & Compare

Putting all of this information together, we can contrast and compare the properties of the two main population groups of stars in our Galaxy and Andromeda: Population I

Population II

When we study other galaxies at greater distances, we will look for these different populations and their relative importance.

Chemical Evolution

Metals form by fusion inside of massive stars The next generation of stars form out of the metal-enriched interstellar gas:

Higher Metal Content in Later Generations.

Clues to Galaxy Formation?

Chemical Evolution only affects populations. Once a star forms, its chemical composition is mostly fixed for life

Hearts of Darkness

Deep in the centers of the Milky Way and Andromeda we find supermassive black holes with masses of many millions of solar masses!

These have been found by the effects of their gravity on the innermost stars in these galaxies:

We can use the motions of the stars and the sizes of the regions in the center to estimate the masses of the central dark objects: Both are located in the dynamical center of their respective galaxies.

Supermassive Black Holes

Such black holes are extremely large: This raises some intersting questions:
  1. What are such large black holes doing at the centers of our Galaxy and Andromeda?

  2. How could such large black holes form?

  3. Do other galaxies harbor similarly large black holes in their centers?
We will see answers to some of these questions in subsequent lectures in this unit.
Return to [ Unit 4 Index | Astronomy 162 Main Page ]
Updated: 2006 February 5
Copyright Richard W. Pogge, All Rights Reserved.