Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 25: A Tale of Two Galaxies:
Readings: Ch 25, section 25-2 & 25-6, see also
Ch 21, section 21-4
The Milky Way & Andromeda
- 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.
- Largest have ~1 Trillion stars (or more)
- Smallest have only ~10 Million stars
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
Nearest bright galaxy to the Milky Way:
Shares many similarities to the Milky Way
- Both are Spiral Galaxies
- Both have similar stellar & gas content
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.
- The disk looks blue (lots of hot stars)
- The spheroid looks red (mostly old stars)
- Could detect many individual stars in both
Baade made H-R diagrams of disk & spheroid stars:
Led to a revolution in our understanding of stellar populations.
- Disk stars had H-R diagrams like open clusters
- Spheroid stars had H-R diagrams like globular clusters.
Stellar Ages (Revisited)
Massive Stars live Short Lives:
- Massive main-sequence stars must be young.
- Low-mass M-S stars can be young or old.
Star Cluster H-R Diagrams:
- Young Clusters: blue main-sequence stars
- Old Clusters: no blue main-sequence stars
"Old" = 10 Gyr or more.
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
- The Disk & in Open Clusters
- Mix of young and old stars
- Metal rich (roughly solar)
- 70% Hydrogen
- 28% Helium
- ~2% "metals"
- Often very gas rich, especially for the young stars.
- The Spheroid & in Globular Clusters
- Oldest stars, >10 Gyr
- Metal Poor (0.1-1% solar)
- 75% Hydrogen
- 24.99% Helium
- ~0.01% metals
- Gas poor, no star formation
The two stellar populations are also distinguished by how they
orbit around the centers of their galaxies:
Pop I Disk Stars:
- Ordered, roughly circular orbits in a plane.
- All orbit in the same general direction.
- Orbit speeds similar at a given radius.
Pop II Spheroid Stars:
- Disordered, elliptical orbits at all inclinations.
- Mix of prograde and retrograde orbits
- Wide ranges of orbital speeds.
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:
- Disk & Open Clusters
- Young & Old Stars
- Blue M-S stars
- Ordered, circular orbits in a plane
- Gas-rich environment with recent star formation.
When we study other galaxies at greater distances, we will look for
these different populations and their relative importance.
- Spheroid & Globular Clusters
- Oldest Stars
- No Blue M-S stars
- Random elliptical orbits in all directions.
- Little or no Gas & Dust, and no star formation.
Metals form by fusion inside of massive stars
The next generation of stars form out of the metal-enriched
- Supernova explosions enrich the interstellar medium with metals.
- Expect that successive generations will become
increasingly metal rich.
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
- Fusion occurs in the deep interiors of stars.
- Except for CNO elements, surface composition remains effectively
unchanged over a stars life.
- Metal content gives us a clue to the formation history of populations
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:
- Stars are orbiting about the centers of these galaxies at speeds
much faster than expected from just the combined gravity of the
- There is also evidence of excess X-ray and radio emission from the very
Both are located in the dynamical center of their respective
- Milky Way: 3.7x106 Msun
- Andromeda: 1-2x108 Msun
Supermassive Black Holes
Such black holes are extremely large:
This raises some intersting questions:
We will see answers to some of these questions in subsequent lectures in
- What are such large black holes doing at the centers of our Galaxy
- How could such large black holes form?
- Do other galaxies harbor similarly large black holes in their centers?
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Updated: 2006 February 5
Copyright © Richard W. Pogge, All Rights Reserved.