If a galaxy has some stars in ordered orbits
Todd Thompson
Department of Astronomy
The Ohio State University Structure of galaxies
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
Central Supermassive Black Holes
What are Galaxies?
A Galaxy is a large assembly of stars, gas, and dust held together by gravity.
- Largest have ~1 Trillion stars (or more)
- Smallest have only ~10 Million stars
For reference, this is comparable to the total number of OREO tm cookies baked since 1912; 362 Billion and counting. Kind of gives new meaning to the phrase "astronomical numbers".
Andromeda (M31)
- 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: Disk: Extended, thin disk of stars, gas, & dust Crossed by spiral arms of blue stars & dust.
Spheroid: 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.
- The disk looks blue (lots of hot stars)
- The spheroid looks red (mostly old stars)
- Could detect many individual stars in both
- Disk stars had H-R diagrams like open clusters
- Spheroid stars had H-R diagrams like globular clusters.
Stellar Ages (Revisited)
- Massive main-sequence stars must be young.
- Low-mass M-S stars can be young or old.
- Young Clusters: blue main-sequence stars
- Old Clusters: no blue main-sequence stars
"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
Location: The Disk & in Open Clusters
Age: Mix of young and old stars
- 70% Hydrogen
- 28% Helium
- ~2% "metals"
Environment: Often very gas rich, especially for the young stars.
Population II
Location: The Spheroid & in Globular Clusters
Ages: Oldest stars, >10 Gyr
- 75% Hydrogen
- 24.99% Helium
- ~0.01% metals
Environment: Gas poor, no star formation
Stellar Orbits
The two stellar populations are also distinguished by how they orbit around the centers of their galaxies:
- Ordered, roughly circular orbits in a plane.
- All orbit in the same general direction.
- Orbit speeds similar at a given radius.
- Disordered, elliptical orbits at all inclinations.
- Mix of prograde and retrograde orbits
- Wide ranges of orbital speeds.
Contrast & Compare
- Disk & Open Clusters
- Young & Old Stars
- Metal-rich
- Blue M-S stars
- Ordered, circular orbits in a plane
- Gas-rich environment with recent star formation.
- Spheroid & Globular Clusters
- Oldest Stars
- Metal-poor
- No Blue M-S stars
- Random elliptical orbits in all directions.
- Little or no Gas & Dust, and no star formation.
Chemical Evolution
- Expect that successive generations will become increasingly metal rich.
Higher Metal Content in Later Generations.
Clues to Galaxy Formation?
- Fusion occurs in the deep interiors of stars.
- Except for CNO elements, surface composition remains effectively unchanged over a stars 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!
- Stars are orbiting about the centers of these galaxies at speeds much faster than expected from just the combined gravity of the stars present.
- There is also evidence of excess X-ray and radio emission from the very central regions.
- Milky Way: 3.7x10 6 Msun
- Andromeda: 1-2x10 8 Msun
Supermassive Black Holes
- Stellar-mass black holes are only expected to be a few 10s of solar masses in size