If a galaxy has some stars in ordered orbits

Todd Thompson
Department of Astronomy
The Ohio State University Structure of galaxies

Key Ideas

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?

• 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)

• Distance=720 kpc

• 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

Population I: Disk and Open Cluster stars

Population II: Spheroid and Globular Cluster stars

Distinguished by: Location, Age, & Chemical Composition

Population I

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

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

• 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?

• 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 of stars.

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 M_sun
• Andromeda: 1-2x10 8 M_sun

Supermassive Black Holes

• Stellar-mass black holes are only expected to be a few 10s of solar masses in size