Astronomy 162: Lecture 24

Astronomy 162: Introduction to Stars, Galaxies, and Cosmology

Todd Thompson
Department of Astronomy
The Ohio State University

Lecture 24:

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.

Sizes:

Largest have ~1 Trillion stars (or more)
Smallest have only ~10 Million stars

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 OREO^tm 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:

Distance=720 kpc

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:

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.

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:

Disk stars had H-R diagrams like open clusters
Spheroid stars had H-R diagrams like globular clusters.

Led to a revolution in our understanding of stellar populations.

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.

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

Composition:

Metal rich (roughly solar)

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

Composition:

Metal Poor (0.1-1% solar)

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:

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: Population I

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.

Population II

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.

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

Supernova explosions enrich the interstellar medium with metals.

The next generation of stars form out of the metal-enriched interstellar gas:

Expect that successive generations will become increasingly metal rich.

Higher Metal Content in Later Generations.

Clues to Galaxy Formation?

Chemical Evolution only affects populations.

Fusion occurs in the deep interiors of stars.
Except for CNO elements, surface composition remains effectively unchanged over a stars life.

Once a star forms, its chemical composition is mostly fixed for 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!

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

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.

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:

Milky Way: 3.7x10⁶ M_sun
Andromeda: 1-2x10⁸ M_sun

Both are located in the dynamical center of their respective galaxies.

Supermassive Black Holes

Such black holes are extremely large:

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

This raises some intersting questions:

What are such large black holes doing at the centers of our Galaxy and Andromeda?
How could such large black holes form?
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.