Lecture 18
April 15, 2008
Milky Way Galaxy

Key Concepts:

What is the Milky Way galaxy made of?
Where is the Sun located inside the Milky Way galaxy?
What does the Milky Way look like at different wavelengths?
What are spiral arms? How do we know whether there are spiral arms in the MW?
How did the MW galaxy form?
How much does the MW galaxy weigh?
What is the future of the MW galaxy?

Zoology of Galaxies ("Hubble Types")

Elliptical vs Spiral/Disk Galaxy

Ellipticals: galaxies of general elliptical shape and smooth featureless appearance (E, SO, cD)

Spirals: galaxies with two or more arms winding out of the center (barred vs. unbarred)

Irregulars: galaxies of irregular appearance with random bright patches

Also depends on the observed wavelength (e.g. NGC 253 in optical vs. near infrared)

Milky Way: Your Perspective

We live in a disk galaxy -- from the band of star light called "Milky Way"

Herschel's Milky Way: W. Herschel (1738-1822) sketch based on number of stars seen in different directions. Correctly deduced that it is a flattened structure, but the Sun was thought to be in the center.
H. Shapley (1885-1972): we live on the edge of the MW -- from the asymmetric distribution of globular clusters (used as a standard candle)

Milky Way: a View from from a spaceship

Quiz18: If we lived in a solar system around a star in the Milky Way halo, what would our night sky look like?

Old and New Things in the Milky Way: Pop I & Pop II Stellar Populations

	Population I ("new")	Population II ("old")
Age	young to old (up to a few billion yrs)	old (~10 billion yrs)
Color	blue to red	red
Location	disk	bulge and halo
Orbit	circular, within the disk	plunging
Metallicity	high (like the Sun)	low (0.1 to 0.001 of the Sun)

Milky Way Formation and Evolution Scenarios

Quiz 18A

Multi-wavelength Views of the Milky Way: Contents and Structure

2MASS near-infrared image of the Milky Way (credit: UMass and NASA)

A Top View of a Spiral Galaxy M51 ("Star-Gas-Star Cycle")

cold gas and dust: dark dust lanes and sites of future star formation (also shocks)

young stars and HII regions: blue star clusters and ionized nebulae (red colored nebulae in this picture)
supernovae and hot bubbles: where young stars used to be and enriched gas being recycled

Face-on View and Spiral Structure

spiral arms are traced in:

cold gas (atomic and molecular gas) and dust
young blue stars and H II regions (e.g. Orion Nebular)

spiral winding problem: they are probably short-lived. [see a nice web discussion by G. Rieke]

spiral arms are spiral density waves. A spiral density wave is an apparent pattern and not a material wave.

Spiral density waves are driven by:

spontaneous formation (e.g., self-propagating star formation)
a tidal interaction (stirred from outside)
a stirring bar (stirred from inside)

mapping the spiral arms in the MW galaxy

a bar in the center of the MW galaxy

Mass of the Milky Way inside the Solar Circle

use Modified Kepler's Law (also review the "Orbital Velocity Law" M = aV²/G)
semi-major axis "a" is 8.5 kpc (about 2 billion AU)
period:
mass:

Quiz 18C: "missing mass problem" and need for dark matter

Cosmic Hide and Seek: the Search for the Missing Mass (by C. Miller)
What's the Matter with the Universe? (by R. J. Nemiroff)

Future of the Milky Way

Reading assignment for next lecture: Unit 74