1. Generations, Birth Rates, and Death Rates

Stars accumulate over time if . . .

. . . death rate is slower than birth rate.

Recall:

Low mass stars live `forever,' i.e. longer than current age of Universe

High mass stars live for almost no time at all
(a few to a few 10's of millions of years)

As a galaxy forms stars over its lifetime,

- low mass stars accumulate

- most prominent as red giants:

composed of stars of a variety of masses and ages;

the most massive are the youngest

At any given moment, hot blue stars are from the current generation of stars just born

Two key ingredients to determining a stellar population:

1. star formation rate:

total amount of mass converted into stars per unit time

2. mass function:

fraction of mass converted into stars of a given mass

2. How Are Ages Determined?

(1) The presence/absence of hot, luminous stars

(massive, Main Sequence stars)

Recall: these stars have short lifetimes

(2) The metalicity of low-mass stars

Recall: we are all made of reprocessed star-matter, including the sun;

Earlier generations of stars have less heavy elements.

Population I

young

metal rich

found in the disks of galaxies

Population II

old

metal poor

found in the bulges and halos of galaxies

and in globular clusters

3. Spiral Arms

Problem: Why don't spiral arms wind up and disappear?

Spiral arms are density waves in galactic disks, moving at different speeds than stars and gas.

Density waves travel

faster in outer regions,

slower in inner regions

relative to rotation of underlying disk.

Density waves move through the disk in a spiral pattern.

These "waves" produce star formation in their wake by collapsing gas

Spiral arms appear as regions of recent star formation in the disks of galaxies.

The waves are an 'instability' in the disk.

Imagine throwing a pebble into a pool of still water, producing ripples.

Next imagine the same, but with the water swirling.

You will get something like a spiral pattern

Neighboring satellite galaxies (like pebbles) may help

produce these instabilities.

4. The Smooth Component

With the exception of globular clusters,

old stars lie in a smooth distribution,

either in the disk or halo of our galaxy

(and other galaxies).

Why is the distribution smooth?

- Dust and gas are sticky (they "dissipate")

(dust and gas particles tend to collide and stick together)

- Stars are not sticky ("collisionless")

(stars rarely collide unless very close)

After star formation occurs in a "sticky" environment

i.e. clumped, and unsmooth distribution,

the stars diffuse, and "relax" into a smoother distribution,

i.e. the stars randomize their motions and positions.

5. Shapes and Stellar Populations in the Galaxy

Where would you expect to find RR Lyraes in the Milky Way?

Where would you expect to find Cepheids?

differentiation of stellar populations

Also, differentiation of structure:

Three distinct (but not separate!) components

.	stars (age, metallicity)	shape (spatial distribution)	kinematics
disk	.	.	.
(+ thick disk)	.	.	.
bulge	.	.	.
halo	.	.	.

Differences in age, spatial distribution, and kinematics

(a) age

(b) presence of red giants

(c) metalicity

(d) kinematics

(e) spatial distribution

(a) they hold the stars together in a spiral pattern.

(b) they shepherd the stars into spiral patterns.

(c) they have nothing to do with producing spiral arms.

(d) they induce star formation in spiral patterns.

(e) they transport bulge stars into the disk.