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. The Milky Way: A Historical Perspective

Before 1920's, Milky Way was believed to be the entire Universe

... and only about 10 kpc x 2 kpc in size

... with the Sun at the center

Three kinds of nebulae:

``emission-line'' nebulae

planetary nebulae

spiral nebulae

-- all thought to be Galactic!

The ``Great Debate:'' Shapley vs. Curtis, 1920

Why so wrong?

No method to measure distances beyond parallax and spectroscopic parallax

What we know today:

- ``spiral nebulae'' lie outside the Milky Way Galaxy

They are galaxies like the Milky Way

- there are other types of extra-galactic ``nebulae'' that represent different types of galaxies

- the Milky Way is roughly 30 kpc in size,

has a flattened disk of young and old stars

a round, centrally located bulge, primarily of old stars,

and a halo of exclusively made of old stars.

- the Sun is about 2/3 the way out from the center of the Milky Way in the disk.

Orientation of Solar System: tilted!

North Celestial Pole vs. North Galactic Pole . . .

. . . off by 60 degrees

3. The distance ladder revisited: Out to Mpc Scales

How did we make this leap in our understanding?

RR Lyrae

- Relatively low-mass, evolved stars

- On the Horizontal Branch

- 10² as luminous as the Sun

- The Sun will be a RR Lyrae in about 5 billion years

- Period and luminosity uncorrelated (``horizontal'')

- But narrow range of luminosity (``horizontal'')

- Plentiful (there are many low-mass stars)

Cepheids

- Massive, extremely luminous evolved stars

- Tomorrow's super novae - Type II

- 10⁴ as luminous as the Sun

- pulsation period and luminosity strongly correlated

Used to map out the galaxy and beyond.

(a) telescopes were inadequate to observe them

(b) there was no available method to infer distances to these 'nebulae'

(c) these 'nebulae' had appearances identical to confirmed Galactic nebulae

(d) there was no philosophical precedence for believing in external galaxies

(e) The size of the Milky Way was unknown