1. Mass - Luminosity - Radius Relations (review)

For stars on the Main Sequence

In general, for stars at constant temperature:

Stars on the Main Sequence have a more complicated Luminosity - Radius relations.

2. Lifetimes (review)

For stars on the Main Sequence

Lifetime Mass/Luminosity Mass^-3

The Main Sequence is a mass sequence where...

hot stars live for only a short time
cools stars live a lot longer

What about stars off of the Main Sequence?

3. Examples of Samples (review)

In every-day life:

Volume limited -

- limited by a fixed distance for all objects

- ``distance limited''

Apparent-brightness limited -

- limited by a fixed apparent brightness for all objects

4. Star Clusters

Premise: All stars in cluster form at once

``laboratories'' for stellar evolution

``snap shots'' in time

open clusters vs. globuar clusters

young vs. old

``stellar populations''

How old are the oldest globulars?

10-15 Gyr!

Use to "date" the universe

5. The Interstellar Medium

Ambient conditions:

dusty and cool today, with scattered clouds and stars towards evening

diffuse neutral hydrogen

21-cm radiation:

spin-flip of electron within Hydrogen atom

dust (dark) clouds

cool: 100-300 degress K

sites of possible present & future star-formation

molecular clouds

cold! 10-50 degress K

sites of possible present & future star-formation

nebulae

a) emission-line nebulae

b) reflection nebulae

star formation sites; different views

Hot: 7000-10,000 degrees K

ionized gas, dust

diffuse ionized gas

hot! 10,000+ degrees K

energy source: stellar radiation, winds, and explosions cosmic rays

6. Formation

(1) kick-off

gravitational collapse

(2-3) fragmentation, and continued collapse

heating via release of gravitational energy

(4) densities rise in the core of the proto-stellar cloud

(5) T Tauri phase - stellar wind

(6) fusion starts

(7) stable burning: gravity and heat+density are in balance

How do we know the above is true?

Direct evidence?

or indirect?

smoking guns

smoldering embers

masked cocoons

(a) visible

(b) ultraviolet

(c) 21-cm

(d) x-ray

(e) infrared

(a) on the Main Sequence

(b) immediately when the interstellar cloud begins to collapse

(c) when it begins to collapse rapidly at constant temperature

(d) just prior to reaching the Main Sequence

(e) when the T Tauri winds occur