1. The importance of star clusters

Times scales for different stages of stellar evolution ...

How can we check?

(i) Observe HR diagrams for clusters

(ii) Measure relative density of stars at different stages

(iii) Combine information for many clusters of different ages

assumption:

The number of stars as a function of mass is constant over a small range of mass

So what are those ``blue stragglers?''

2. Binaries and Mass Transfer: Stellar Thieves

Mass from one star gravitationally 'stolen' by companion:

Nova:

surface-fusion of Hydrogen on a white dwarf

(non-destructive for white dwarf)

Super-nova:

catastrophic explosion

Nova vs. Super-nova:

- different light curves

- different amounts of energy released

3. High Mass Stellar Evolution

Short lives

Heavy element production in ``onion skin'' layers of star

- off the Main Sequence

The most massive stars ``burn'' up to iron in the core

Iron is the ``limit'' for exothermic fusion

- i.e., iron cannot fuse with anything to produce energy

- iron nuclei are the most stable of heavy elements

So what happens after the iron core is formed ?

uh oh ...

4. Core Collapse

. . . beyond white dwarf densities

Here's the scenario:

(a) Collapse heats core

(b) Nuclei are broken apart n, p⁺

+ all the e^- already there

= hot, dense particle soup

(c) electrons (e^-) more and more degenerate

(d) protons (p⁺) and neutrons (n) also degenerate,

- but not as much as e^-:

``Fermi Sea'' isn't as high

Why: They are not fundamental particles

More states per given energy level

(e) Electron ``Fermi Energy'' high enough for e^- to form heavier particles:

e^- + p⁺ n +

`` inverse beta decay ''

(f) e^- degeneracy decreases

(g) n degeneracy increases

(h) collapse comes to ``screeching'' halt

(i) core bounces

(j) shock wave rips apart outer layers of star

Super-nova explosion (Type II)

5. Super novae types

Two types:

Type I

observed: Hydrogen poor

white-dwarfs ``over the limit'' (mass limit)

explosive Carbon fusion in core

``Carbon Flash'' - a deflagration

entire star is ripped apart

Type II

observed: Hydrogen rich

massive stars with iron cores

core bounce

formation of a neutron star

Type I mass limit: white dwarf ``critical mass''

Nobel prize winner

Died: 1995

Over this mass, white-dwarf core collapses;

electron degeneracy pressure overwhelmed by gravity.

(a) the electron Fermi Energy becomes high enough for electrons and protons to form neutrons

(b) the neutron Fermi Sea compresses the electrons into the protons

(c) degeneracy is a state where one particle transforms into another particle

(d) the iron nuclei in the core interact with the electrons, and transform them into neutrons

(e) as the iron nuclei are ripped apart, this forces the electrons to become more degenerate

(a) stellar progenitors

(b) composition

(c) light curves

(d) remnants

(e) brighter than a nova