# PowerPoint Presentation - 8 - Stellar Evolution

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```					8 - Stellar Evolution
STELLAR STRUCTURE MODELS
Mathematical descriptions of what the stars should be like inside

Hydrostatic Equilibrium - balance of gravity & pressure
Thermal Equilibrium - balance of heat energy everywhere
Energy Transport (how does it get from the core to the surface?
•conduction - direct transfer through physical contact
•convection - transfer of energy through fluid motion
Equation of State - relates pressure, density, temperature
Opacity - how photon transfer are affected by the gas
Energy Generation - energy production vs. temperature, density, etc.
“Boundary Conditions” - L, M, R, etc. to look like a real star!
(this slide should look familiar!)
Consequence of short lifetimes of massive stars:
We should expect
any very massive
star to be young, and
found near its place
of formation.
For a group of stars formed at the same approximate time,
the more luminous ones should evolve faster than the less-
luminous ones.
Stellar Evolution:
1 solar mass star

Zero-Age Main
Sequence (ZAMS)
ZAMS                    Star “burns” (fuses)
H  He in its core
Low-mass Stars:
•Convective envelopes
Since reaching the
Main Sequence, H
has been depleted
in the core, while He
has been built up
there

We do not see this
on the surface!
After H fuel used up in
core, “shell H-burning”
begins
H-burning shell
narrows, star
leaves MS
Stellar Evolution:
5 solar mass star

Core contraction
phase & H shell
burning
Massive Stars:
•Convective cores
H-burning shell
narrows…
Evolutionary
Tracks
Isochrone
A F
Young  Old

“Absolute
Magnitude” MV
is L (Watts)
expressed as a
magnitude
(specifically
MV=mV for
d=10 pc)
“to flash, or not to flash….”

•Lower-mass stars - He-ignition begins in “electron
degenerate core” - affects “equation of state”
•Heating does not expand the gas in the core, merely
raises T, increasing reaction rates
•Explosive reaction ensues - “He (core) Flash” - core
briefly reaches ~1011 Lsun??
•Degeneracy removed and “normal” He-burning follows
•Higher mass stars - He-burning begins in non-
degenerate core
“Asymptotic Giant Branch” (AGB)
“Horizontal Branch”

4He12C   (and 16O)
4He12C   (and 16O)
Instability Strip (Cepheids & RR Lyrae Stars)
Period-Luminosity Relation for
Cepheid Variable Stars

Average                period
brightness &
distance 
average L

Type I -                                    Type II -
solar-like                                  “metal poor”
abundances                                  compared to
Sun
Thermal Pulsing
Dredge-up of C,O
via convection
Superwind & shell
Mass in H-                        ejection
rich envelope

“dead” core becomes white dwarf star

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 views: 5 posted: 1/27/2012 language: English pages: 21