Ast 228 - 4/13
-G & Mya on Molecule Formation (focus on T of early star
formation)
-What does SF look like on HRD
-How do L, T, R change
-What about mass/time?
-what governs
-Now on HRD...what governs? EQ of SS! How do T/l/Cahange
Star Formation Process: What does this look like on
the HRD diagram?
Cloud
Protostar
Disk
Star!
Pre-Main Sequence Evolution on the HRD
Hyashi Tracks (mass dependent)
What happens to Lower mass objects?
H-R Diagrams: NGC 2071
50 M-type Cluster Members
Median Age: 0.4±0.2 Myr
Median Age: <1 Myr
Mass Range: 0.02- 0.82 Mu,
8 BDs
Brown Dwarfs (BDs)
Low mass, low luminosity objects unable to
sustain nuclear hydrogen burning (M < 0.08 Mu )
GL 229A – 0.3-0.45Mu
GL 229B – >0.007Mu
(Leggett et al. 2002)
Brown Dwarfs (BDs)
Brightest when young; L,T decrease with time
“Brown dwarfs cool
like rocks.”
(Burrows et al. 1997)
Vogt-Russell Theorem
The structure and evolution of a star are uniquely
determined by the star’s mass and composition
Holds all of the way through from preto post- main sequence evolution!!!!!!!
Equations of Stellar Structure
Hydrostatic
Equilibrium
Mass Continuity
Energy
Generation
Energy Transport
(Convection)
What are the dependent/independent variables?
Equations of Stellar Structure - Energy Generation
Energy
Generation...
The structure and evolution of a star are uniquely
determined by the star’s mass and composition
In low mass
stars:
ε by Proton-Proton Chain
ε ∝T4
In high mass
stars:
ε by CNO Cycle
ε ∝T20
MS lifetimes ↓ with mass!!!
Equations of Stellar Structure - Energy Transport
Energy Transport
(2 ways)
κ - opacity
(Convection)
Radiation or Convection?
Depends on κ opacity.....
...opacity depends on T and ρ
...which depend on R and MASS!
The structure and evolution of a star are uniquely
determined by the star’s mass and composition
O
B
A
F
G
K M (L T)
Internal Structure by Mass
Energy Transport
(2 ways)
(Convection)
Depends on Mass!!
Results from Stellar Models:
Models predict M-L relation:
(Main Seq. stars ONLY)
L: 5x10-4 L⊙ ➔ 1x106 L⊙
(9 orders of mag!)
M: 0.1 M⊙ ➔ 100 M⊙
(3 orders of mag!)
How do we know the Stellar Structure Equations are
correct? By modeling stars & comparing to HRD!
Results from Stellar Models:
Models Predict Minimum and
Maximum mass for a
“normal” star!
Minimum Mass: ~0.072 M⊙
(Below, can’t burn H)
Maximum Mass: ~100 M⊙
(Above, unstable on order
of hours!)
How do we know the Stellar Structure Equations are
correct? By modeling stars & comparing to HRD!
Stellar Exceptions: Ultra-massive Stars!
Luminous Blue Variables (LBV) - Mass > 100 M⊙
Pistol Star:
K3-50 (UC HII Reg.)
200 M⊙ ??!
Stellar Exceptions: Population III Stars
The First Stars: NO Metal, Mass > 100-200 M⊙
(Simulated Image from WMAP)
Lifetime?
Imagine an alien from a distant galaxy shows
up to survey stellar masses in the MW - which
type of star is the alien most likely to see??
O
B
A
F
G
K
M
Protostar
MS Star
Old Fogie
Example: Globular Clusters in MW
M3
Main Sequence Evolution
What happens
to density,
composition,T
and L as star
evolves on MS?
M3
Main Sequence Evolution
What happens
to Te , L, R as
star evolves on
MS?
Globular Cluster
M3
Te , L, R increase
(slowly)
Late Stages of Stellar Evolution
What happens
to Te , L, and R,
as star evolves
off MS?
Globular Cluster
M3
Depends on Mass!
Evolution of the Sun (1 M⊙, low mass star)
(ttozams ~ 100 Myr)
G2 V
Globular Cluster
M3
tms ~ 9.8 Gyr
Evolution of the Sun (1 M⊙, low mass star)
tms ~ 9.8 Gyr
G2 V
•Core Hydrogen exhausted,
core begins to collapse:
ρc ↑, Tc ↑, εgrav ↑
-Gravitational Radiation
Tshell ↑, R* ↑ -- L* ↑, Te ↓
Radiates Gravitational Energy!
Star is now a Subgiant
Evolution of the Sun (1 M⊙, low mass star)
tshellburning ~ 2.4 Gyr
•H-Shell burning; shell is in
hydrostatic equilibrium
Tshell ↑, R* ↑ -- L* ↑, Te↓(slow)
ρc ↑ (partially DEGENERATE),
Tc ↑ (slow)
Star is now a Red Giant
What is Degeneracy?
How does εshell compare to εcore?
Where does εshell go?
εshell > εcore !
Etot = 1/2 U; K = -1/2 U
SG (Subgiant) Branch
(Toasty
Terrestrials!)
Evolution of the Sun (1 M⊙, low mass star)
tHeburning ~ 30 Myr
•Tshell reaches 108K; Helium Flash
(L~1011Lsun in a few seconds!);
He burning begins via triplealpha process; He → C & O
L* cst, Te ↑
ρc ↑ (degenerate)
Star is on the Horizontal
Branch (HB)
C & O core is Degenerate
tHBsun ~ 30 Myr
Horizontal Branch
Evolution of the Sun (1 M⊙, low mass star)
(ttozams ~ 100 Myr)
G2 V
Globular Cluster
M3
tms ~ 9.8 Gyr
HB (5 solar mass)
Evolution of the Sun (1 M⊙, low mass star)
AGB/Post AGB Stars
•Core He exhausted, H/He shell
burning causes star to expand
L* ↑, Te ↓
ρc degenerate
Star is on the Asymptotic Giant
Branch (AGB)
Star experiences Mass Loss
(10-6 M⊙/yr, evolving to10-4M⊙/yr)
Pulsations
AGB Stars Example - Mira
LPV - Long Period
Variable
Mira varies with P~331 days,
Delta M ~ 6 magnitudes
Delta R ~ 20%!
Delta Teff ~ 1900-2600 K
Galex - UV
Death of Low/Intermediate Mass
Stars
Poof!
•Outer layers expand into a shell -
Planetary Nebula
•DEGENERATE Carbon core cools
and becomes a White Dwarf
PRESSURE
Cat’s Eye Nebula
Stellar Evolution:
Solar Type Stars
Evolution of a 5 M⊙ star (intermediate mass)
Late B
(B5V)
•Similar to Solar Mass but
MUCH shorter timescale:
tms ~ 93 Myr
(vs 9.8Gyr)
tshellburning ~ 2.3 Myr
(vs. 2.4 Gyr)
Globular Cluster
M3
-He fusion starts before core
becomes degenerate
implication?
Evolution of a 5 M⊙ star (intermediate mass)
•Similar to Solar Mass but
MUCH shorter timescale:
tms ~ 93 Myr
(vs 9.8Gyr)
tshellburning ~ 2.3 Myr
(vs. 2.3 Gyr)
-He fusion starts before core
becomes degenerate
implication? NO He Flash
(limit is 2.25 solar masses)
Red Supergiant!
tHe ~ 100,000 y
Evolution of a 5 M⊙ star (intermediate mass)
•Similar to Solar Mass but
MUCH shorter timescale:
tms ~ 93 Myr
(vs 9.8Gyr)
tshellburning ~ 2.3 Myr
(vs. 2.3 Gyr)
Red Supergiant!
tHe ~ 100,000 y
tC < 100,000 y
Red Supergiants - Example: Betelgeuse
Red Supergiant!
(high mass 13-17 Msun!)
Intermediate Mass Variables: Cepheids
Bright Variable Stars Useful for
Distance Determinations!
(Henrietta Leavitt)
Death of Low/Intermediate Mass
Stars
Poof!
•Outer layers expand into a shell -
Planetary Nebula
•Carbon core cools and becomes a
White Dwarf
PRESSURE
From AGB to Planetary Nebulae/WD
AGB/Post AGB Stars
Star is on the Asymptotic Giant
Branch (AGB)
Star experiences Mass Loss
(10-6 M⊙/yr, evolving to10-4M⊙/yr)
Pulsations
Radiation pressure from stellar
wind pushes envelope out - Poof!
Planetary Nebula
Exposed stellar core is white
dwarf