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 pre- to 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 ε by Proton-Proton Chain stars: ε ∝T4
In high mass ε by CNO Cycle stars: 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)
-4 6 L: 5x10 L⊙ ➔ 1x10 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 Protostar A MS Star F Old Fogie G K M 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?
Te , L, R increase Globular Cluster (slowly) M3 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)
t ~ 9.8 Gyr G2 V ms
Globular Cluster M3 Evolution of the Sun (1 M⊙, low mass star)
tms ~ 9.8 Gyr
•Core Hydrogen exhausted, G2 V core begins to collapse: ρc ↑, T c ↑, 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? εshell > εcore !
Where does εshell go? 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
8 •Tshell reaches 10 K; Helium Flash (L~1011Lsun in a few seconds!); He burning begins via triple- alpha 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)
t ~ 9.8 Gyr G2 V ms
Globular Cluster M3 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 -6 -4 (10 M⊙/yr, evolving to10 M⊙/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)
•Similar to Solar Mass but Late B MUCH shorter timescale: (B5V) tms ~ 93 Myr (vs 9.8Gyr)
tshellburning ~ 2.3 Myr (vs. 2.4 Gyr)
-He fusion starts before core Globular Cluster becomes degenerate
M3 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 -6 -4 (10 M⊙/yr, evolving to10 M⊙/yr) Pulsations
Radiation pressure from stellar wind pushes envelope out - Poof! Planetary Nebula
Exposed stellar core is white dwarf