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Ast 228 - 4/13 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.
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