Chap 20 Stellar Evolution Evolution of LowMass Star
- Slides: 41
Chap. 20 Stellar Evolution
Evolution of Low-Mass Star : Main Sequence Evolution of stars from the main sequence to red giant Red Giant branch Subgiant branch Main sequence turnoff point
Evolution of Low-Mass Star : Main Sequence
Evolution of Low-Mass Star : Main Sequence Main-sequence 1. Main-sequence : steady core H burning by pp chain 2. Main-sequence lifetime : 90% of entire lifetime of star 3. Main-sequence evolution mass Mass L, Teff , t. MS (early spectral type stars) Mass L, Teff , t. MS (late spectral type stars) 4. Main-sequence evolution chemical composition Mass , metal abundance CNO cycle Mass , metal abundance pp chain
Evolution of Low-Mass Star : Main Sequence
Evolution of Low-Mass Star : Main Sequence 5. Upper main-sequence convective core, radiative surface Lower main-sequence radiative core, surface convection zone 6. Nuclear burning change of the composition of the star's interior (i. e. , H He) 7. End of the MS phase complete depletion of H in the core (~ 10% of total mass of the star) H core He core : gas pressure weaken slow He core contraction greater energy production by gravitational contraction L Main sequence Turnoff (MSTO)
Evolution of Low-Mass Star : Main Sequence Lower MS Upper MS
Evolution of Low-Mass Star Evolution of stars from the main sequence to red giant Main sequence turnoff point
Evolution of Low-Mass Star : Subgiant Branch 1. H shell burning : reactions go faster and produce more energy 2. He core collapse Tc , P(outer core) heats up the H envelope expanding (though core contract !) R , Teff L ~ costant. (Recall Stefan-Boltzman Eq !) Subgiant
Evolution of Low-Mass Star Evolution of stars from the main sequence to red giant Subgiant branch
Evolution of Low-Mass Star : Subgiant Branch
Evolution of Low-Mass Star : Redgiant Branch 1. As Tc L , R , Teff (red color) fully convective, small dense He core H burning shell 2. The outer portions of the star are expanding & cooling 3. Up to 100 solar radius, Teff ~ 3000 K, Tc ~ 50 million K
Evolution of Low-Mass Star : Redgiant Branch 4. mass loss during red-giant branch strong stellar wind 20 - 30 % of the original stellar mass (cf) core contraction until degenerate state
Red giant star expand
Evolution of Low-Mass Star Evolution of stars from the main sequence to red giant Red Giant branch
Evolution of Low-Mass Star (cf) Electron degenerate dense gas electrons in a neutral atom occupy certain allowed states Pauli exclusion principle no two electrons can be together in exactly the same energy state For very high density gas (> 108 kg/m 3) degenerate electron gas degenerate gas pressure independent of the temperature when electrons are degenerate, they conduct heat very efficiently, and temperature variations are quickly smoothed out degenerate cores have the same temperature throughout
Electron degenerate dense gas
Evolution of Low-Mass Star Helium Flash 1. Density of He core , Tc degenerate core electron degeneracy pressure P is independent T As Tc , P ~ constant core does not expand Tc ~ 108 K core He burning by 3 process unstable core (out of control) He flash : very short time (~ a few minutes) He core ignition
Evolution of Low-Mass Star Evolution of stars from the main sequence to red giant Helium Flash
Evolution of Low-Mass Star After Helium Flash 1. When the core temperature finally reaches about 350 million K, the electrons become nondegenerate expanding core nondegenerate stable He core in HR diagram : L running to Horizontal-Branch Star
Evolution of Low-Mass Star Horizontal-Branch (HB) Star stable He core burning (3 process) + H shell burning : 'He main-sequence'
Horizontal-Branch Star He core & H shell burning
Evolution of Low-Mass Star Helium Flash
Globular Cluster Asymptotic giant Red giants Horizontal branch Turnoff point Main Sequence M 13
Evolution of Low-Mass Star Asymptotic Giant Branch Star 3 process in He core central He consumed C core shrink C core + He shell + H shell envelope expands R , L * similar evolutionary track of red giant branch
Evolution of Low-Mass Star
Evolution of Low-Mass Star Main Sequence Asymptotic Giant Branch Horizontal Branch
Death of Low-Mass Star 1. Can C burning occurs? No ! the temperature never reaches the point of C burning (at least Tc ~ 6 108 K)
Death of Low-Mass Star: Planetary Nebular 2. Planetary Nebular inner C core highly compressed and dead outer He, H shell burning outer envelope expands and cool separated from the core thin shell (3 dimensional envelope) planetary nebular
Death of Low-Mass Star Planetary Nebular
Death of Low-Mass Star : White Dwarf 3. White Dwarf expanding envelope of planetary nebular more diffuse and cooler merging with interstellar space highly compressed small size core (earth size) shrinking and attain an enormous density white dwarf no energy source continue dime and cool No white dwarf could be stable against gravitational collapse if it exceeded the mass of 1. 4 - 1. 5 solar masses Chandrasekhar limit Subrahmanuan Chandrasekhar
Death of Low-Mass Star : White Dwarf hydrostatic equilibrium : degenerate electron pressure = gravity Degenerate electron pressure does not depend on the temperature, but only on the density High density by Gravitational collapse of WD depends on the mass the more massive the WD, the smaller its size
Death of Low-Mass Star : White Dwarf
Death of Low-Mass Star White Dwarf Red Giant Horizontal Branch Star Asymptotic Giant Branch Star Globular Cluster M 4 White Dwarf
Death of Low-Mass Star
Evolution of Close Binary-Star System 1. If the two stars of binary system are close gravitational interaction affect the stellar evolution feature 2. Roche lobes = “zone of influence” of gravitational force of binary system Lagrange point = a place where the gravitational pulls of the two stars exactly balance the rotation of the binary system the Roche lobes of the two stars meet at a Lagrange point
Evolution of Close Binary-Star System 3. Type of close binary system (1) Detached binary system (2) Semi-detached binary system (mass-transfer binary) (3) Contact binary system (common-envelope binary)
Nova In a binary system, one star is a normal star and the other is a white dwarf Matter accretes in a thin layer on the surface of the white dwarf and eventually ignites in a thermonuclear explosion
Globular Cluster Asymptotic giant Red giants Horizontal branch Turnoff point Main Sequence M 13
Cluster Evolution
Cluster Evolution
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- Chap a to z
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- Rivalry 1 chapter 6
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- Find the passage
- Swapping chap 21
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- Durbin chap