Sternentwicklung Stellar evolution Vorlesung Spätstadien der Sternentwicklung, WS 07/08 Lebzelter & Hron

Early concepts Lord Kelvin: source of solar energy is gravitational contraction. Age of the sun is 100 million years Charles Darwin: age of the earth is several billion years. Ernest Rutherford: Radium possible long time energy source O. Gingerich, 1999, Ap&SS 267, 3

Early concepts Star form out of meteoritic particles, become first red giants, contract (hot stars), and then cool down (red dwarfs). Lockyer (1890), Russell (1925)

Milestones Understanding of the stellar structure (Eddington) Understanding of the star‘s composition (Payne, Unsöld) Understanding of the energy source (Atkinson, Bethe, von Weizsäcker)

What is a star?

Energy Production

Nuclear Timescale 10% of the sun involved 0.7% of the mass of a hydrogen core will be converted into energy E = m c 2 t nuclear ~ years

core hydrogen burning

Effect of hydrogen burning 4 H transformed into 1 He mean molecular weight increases according to the ideal gas law density and T have to increase: core contracts  energy production increases and opacity decreases  L, R, T increase

Iben 1967

Core after H burning no H  no energy production energy production in shell, core becomes isothermic H burning shell  core increases in mass maximum core mass ~ 10% stellar mass  core collapse low mass stars: core degenerates first

Core and Envelope L bottom = L out  star is in TE L bottom increases  L out has to increase R increases  more surface  more L can be emitted  again TE R increases  T decreases  at some point opacity increases  NO TE  star becomes red giant (Hertzsprung-gap!) Runaway stops at Hayashi-track

TE Energy trapped no TE runaway phase convection Hayashi line TE

Iben 1967

First Dredge Up Convective zone reaches layers with processed material Abundance changes on the surface: 12 C decreasesLi decreases 14 N increases 3 He increases O ~ constant 12 C/ 13 C drops from 90 (solar) to ~20

Evolution on the RGB Luminosity provided almost exclusively by thin H burning shell (0.001 – M solar ) burning rate of H shell determined by size and mass of core  He core mass – luminosity relation

Helium core flash core T increases until He ignition temperature (10 8 K) – approx. 0.5 M solar core material degenerated  gas pressure not sensitive to T  no cooling by extension  thermonuclear runaway Duration approx. 1 Mio years Most of the E does not reach the surface not for stars above 2.25 M solar

Helium Burning

4 He + 4 He  8 Be 8 Be + 4 He  12 C Energy production  T 40 Energy release per nucleus one order of magnitude less than for H burning 12 C +   16 O +  16 O +   20 Ne + 

He exhausted  2nd ascent on the giant branch (Asymptotic Giant Branch, AGB)

Our Sun Aus Sackmann et al Birth 0 Milliarden Jahre Today 4.5 Billion years 10 Billion years Hydrogen exhausted 12 Billion years Helium ignition 12.4 Billion years Ejection of outer shell

CMD of Stellar Clusters

Isochrones Bertelli et al. 2000

Isochrones

Literature Salaris & Cassisi: Evolution of Stars and Stellar Populations Kippenhahn & Weigert: Stellar Structure and Evolution Renzini et al. 1992, ApJ 400, 280

Miras Innenleben

helium burning

Aus: James Kaler, Sterne

Beiträge zum ISM Sedlmayr 1994

Thermische Pulse PDCZ...Pulse driven convection zone

Thermische Pulse continuous line...surface luminositydashed line...H-burning luminosity dotted line...He-burning luminosityWood & Zarro 1981

Vassiliadis & Wood 1993

Wood & Zarro 1981

shell hydrogen burning