Life and Death of Stars I

1. Life and Death of Stars I Low Mass Stars Evolution Lancelot L. Kao

2. Outline • Main-Sequence Stars • Structure of Main-Sequence Stars • Star Formation • Stellar Evolution • Low-Mass Star Evolution • Our Sun Evolution • Planetary Nebula & White Dwarfs

3. Main-Sequence Stars • Mass-luminosity Relation • Sequence of Masses • Hydrostatic Equilibrium • Balance between weight (due to gravity) and gas pressure • Increase density and temperature inward • Energy Source • Thermonuclear fusion of hydrogen nuclei to form helium nuclei in the core region • Temperature exceeding 8 million Kelvin • Mass Range • Low-mass MS star 0.08 – 3 solar masses • Intermediate-mass MS star 3 – 8 solar masses • High-mass MS star 8 – 100 solar masses

4. Structure of Main-Sequence Stars

5. Star Formation • Giant molecular cloud • Dense cold cores (~10 K) • Gravitational contraction • Accretion disk formation • Central core growth (size and temperature) • Protostar • T-Tauri phase/stage • Zero Age Main-Sequence star • Trigger mechanism

6. Star Formation

7. Star Formation

8. Star Formation ^ Bipolar Outflow and Herbig-Haro Objects (HH1 & HH2)  Infrared image of the newborn stars inside the Orion Nebula (~0.1 pc in size).

9. Star Formation

10. Star Formation

11. Star Formation Star Cluster NGC 2264

12. Star Formation Pleiades Evolutionary Track to Main-Sequence

13. Stellar Evolution • What drives stellar evolution? • No longer in hydrostatic equilibrium • Main-Sequence Lifetime • Hydrogen-burning timescale • Nuclear physics + stellar model • Basic recipe • Change in energy source(s) • Change in gas pressure • Change in stellar structure • Synthesis of chemical elements • Mass loss and recycle of chemical elements

14. Stellar Evolution

15. Low-Mass Star Evolution • Protostar • Main-Sequence Stage • Post Main-Sequence Stages • Subgiant to Red Giant • Helium Flash to Helium-burning (Horizontal Branch) • Pulsating Yellow Giant • 2nd Red Giant (Supergiant – Asymptotic Giant Branch) • End Stage • Planetary Nebula • White Dwarf

16. Our Sun • Protostar ~ 200 million years • Main-sequence ~ 10 billion years • Subgiant to Red Giant before Helium Flash ~ 200 million years to 2 billion years • Helium Flash ~ few seconds • Helium-burning + Pulsating Yellow Giant ~ 2 billion years • 2nd Red Giant ? • Planetary Nebula ~ few hundreds thousand years

17. Our Sun – Main-Sequence Stage • Hydrogen abundance decreases as helium abundance increases as the Sun aged • Sun slowly expands

18. Our Sun (Subgiant to Red Giant) • Hydrogen-burning ends in the core • Core temperature temporary decreases • Core pressure decreases • Core Contraction (loss hydrostatic equilibrium) • Releasing gravitational potential energy • Heating the core and the surrounding layers • Core temperature increases but pressure does not increase (partial degenerate matter) • Core continues to contract • Surrounding layers temperature increases • Pressure increases in the surrounding layers • Expansion of surrounding layers • Hydrogen-burning shell develops • Additional energy pushes the layers to expand more rapidly

19. Our Sun (Subgiant to Red Giant)

20. Our Sun (Helium-burning Stage) • Helium Flash • Ignition of helium • Removing partial degeneracy • 100 million Kelvin • Helium-burning in core • Hydrogen-burning shell • Pulsation

21. Our Sun – Helium-burning Stage

22. Our Sun (After Helium-burning) • Helium exhaustion • Core contraction • Expansion of outer layers • Ignition of helium-burning and hydrogen-burning shell

23. Our Sun (After Helium-burning Stage)

24. Our Sun (Planetary Nebula Stage) • Thermal pulses from helium shell flashes • Outer layers are slowly loss to space • Mass loss • White dwarf • 0.5 solar mass • Carbon and oxygen core

25. Our Sun (Planetary Nebula Stage)

27. Planetary Nebula

28. Planetary Nebula MyCn 18 at 2.5 kpc

29. White Dwarfs • Mass-Radius Relation • Chandrasekhar Mass Limit –the maximum mass for a white dwarf is 1.4 solar mass