• Childhood • Adulthood • End of life: ... of low mass stars ... of massive stars

1. Life and death of stars • Childhood • Adulthood • End of life: ... of low mass stars ... of massive stars

2. log (L/L ) +4 B +2 C E 0 D A −2 20000 5000 10000 2500 Teff Childhood Evolution towards the main sequence Example: 1 M star A: start of gravitational collapse, thermal emission B: 100 years, maximum luminosity by thermal emission C: 100 000 years D: 1 million years E: 10 millions years, start of nuclear reactions

3. Childhood - 2 Effect of initial mass If the initial mass is higher: • each evolutionary stage is shorter • position on the main sequence is hotter and more luminous Maximal mass ~ 100 M Above: radiation pressure is too high and blows gas away IC1396 globule and hot stars (CFHT)

4. Spec. typeM(M ) L(L ) T(K)t (109 yrs) O7 25 90000 35000 0.003 B0 15 10000 30000 0.015 A0 3 60 11000 0.5 F0 1.5 6 7000 2.5 G2 1.0 1 5800 10 K0 0.8 0.6 5200 13 M0 0.6 0.02 3900 200 Adulthood Life on main sequence Lifetime on the main sequence: (t in billion years if M and L in solar units)

5. L evolution zero age main sequence (ZAMS) Teff Adulthood - 2 Evolution on main sequence Helium accumulates in the core→ hampers hydrogen fusion → one might imagine energy production decreases This is not the case because: • central pressure goes down → the core contracts → T increases → (1) there is more matter in the core (2) the reaction rate increases → L increases • external layers expand → Teff decreases

6. Adulthood - 3 Evolution of the Sun on the main sequence On the zero age main sequence, the solar luminosity was ~ 70% of its present value (but greenhouse effect on Earth was probably more important) In 5 billion years, it will reach twice the present value: 2 L In 1 billion years, our planet will probably be too hot to sustain life T ~ L1/4 Now: T ~ 10 °C In 5 × 109 years: T ~ 60 °C

7. log (L/L ) I +4 G +2 H F 0 E J −2 K 80000 40000 20000 10000 5000 2500 Teff End of life of low mass stars Evolution after main sequence Example: 1 M star E : end of main sequence life (109 years) F: +200 Myr (million years), beginning of red giant phase G: +300 Myr, helium flash H: +100 Myr, horizontal branch I: +400 Myr, red supergiant J: +a few Myr, planetary nebula J → K: + ~ 100 billion years, slowly cooling white dwarf

8. L F E Teff End of life of low mass stars - 2 Towards the red giant branch E → F: amplified version of main sequence evolution • helium accumulates in the core and hampers hydrogen fusion → central pressure decreases → core contraction → T increases → (1) more matter in the core (2) reaction rate increases • L does not increase at once because the energy release is too fast to immediately reach surface • energy accumulates in the core → external layers expand → Teff decreases at ≈ constant L

9. L G F E Teff End of life of low mass stars - 3 On the red giant branch F → G: huge amount of energy accumulated in the core → transport by radiation is not efficient enough → the envelope becomes fully convective → external luminosity finally reflects energy production → star goes up in the HR diagram → red giant For the Sun: L ≈ 100 L R ≈ 20 R

10. L G H F E Teff End of life of low mass stars - 4 Towards horizontal branch G → H: the core temperature continues increasing 108 K → (1) helium fusion (through ‘triple alpha’ reaction) (2) 12C + 4He → 16O He fusion can happen very fast: helium flash → strong increase of stellar wind → outer layers are ejected → loss of a significant fraction of stellar mass

11. H → He He → C End of life of low mass stars - 5 Horizontal branch H: the helium flash causes expansion of the stellar core → T decreases → the star finds a new equilibrium state similar to the main sequence but for He fusion instead of H (horizontal branch) The star has a layered structure: • in the core: He → C • in a shell: H → He • in the envelope: no nuclear reactions Image: not to scale

12. I L G H F E Teff End of life of low mass stars - 6 The asymptotic giant branch H → I: scenario ≈ main séquence → red giant • accumulation of carbon in the core hampers helium fusion → the core shrinks → T increases → reaction rate increases → L and R increase → redsupergiant (or AGB star) • huge envelope → (1) irregular shape (2) becomes unstable → pulsations with ejection of matter

13. End of life of low mass stars - 7 Planetary nebulae Matter ejected by supergiants No relation with planets (called planetary because they appear as coloured disks in small telescopes) Diameter ~ 1 light year Lifetime ~ 10 000 years Number ~ 10 000 in our galaxy Planetary nebula IC418 (HST)

14. End of life of low mass stars - 8 A gallery of planetary nebulae Planetary nebula M57 (HST)

15. End of life of low mass stars - 9 A gallery of planetary nebulae Planetary nebula NGC2392 (HST)

16. End of life of low mass stars – 10 A gallery of planetary nebulae Planetary nebula ‘hourglass’ (HST)

17. End of life of low mass stars – 11 A gallery of planetary nebulae Helix nebula NGC7293 (HST)

18. End of life of low mass stars – 12 A gallery of planetary nebulae NGC7293 zoom up (HST)

19. End of life of low mass stars – 13 White dwarfs Stellar core after ejection of the outer layers: not massive enough to sustain nuclear reactions from `ashes´ Accumulates energy by gravitational contraction then slowly cools down R ~ 10 000 km ~ planet M < 1.4 ML ~ 0.001 L Sirius B : 1st white dwarf discovered (1862) identified as such in 1915 T≈ 25 000 K M≈ 1.03 M R ≈ 0.92 REarth Sirius A and B

20. End of life of low mass stars – 14 White dwarfs Density ρ~ 1 ton/cm3→ huge pressure → individual atoms are ‘crushed’, e− are not bound to a nucleus but free as in a metal: degenerated matter Pauli exclusion principle: max 2 e− per energy level ρ↑ → E ↑ → P ↑ → degeneracy pressure stops contraction (if M < 1.4 M ) Relation mass – radius: M ↑ → R↓ Planetary nebula M27

21. H H → He He → C C O Si Fe End of life of massive stars Evolution of stars with mass > 8 M First phases similar to those of lower mass stars But no helium flash (if M > 2 M→ slow combustion of He) P and T are high enough to go over C and O → successive combustions up to Fe • nuclear reaction ashes accumulate in the core • the next reaction starts → the stellar core has a layered structure (↔ onion)

22. End of life of massive stars - 2 The iron catastrophe 56Fe = most stable nucleus → no energy production by fusion → nothing can stop contraction of the Fe core (even degeneracy pressure is not strong enough) → P↑ until e− combine with protons to form neutrons → the core is transformed in neutronic matter (ρ ~ 1017 kg/m3) Very fast contraction → goes over equilibrium density → rebound of the core → shock wave Conservation of impulse → the wave accelerates when reaching lower density layers

23. End of life of massive stars - 3 Type II supernovæ The shock wave expells the external layers of the star → sudden increase of luminosity (~1010× L ) (~ galaxy bulge or small galaxy) followed by progressive decrease of L (~ a few weeks or months) The stellar core generally remains → neutron star: R ~ 10 km ρ ~ 1017 kg/m3 (1 cm3 weights 100 millions tons!) SN1994D in NGC4526

24. End of life of massive stars - 4 Naked eye supernovæ YearConstellationmV 185 Centaurus −7 393 Scorpio 0 1006 Wolf −8 1054 Taurus −4 (Crab) 1181 Cassiopea 0 1572 Cassiopea −3 (Tycho) 1604 Ophiuchus −2 (Kepler) 1987 Doradus +3.5 Crab nebula (HST)

25. End of life of massive stars - 5 SN1987A 1st naked eye supernova since invention of the telescope Discovered by Ian Shelton on 23 February 1987 Explosion of a supergiant star (L ~ 60 000 L ) Detection of 19 neutrinos → ~1058 neutrinos produced by fusion of electrons and protons in the stellar core Neutrinos carry ~99% of the energy Kinetic energy: ~1% Photon energy: ~0.01% SN1987A and the Tarentula nebula

26. End of life of massive stars - 6 Stellar winds Radiation pressure → all stars loose some matter (winds) Mass loss during main sequence phase: ~0.1% for the Sun ~20% for M ~ 20 M ~90% for M ~ 100 M → in some cases, matter processed by nuclear reaction can be observed on the stellar surface → Wolf-Rayet stars Nebula around WR124 (HST)

27. End of life of massive stars - 7 Pulsars 1967: Jocelyn Bell detects a radiosource emitting pulses every 1.33730113 seconds → code name LGM-1 Variations ~1 s → size < ~100 000 km → planet, white dwarf, neutron star? • Planet? no (not enough energy) •Stellar pulsation? – period too high for a white dwarf – too low for a neutron star •Rotation? – too fast for a white dwarf – OK for a neutron star Jocelyn Bell

28. End of life of massive stars - 8 Neutrons stars 1968: a pulsar is discovered at the centre of the Crab nebula (supernova remnant) → hypothesis pulsar = neutron star confirmed Rapid rotation because of angular momentum conservation Intense magnetic field → charged particles at the star’s surface have spiral trajectories along field lines → emission of synchrotron radiation along the magnetic axis Magnetic axis ≠ rotation axis → the beams sweeps space Pulsar (artist view)

29. End of life of massive stars - 9 The Schwarzschild radius If the mass of a neutron star > 3 M → escape speed > c → nothing can escape from the star → vlib = c → = Schwarzschild radius RS (km) = 3 M (M ) Black hole surrounded by a luminous disk

30. End of life of massive stars - 10 Black holes If R < RS→ matter is more and more compressed because of spacetime curvature → black hole Singularity of spacetime? If ρ > 1093 kg/m3 (Planck density) → we would need a theory of quantum gravity → ??? Stellar black hole in front of the Southern sky

31. End of life of massive stars - 11 Detection of black holes (1) By bending of light rays (gravitational mirage): Multiple deformed images of background sources (2) In binary systems: by mass transfer from the stellar companion onto the black hole Accretion disk Strong heating of matter in the inner disk → emission of high energy radiation (X,…) Energy efficiency (10 to 20%) much higher than nuclear reactions (< 1%) Black hole in a binary system