Transcript Lecture 7
The Sun and the Stars The Sun and the Stars Dr Matt Burleigh The Sun and the Stars A star like our Sun never becomes hot enough in core to burn heavier elements!! Evolutionary tracks for stars of differing mass II evolution of 5 solar mass star (pop I) For a M> 1Msun core hot enough (>10 million K) for CNO cycle to operate 12 C 1H 13N 13 N 13C e C 1H 14 N 13 14 15 15 CNO cycle N H O 1 15 O15N e N 1H 12 C 4He Net effect, 4 H 1He (CNO just catalysts) Enuc Tc 18 c.f. Enuc Tc 4 for PP chain (Enuc = energy released per unit mass) O stars only spend 106 years on main sequence!! Dr Matt Burleigh The Sun and the Stars Stellar structure : Lower Main Sequence (Eg Solar masses) Upper Main Sequence (Eg 5 Msun) radiative core, convective outer layers well-mixed convective core, radiative envelope energy via PP chain on MS energy mostly by CNO cycle on main sequence CNO dominates above 20million K Dr Matt Burleigh The Sun and the Stars NB UMS stars are convective in the core, materials are well mixed Supernova? Log(L/Lsun) Once H in core virtually exhausted, core contracts due to drop in pressure, H burns in shell around He core. Burnt material (He) added to core, density increases. Eventually core becomes so dense contracts and heats up energy generation in shell accelerates, outer envelope expands, surface temperature drops (moves to right on HR diagram). Lower temperature increases opacity, convection in envelope increases transport of energy to surface, luminosity rises RGB ~1.34Msun PN ejected to WD 4 AGB He H Thermal Pulse begins C+O He ignition Helium Flash He RGB 2 0 WD cooling 4.2 3.8 3.4 Log Teff (K) Core continues to contract (T> 100million K) Triple-alpha process in core. He ignites, but no helium flash) Dr Matt Burleigh The Sun and the Stars NB UMS stars are convective in the core, materials are well mixed Supernova? Log(L/Lsun) Rapid expansion of core leads to reduction in luminosity (much less dramatic in this case). He burning core H burning shell. PN ejected to WD 4 He H Eventually He burning dominates, surface temp rises (moves to left on H-R diagram) 2 Finally He in core exhausted, C core contracts, now He burning in shell, and H burning in shell, heat from contraction, accelerates process, outer envelope expands AGB 0 12 Thermal Pulse begins C+O He ignition Helium Flash He RGB WD cooling After this evolution uncertain, thermal pulses drive strong wind leaving PN and exposed core, or star may become Supernova 5-8Msun stars can burn Carbon to Oxygen via (2nd loop on H-R diagram) AGB 4.2 3.8 3.4 Log Teff (K) C 4He 16O Serious mass loss via strong superwinds Dr Matt Burleigh The Sun and the Stars III Evolution of >8 solar mass star Apart from an initial rise on main sequence, evolution is almost horizontal. i.e. evolution occurs at almost constant luminosity (blue giant red-giant blue giant etc) Large stellar winds/mass loss even on main sequence (~10-6 to 10-7 Msun/yr). Usual reactions - PP-chain+CNO 4 H 4 He 3 4 He 12 C 12 But also T~107 K T~108 K C 4He16O 16 O 4He 20Ne 12 C 12C 23Na H 23Mg n T~108 K T~6x108 K 24 Mg 16 O 16O 24 Mg 2 4 He T~6x108 K 28Si 4 He 31P H 31S n 32 S Dr Matt Burleigh The Sun and the Stars and, -processes, e.g. 28 Si 4He 32 S 32 S 4He 36 Ar 56 Fe 56 Ni -process must end at Fe because Fe is at peak of BE curve All reactions beyond Fe endothermic i.e. require more energy than they produce Dr Matt Burleigh The Sun and the Stars Ignition of each new burning process is preceded by contraction and heating up of core. Stellar evolutionary track oscillates on HR diagram as each new energy source becomes available H+He envelope H burning He burning C burning O burning Si burning Fe core NB burning timescales very short at end (e.g. HeC 106 years, CO 103 yrs, SiFe in just a few days) If at the end of -process,core mass exceeds 1.4Msun (Chandrasekhar limit), electron degeneracy pressure can no longer support star and core collapses (1~sec). When T> 6x109K, photodisintegration occurs 56 Fe 134 He 4n 4 Endothermic!! requires 100Mevcollapse accelerates He 2 p 2n p e n e Energy removed rapidly from core, core contraction accelerates Dr Matt Burleigh The Sun and the Stars Core: R~ 0.01Rsun, ~1015 g/cm3, M~1.4Msun Eventually neutron degeneracy pressure opposes collapse (can be exceeded by ~50%), core-bounce – contraction of core heats outer layers which burn explosively, star literally explodes as a Type II Supernova leaving dense core of neutrons (R~5km) – a neutron-star Simulations suggest initial shock wave may stall, neutrino trapping may help drive off outer layers. r-processes (rapid) manufacture heavy elements L~109 Lsun (Mv= -20) If M>25 Msun , neutron degeneracy pressure cannot halt collapse – star becomes a black-hole Dr Matt Burleigh Type II Supernovae Dr Matt Burleigh Evolutionary phases of a massive star Stage Physical processes Protostar Dust and gas cloud collapses rapidly, accompanied by heating of the interior and ionisation of atoms PMS Semihydrostatic equilibrium; contraction and heating continue ZAMS Hydrogen burning commences Initial evolution on the main sequence Hydrogen consumed in the core; some contraction occurs Evolution off the main sequence Hydrogen depleted in the core, isothermal helium core and hydrogen-burning established Evolution to the right in the H-R diagram Core rapidly contracts, envelope expands, hydrogen-burning shell narrows Red giant Energy output increases, convective envelope forms, helium burning begins Cepheid Convective shell contracts, core helium burning becomes the major energy source Supergiant Helium-burning shell forms Dr Matt Burleigh Type II Supernovae • • • • Explosion of stellar core stellar core to form neutron star Absolute magnitudes from –16 to –20 (energy ~1044J) – e.g. China, SN of 1054 reached mV=-6 (remnant is Crab Nebula) Ejects a large fraction of original mass with v~5000-10000 km s-1 Seen in spiral galaxies only, especially in spiral arms… Population I stars SN 1987A in LMC Dr Matt Burleigh Dr Matt Burleigh Type Ia Supernovae • • • Seen in both elliptical and spiral galaxies… Population I & II stars Progenitors are H-deficient, highly evolved stars Mechanism not well understood – Single degenerate: Accretion onto a white dwarf from a companion star increases MWD > Chandrasekhar limit – Double degenerate: Merger of two WDs to give M > 1.4M – Both mechanisms may operate Dr Matt Burleigh SN2014J in M82: closest SNIa for 42 years Discovered by Dr Steve Fossey & students at University College London’s Mill Hill Observatory (0.35m telescope) on 21st January 2014 Dr Matt Burleigh Supernovae: Key Points • • • SN responsible for nucleosynthesis of element above 56Fe Remnant neutron stars… sometimes revealed as pulsars Shockwave heating of interstellar medium… Supernova Remnants Dr Matt Burleigh Vela Supernova Remnants Crab Nebula Dr Matt Burleigh Supernova Remnants Cassiopiea A Dr Matt Burleigh Supernova expansion Dr Matt Burleigh The Sun and the Stars Example SN light-curves, type I and type II Crab nebula 1054 SN Dr Matt Burleigh The Sun and the Stars IV Evolution of very low mass stars Stars < 0.08Msun never make it onto the main sequence. Gravitational contraction does not heat the gas efficiently, and their cores are degenerate long before they are hot enough to start thermonuclear reactions. These “failed stars” simply cool as Brown dwarfs Dr Matt Burleigh