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The Sun and the Stars. The Sun and the Stars. 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)
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The Sun and the Stars The Sun and the Stars
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 CNO cycle Net effect, 4 H 1He (CNO just catalysts) for PP chain c.f. (Enuc = energy released per unit mass) O stars only spend 106 years on main sequence!!
The Sun and the Stars Stellar structure : Lower Main Sequence Upper Main Sequence (Eg 5 Msun) (Eg Solar masses) radiative core, convective outer layers energy via PP chain on MS well-mixed convective core, radiative envelope energy mostly by CNO cycle on main sequence CNO dominates above 20million K
The Sun and the Stars NB UMS stars are convective in the core, materials are well mixed ~1.34Msun • 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 • Core continues to contract (T> 100million K) • Triple-alpha process in core. He ignites, but no helium flash) He ignition
The Sun and the Stars NB UMS stars are convective in the core, materials are well mixed Rapid expansion of core leads to reduction in luminosity (much less dramatic in this case). He burning core H burning shell. Eventually He burning dominates, surface temp rises (moves to left on H-R diagram) 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 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) Serious mass loss via strong superwinds He ignition
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 T~107 K T~108 K But also T~108 K T~6x108 K T~6x108 K
The Sun and the Stars and, -processes, e.g. -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
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! 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 Endothermic!! requires 100Mevcollapse accelerates Energy removed rapidly from core, core contraction accelerates
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 IISupernova 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
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 Type II Supernovae SN 1987A in LMC
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 Type Ia Supernovae
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
SN responsible for nucleosynthesis of element above 56Fe Remnant neutron stars… sometimes revealed as pulsars Shockwave heating of interstellar medium… Supernova Remnants Supernovae: Key Points
Vela Crab Nebula Supernova Remnants
Supernova Remnants Cassiopiea A
The Sun and the Stars Example SN light-curves, type I and type II Crab nebula 1054 SN
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