Evolution of High Mass Stars

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1 Luminosity (L sun ) Evolution of High Mass Stars High Mass Stars O & B Stars (M > 4 M sun ): Burn Hot Live Fast Die Young Main Sequence Phase: Burn H to He in core Build up a He core, like low-mass stars Lasts for only ~ 10 Myr Red Supergiant Phase Crossing the Supergiant Branch After H core exhaustion: Inert He core contracts & heats up H burning in a shell around the He core Huge, puffy envelope ~ size of orbit of Jupiter Red Supergiant Moves horizontally across the H-R diagram: Takes ~1 Myr to cross H-R diagram ,000 20,000 10,000 5,000 2,500 Temperature (K) Red Supergiant Star Helium Burning H Burning Inert He Core Cool, Extended Envelope Not to Scale Core Temperature reaches 170 Million K Ignites Helium burning to C & O: Rapid Phase: ~ 1 Myr He burning in the core H burning in a shell Start building a C-O core Star becomes a Blue Supergiant.

2 Luminosity (L sun ) Luminosity (L sun ) Blue Supergiant Blue Supergiant ,000 20,000 10,000 5,000 2,500 Temperature (K) He Core Exhaustion When He runs out in the core: Inert C-O core collapses & heats up H & He burning moves into shells Becomes a Red Supergiant again C-O Core collapses until: T core > 600 Million K density > 150,000 g/cc Ignites Carbon Burning in the Core. End of Helium Burning Carbon Burning: Nuclear reaction network: 12 C+ 12 C fuses to: 24 Mg 20 Ne + 4 He 16 O He Build up an inert O-Ne-Mg core Lasts only ~1000 years before C runs out. 40,000 20,000 10,000 5,000 2,500 Temperature (K) End of Carbon Burning Phase: Intermediate Mass Stars H Burning He Burning C Burning Inert O-Ne-Mg Core Red Supergiant Envelope Stars with 4 < M < 8 M sun After 1000 years: Inert O-Ne-Mg core contracts & heats up C, He, & H burning shells Thermal pulses destabilize the envelope: Eject the envelope in a massive stellar wind Leave O-Ne-Mg white dwarf core behind

3 High Mass Stars: M > 8 M sun At the onset of Carbon Burning: Evolution is so fast that the envelope can no longer respond. Should see little outward sign of the inward turmoil to come. Neon Burning O-Ne-Mg core contracts & heats up until: T core ~ 1.5 Billion K density ~ 10 7 g/cc Ignite Neon burning: reaction network makes O, Mg, & others Huge neutrino losses Builds a heavy O-Mg core Lasts for a few years before Ne runs out. Oxygen Burning Ne runs out, core contracts & heats up until: T core ~ 2.1 Billion K density ~ few x 10 7 g/cc Ignite Oxygen burning: reaction network making Si, S, P, & others Huge neutrino losses Builds a heavy Si core. Lasts for ~1 year before O runs out. Silicon Burning O runs out, Si core contracts & heats up until: T core ~ 3.5 Billion K density ~ 10 8 g/cc Ignite Silicon burning: Builds a heavy Ni/Fe core. Lasts for ~1 day... End of Silicon Burning Phase: H Burning He Burning C Burning Ne Burning O Burning Si Burning Inert Fe-Ni Core Onion Structure Core Radius: ~1 R earth Envelope: ~ 5 AU

4 The Nuclear Impasse Fusion of light elements releases nuclear binding energy. Iron (Fe) is the most tightly bound nucleus: Fusion of nuclei lighter than Fe release energy. Fusion of nuclei heavier than Fe absorb energy. Once an Fe core forms, there are no new fusion reactions left for the star to tap. End of the Road At the end of the Silicon Burning Day: Star builds up an inert Iron core Series of nested nuclear burning shells Finally, the Fe core exceeds M sun : Fe core begins to contract & heat up. This collapse is final & catastrophic Last Days of a Massive Star Burns a succession of nuclear fuels: Hydrogen burning: 10 Myr Helium burning: 1 Myr Carbon burning: 1000 years Neon burning: ~10 years Oxygen burning: ~1 year Silicon burning: ~1 day Builds up an inert Iron core in the center. Iron Core Collapse Iron core grows to a mass of M sun Collapses & begins to heat up T>10 Billion K & density ~10 10 g/cc Two energy consuming processes kick in: Nuclei photodisintegrate into He, p & n Protons & electrons combine to form neutrons & neutrinos neutrinos escape & carry away energy Makes the core collapse faster Catastrophic Collapse Start of Iron Core collapse: Radius ~ 6000 km (~R earth ) Density ~ 10 8 g/cc 1 second later: Radius ~50 km Density ~10 14 g/cc Collapse Speed ~0.25 c! Core Bounce Core collapses until its density is ~2.4x10 14 g/cc, the density of an atomic nucleus! Any further contraction is resisted. Inner 0.7M sun of the core: comes to a screeching halt & springs back a bit (bounces) Infalling gas hits the bouncing core head-on!

5 Shockwave Blastwave smashes out through the star: Explosive nuclear fusion in its wake produces more heavy elements Heats up and accelerates the envelope Shock breakout a few hours later Breakout speed ~0.1c! Nucleosynthesis in Stars Start with Hydrogen & Helium: Fuse Hydrogen into elements up to Iron and Nickel: controlled nuclear fusion. Accumulate in the core layers of stars Supernova Explosion: explosive nuclear fusion builds more light elements up to Iron & Nickel. neutron reactions build Iron & Nickel into heavy elements up to 254 Cf supernova explosion is the ONLY PLACE to make heavy elements like gold & silver Supernova! At shock breakout: Star brightens to ~10 Billion L sun in minutes Can outshine an entire galaxy of stars! Outer envelope is blasted off: accelerated to a few 10,000 km/sec gas expands & cools off Only the core remains behind Supernova 1987a

6 Historical Supernovae Crab Nebula Remnant of Supernova in AD: Guest Star in Taurus Observed by the Chinese (Song dynasty) Visible in daylight for 23 days 1572: Tycho Brahe s Supernova 1604: Johannes Kepler s Supernova BC: Vela supernova Observed by the Sumerians; appears in legends about the god Ea. Supernova Remnants Scraps of an old Supernova Remnant What happens to the envelope? Enriched with metals in the explosion Expands at a few 10,000 km/sec Supernova Blast Wave: Plows up the surrounding interstellar gas Heats & stirs up the interstellar medium Shines as ionized nebulae up to a few thousand years after the explosion Supernova Remnants have Non-thermal Emission X-ray Radio Stardust Metal-enriched gas mixes with interstellar gas Goes into the next generation of stars. Successive generations are metal rich. Sun & planets (& us): Contain many metals (iron, silicon, etc.) Only ~5 Gyr old The Solar System formed from gas enriched by previous generations of massive stars.

7 Questions: Why don t stars have just any Luminosity and Temperature? Why is there a distinct Main Sequence? Lifetime What makes one M-S star different from another? Mass Were giants, supergiants, and white dwarfs born that way, or is something else going on? Evolution Patterns on the H-R Diagram are telling us about the internal physics of stars. A sequence of nuclear burning in the core of a massive star continues only up to a limit. The element that is produced when this limit is reached is: A) Uranium B) Iron C) Silicon D) Oxygen After the material in the core of a massive star has been converted to iron, further energy can be released to heat the core only by: A) Fusion of iron into heavier elements B) Splitting of iron by gamma rays C) Absorption of neutrinos D) Gravitational contraction Heavy elements beyond iron are produced: A) By nuclear fusion in the cores of massive stars B) By nuclear fusion in the cores of low mass stars C) In supernova explosions D) In planetary nebulae

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