Lecture 26. High Mass Post Main Sequence Stages

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1 Lecture 26 Fate of Massive Stars Heavy Element Fusion Core Collapse Supernova Neutrinoes Gaseous Remnants Neutron Stars Mar 27, 2006 Astro 100 Lecture 26 1 High Mass Post Main Sequence Stages For M(main sequence) > 8 M sun, temperature -> 600 million K, starting core Carbon fusion: 12 C + 12 C -> 20 Ne + 4 He (Neon) After carbon runs out in the core, start carbon shell fusion, core collapses etc, etc: 10 9 K 20 Ne + 4 He -> 24 Mg (Magnesium) 1.5x10 9 K 16 O + 16 O -> 28 Si + 4 He (Silicon) 3x10 9 K 28 Si + stuff -> all the way up to 56 Fe (Iron) Star wanders between blue and red supergiant Mar 27, 2006 Astro 100 Lecture 26 2

2 The Massive Star Evolution This is the end: There is no energy available from Iron, since it is the most stable element. Timescale: These happen with increasing speed, since each successive step yields less energy. For 25 M sun star: Carbon core fusion: 600 years Neon " " : 1 year Oxygen " " : 6 months Silicon " " : 1 day! For less massive stars, stages take longer, since pressure (temperature required) is much lower. Sample star: Betelgeuse (alpha Ori). M = 10 M sun. Probably in carbon core fusion stage, maybe lasting 100,000 years. Mar 27, 2006 Astro 100 Lecture 26 3 Supernova! Next: Burned-out Fe core collapses into something very compact (is beyond Chandrasekhar Limit), releasing enough energy to expel the envelope (with its heavy elements) in Supernova explosion Current ideas on what happens in a supernova (actually "Core Collapse" supernova - there are others): More than 1.4 M sun of fusion ash accumulates: electron degeneracy fails and core collapses (in 0.25 sec!) When core temperature reaches 5 billion K, electron + proton -> neutron + neutrino All electrons are turned into neutrons Neutrinos almost all escape star, carrying away most of remaining energy Mar 27, 2006 Astro 100 Lecture 26 4

3 Core Collapse Core neutrons collapse to a density 4x10 14 gm/cm 3, the density inside an atomic nucleus (core is few 10's of km across). Collapse halts under nuclear forces. Rest of inside of star crashes down on core generating shock wave, which (in some models) escapes the star, blowing off envelope. This causes the visible explosion. Leaves expanding gaseous remnant. (note: a very difficult calculation. But it must happen, since we see the explosion!). Core settles into a collapsed remnant. Either: a) Neutron star. Supported by neutron degeneracy. Radius 10 km. Like white dwarfs, there is an upper mass limit, maybe 3 M sun. Above this, get a b) Black hole. Surface gravity so large that light can't escape. Disappears except for gravity Mar 27, 2006 Astro 100 Lecture 26 5 Naked Eye Supernovae There have been three supernovae in modern history that have been visible to the naked eye, in 1572, 1604, and 1987 (Five others, in AD 185, 393, 1006, 1126, and 1054 have been reported by historians, mostly in China and Japan). Name PKS Crab Cas (Tycho s) Oph (Kepler s) SN1987A Date Mag Mar 27, 2006 Astro 100 Lecture 26 6

4 SN 1987A The only naked-eye SN observed with modern techniques: Location: in Large Magellanic Cloud ("LMC"), a nearby galaxy Light Curve: peaked at 3 mag after 85 days; now fainter than 15th. Pre-SN star. The star that exploded has been identified in old photographs and surveys of LMC. It was Sk , a 13th mag B3I supergiant (10 5 L sun ). Theoreticians best estimates: on MS was 20 M sun. Shed mass by mass loss. Just before SN, had maybe 5 M sun Hydrogen in envelope, and 6 M sun Helium, 2 M sun heavier elements, including 1.5 M sun iron in core. Neutrinos. For 0.1 sec, power calculated to be Watts. SN1987A was the first (except for sun) astronomical object seen in neutrinos. 3 hours before reported in visible light, 19 neutrinos seen in Japan and US. Just the right amount and timing! Mar 27, 2006 Astro 100 Lecture 26 7 (Gaseous) Supernova Remnants (SNR's) Supernovae are visible for tens of thousands of years through visible and radio light emitted by ejected gas. The emission lines from the nebula are often very rich in lines from heavy elements, as expected. The time of an SN can be estimated from the current expansion of nebula (using doppler shift of emission lines). Some are identifiable with historical SN's ( Crab Neb - SN 1054 SN1987A Origin of the Heavy Elements. All identified SNR s are rich in heavy elements CNO.. Fe.. presumably produced in the fusion leading up to the explosion. By a rough count of SN s/year, these account for most of the elements heavier than Helium in the universe. => We are made of supernova remnants! Mar 27, 2006 Astro 100 Lecture 26 8

5 Neutron Stars What else is left? Collapsed remnants: neutron star, black hole Neutron stars: degenerate gas stars, but electrons and protons are combined into neutrons. Recall for White dwarfs: electron degeneracy works up to M < 1.4 M sun ( Chandrasekhar Limit ) Neutron Stars: neutron degeneracy works up to M < (roughly!) 3 M sun The supernova scenario suggests neutron-degenerate objects may be formed in the evolution of the most massive stars. They would be very exotic objects. For instance Mass 1.3 M sun Radius: 16 km. Surface area small: thermal radiation too tiny to observe (until recently). Central Density: 4x10 14 gm/cm 3 (the density of an atomic nucleus) Escape velocity: 1/2 speed of light Mar 27, 2006 Astro 100 Lecture 26 9 High Mass Evolution Figure 7.21, p233, Arny Mar 27, 2006 Astro 100 Lecture 26 10

6 Massive Star before Core Collapse Figure 7.18, p230, Arny Mar 27, 2006 Astro 100 Lecture Supernova in the galaxy NGC 4725 Figure 7.19, p231, Arny Mar 27, 2006 Astro 100 Lecture 26 12

7 Gaseous Supernova Remnants B Crab A C Vela Cas Mar 27, 2006 Astro 100 Lecture 26 Figure 7.20, p232, Arny 13 SN1987A in the Large Magellanic Cloud Mar 27, 2006 Astro 100 Lecture 26 14

8 SN1987A Neutrino Mar 27, 2006 Astro 100 Lecture SN1987A Remnant Mar 27, 2006 Astro 100 Lecture 26 16

9 SN1987A Remnant - Animation Mar 27, 2006 Astro 100 Lecture SN1987A Light Echo Mar 27, 2006 Astro 100 Lecture 26 18

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