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More Nucleosynthesis –final products are altered by the core collapse supernova shock before dispersal to the ISM hydrogen, helium, and carbon burning.

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Presentation on theme: "More Nucleosynthesis –final products are altered by the core collapse supernova shock before dispersal to the ISM hydrogen, helium, and carbon burning."— Presentation transcript:

1 More Nucleosynthesis –final products are altered by the core collapse supernova shock before dispersal to the ISM hydrogen, helium, and carbon burning products are largely left unaltered a sizeable fraction of oxygen burning products are further processed no silicon products are returned to the ISM –known to be the dominant sources of oxygen neon - sulfur Nova nucleosynthesis products of explosive hydrogen burning –lithium-7 –nitrogen-15 –sodium-23 –aluminum-26 –not enough mass in nova envelopes to make significant contributions to most CNO process nuclei Supernova nucleosynthesis core collapse supernova shock waves cause explosive nucleosynthesis –processes all matter below the bottom third of the oxygen shell to intermediate mass (like Ca) and iron peak (like Ni) elements

2 More Nucleosynthesis (cont.) –dominant source of elements and isotopes from Ca-Zn, except for Mn-Cu core collapse supernovae are the most likely source of the r-process –rapid neutron capture is when the amount of time to capture a neutron << the time for the more stable radioactive isotopes to decay any nucleus can capture several neutrons before decaying –rapid neutron capture can occur above the proto-neutron star after collapse by the fraction of free neutrons available in the gas –problem is how to mix from below the oxygen shell to above, which we know hapens from observations Type Ia supernovae also fuse material to iron-peak –burn CO white dwarf to mostly iron peak, with outer layer of intermdiate mass elements and isotopes –dominant source of Mn-Cu Cosmic-ray Nucleosynthesis cosmic rays are highly energetic particles now known to be emitted by supernova ejecta very energetic particles can either fuse with nuclei, scatter off nuclei, or break (as into pieces) nuclei cosmic rays which are oxygen nuclei can create rare isotopes by hitting other oxygen nuclei –dominant source of lithium-6, beryllium-9. boron-11 in our Galaxy

3 The Cycle of Stellar Evolution Having a knowledge of nucleosynthesis, we see that continued generations of stars will enrich the ISM with their nuclear ashes The whole enrichment process can be described in three steps star formation occurs in a molecular cloud out of whatever composition is there stellar evolution occurs –high mass stars live and die before low mass stars even finish the formation process –low mass stars eventaully enrich the ISM through planetary nebulae interstellar shock waves help distribute new elements throughout the Galaxy Some other thoughts each successive generation of stars depletes hydrogen isotopes in favor of heavier nuclei each successive generation of stars leaves behind a non-negligable fraction of mass in a blackhole or neutron star –a build-up of a so-called dark matter component –eventually our Galaxy will run out of matter to form stars with

4 Neutron Stars Remember that core collapse supernovae with initial masses < 25 M sol, leave a remnant of their cores electrons are pushed into protons during core collapse, making neutrons neutron degeneracy pressure causes a bounce of the core and the generation of a shock wave core of neutrons remains bound together after shock causes the rest of the envelope to explode first theorized in 1933 by Paul Dirac first observed in 1967 by Jocelyn Bell Neutron stars are extremely small and dense size during formation ~ 100 km size after explosion ~ 10 km –something the mass of the Sun packed into a space about 6 miles across density ~ 10 14 gm/cc –a billion times denser than a white dwarf –one cm of neutronium as some call it, would contain ~ 100 million tonnes about the mass of a terrestrial mountain

5 Neutron Stars (cont.) gravity is extremely strong –by inverse square- law, it should be at least 5 billion times stronger than at the surface of the Sun –average person would be squashed to less than 1 mm tall most rotate very fast –rotation periods often less than 1 second –due to conservation of angular momentum most have extremely strong magnetic fields –there is an inverse square law for magnetic field strength as well, so we expect a billion fold increase over the Sun

6 Pulsars In 1967, Jocelyn Bell observed an object lying within the Crab Nebula that emitted radio waves in short bursts about 1.34 seconds apart pulses were so regular, that they were better than most clocks over 1000 have been discovered and are now known as pulsars Generic pulsar properties include accurate pulsing of radiation most pulses appear in radio, but some emit in all parts of the EM spectrum rotation periods are short –most range from 0.03 seconds to 0.3 seconds some are associated with supernova remnants –Crab Nebula pulsing can be seen in the optical a neutron star from a supernova in 1054 AD –Vela Remnant Some properties can only be explained by association with neutron stars only rotation can create such a regular signal

7 Pulsars (cont.) Only a small object can create such a short pulse –duration of pulse can be no larger than the light travel time across the emitting region Best model is known as the lighthouse model two spots on the north and south magnetic poles of the neutron star emit radiation –results in a lighthouse effect charged particles thought to interact with the strong magnetic fields produce the radiation if the beams are in the direction of the Earth, than we see them –this means we only see a very small fraction of the actual number of pulsars in our Galaxy Not all neutron stars are pulsars rotation rate and magnetic fields decay with time expect a typical lifetime to be about 10 7 - 10 8 years most astronomers expect all neutron stars to be born as pulsars in Type II supernovae, but later fade

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