Galactic Enrichment through Nucleosynthesis

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 Msol, 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 ~ 1014gm/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 107 - 108 years • most astronomers expect all neutron stars to be born as pulsars in Type II supernovae, but later fade