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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 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
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
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
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
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
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
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