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More Nucleosynthesis
– final products are altered by the core collapse supernova
shock before dispersal to the ISM
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hydrogen, helium, and carbon burning products are largely
left unaltered
•
a sizeable fraction of oxygen burning products are further
processed
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no silicon products are returned to the ISM
– known to be the dominant sources of
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oxygen
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neon - sulfur
Nova nucleosynthesis
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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
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Supernova nucleosynthesis
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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
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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
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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
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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
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Cosmic-ray Nucleosynthesis
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cosmic rays are highly energetic particles now known
to be emitted by supernova ejecta
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very energetic particles can either fuse with nuclei,
scatter off nuclei, or break (as into pieces) nuclei
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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
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Having a knowledge of nucleosynthesis, we see that
continued generations of stars will enrich the ISM
with their nuclear ashes
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The whole enrichment process can be described in
three steps
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star formation occurs in a molecular cloud out of
whatever composition is there
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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
•
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interstellar shock waves help distribute new elements
throughout the Galaxy
Some other thoughts
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each successive generation of stars depletes
hydrogen isotopes in favor of heavier nuclei
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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
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Remember that core collapse supernovae with initial
masses < 25 Msol, leave a remnant of their cores
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electrons are pushed into protons during core
collapse, making neutrons
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neutron degeneracy pressure causes a bounce of the
core and the generation of a shock wave
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core of neutrons remains bound together after shock
causes the rest of the envelope to explode
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first theorized in 1933 by Paul Dirac
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first observed in 1967 by Jocelyn Bell
Neutron stars are extremely small and dense
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size during formation ~ 100 km
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size after explosion ~ 10 km
– something the mass of the Sun packed into a space
about 6 miles across
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density ~ 1014gm/cc
– a billion times denser than a white dwarf
– one cm of neutronium as some call it, would contain ~
100 million tonnes
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about the mass of a terrestrial mountain
Neutron Stars (cont.)
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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
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most rotate very fast
– rotation periods often less than 1 second
– due to conservation of angular momentum
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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
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In 1967, Jocelyn Bell observed an object lying within
the Crab Nebula that emitted radio waves in short
bursts about 1.34 seconds apart
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pulses were so regular, that they were better than
most clocks
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over 1000 have been discovered and are now known
as pulsars
Generic pulsar properties include
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accurate pulsing of radiation
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most pulses appear in radio, but some emit in all parts
of the EM spectrum
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rotation periods are short
– most range from 0.03 seconds to 0.3 seconds
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some are associated with supernova remnants
– Crab Nebula
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pulsing can be seen in the optical
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a neutron star from a supernova in 1054 AD
– Vela Remnant
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Some properties can only be explained by
association with neutron stars
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only rotation can create such a regular signal
Pulsars (cont.)
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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
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Best model is known as the lighthouse model
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two spots on the north and south magnetic poles of
the neutron star emit radiation
– results in a lighthouse effect
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charged particles thought to interact with the strong
magnetic fields produce the radiation
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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
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Not all neutron stars are pulsars
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rotation rate and magnetic fields decay with time
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expect a typical lifetime to be about 107 - 108 years
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most astronomers expect all neutron stars to be born
as pulsars in Type II supernovae, but later fade