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Announcements• Pick up graded homework (projects, tests still in
progress)• Transit of Mercury (crossing in front of Sun), this
afternoon, roughly noon-5:00. We’ll have telescopes set up at observatory for viewing (weather permitting).
Neutron Star Formation
• When the mass of the iron core exceeds the Chandrasekhar limit it implodes, again converting gravitational energy into thermal energy.
• As the hot material is crushed to nuclear density, protons and electrons combine to form neutrons.
• The result is a “neutron star”: a ball of neutrons heavier than our sun, roughly 30 kilometers across (first predicted in 1932).
• The left-over energy blows the rest of the star outward in a type-II supernova explosion.
proton
electron
neutron
neutrino
Pulsars
• Observed as periodic bursts of radio waves (“static”)
• Typically about 1 second between bursts, some as fast as 1 millisecond
• Also faintly visible at other wavelengths
• A few hundred are now known• What are they? Rapidly
spinning neutron stars, whose strong magnetic fields accelerate plasma to produce the beam of radio waves
Deaths of Stars (summary)
• If less than 10 solar masses, nuclear fusion stops with carbon and oxygen; outer layers are ejected as a planetary nebula; core becomes a white dwarf.
• If greater than 10 solar masses, nuclear fusion proceeds all the way to iron formation; core collapse results in type-II supernova explosion; end result is (often) a neutron star
What is the solar system made of?
• 74% hydrogen• 25% helium• 1% everything else (esp.
carbon, oxygen, silicon, iron)
Where did these heavy elements come from?
Elemental abundances are (roughly) what results from supernova explosions:
Medium-weight elements (C, N, O, . . . up to Fe) are produced in fusion reactions before the explosion.
Still heavier elements are produced in the explosion itself.
We are stardust,
We are billion year old carbon.
-- Joni Mitchell, 1969