Transcript Introduction to Astronomy

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 Stars
8 November 2006
Today:
• Final days of a massive star
• Neutron stars and how they form
• What we observe: Pulsars
Neutron Star Formation
proton
electron
neutron
neutrino
• 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.
Neutrinos from Supernova 1987a
Size of a neutron star
The discovery of pulsars (1967)
Radio antenna array, near Cambridge, England
Jocelyn Bell
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
The Crab Pulsar (from SN 1054)
Blinks on and off 30 times per second
X-ray images
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