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Outline
• Core-Collapse Supernova – White Dwarfs and the Chandrasekhar Limit
• Supernova Remnants – Neutron Stars – Pulsars – Black Holes
• Type Ia Supernova
Above the Chandrasekhar Limit
• Electrons can no longer hold up the star • Atoms collapse under gravity to make
neutrons • Create a Neutron Star
• Collapse releases energy, which makes a Supernova
Types of SN
• Alex Heger No Core Collapse
Type II
Core Collapse
Type Ia
Type Ib/c
Core-Collapse Supernova
• Center of star (>8 M¤) burns to Fe • No more energy, starts to collapse • Too Massive to stop at white dwarf
Core-Collapse SN
• Core converts into neutron star, releases 1053 ergs of neutrinos
• Infalling material “bounces” off the neutron star
• Creates Explosion + nuclear burning
• Explosion has 1051 erg of energy
Core-Collapse SN
Light Curve
• Brightens over a few days • Fades over a few hundred • Powered by radioactive
decay of Ni and Co
SN 1937C, Baade & Zwicky 1938
Supernova Remnants
• Phases: – Coasting – Blastwave – Snowplow – Termination
SN 1604, Kepler‘s Supernova
Coasting Phase
• Material expands ballistically 1987A
Coasting Phase
• Material expands ballistically 1987A
Coasting Phase
• When does coasting end?
Coasting Phase
• When does coasting end?
• As SN expands, it sweeps up mass • Eventually, mass swept up is about the same
as the initial mass ejected • Starts to decelerate • Happens after ~100-1000 years, depending
on conditions
Blastwave
SNR 0509 in LMC
Blastwave
• Over-pressured shock wave moving outward
• Energy is conserved • Self-similar solution can
be found • Worked out by Taylor
(1950) and Sedov (1959)
Types of SN
• Alex Heger No Core Collapse
Type II
Core Collapse
Type Ia
Type Ib/c
Neutron Stars
• Leftovers of supernovae
• Super-dense, made of mostly neutrons
• Like a giant nucleus
Neutron Stars
White Dwarfs
Properties of Neutron Stars • Neutron star is supported by degenerate neutron pressure: so densely packed that resist any further compression
• Interior structure: Solid iron crust + sea of densely packed neutrons (superfluid) • Rotate rapidly! E.g. while The Sun and other stars take weeks to make a single rotation around their axis, the Crab pulsars rotates 30 times per second • Extreme magnets: 1012 to 1015 times stronger than the Earth’s magnetic field à radiate like a lighthouse, only in two narrow beams along north and south magnetic poles!
Pulsars from Neutron Stars
• Jocelyn Bell, a graduate student working with a group of English astronomers discovered a periodic signal in the radio part of the spectrum, coming from a distant galaxy (1967)
• Astronomers considered (briefly) the possibility of an alien civilization sending the regular pulses! “LGM1”
• Several thousands of pulsars known today
PSR 0329+54 Interval between pulses=0.714 seconds
Pulsars are like cosmic lighthouses
• Radiation permitted to escape only along magnetic poles.
• Most often, rotation axis ≠ magnetic field axis è this produces a lighthouse effect
Pulsar Beaming
• If these jets are pointed at Earth, we can detect neutron stars. • As the star spins, the jets can sweep past earth, creating a signal that looks like
a pulse. • Neutron stars can spin very rapidly, so these pulses can be quite close together
in time! But they are precise, the best clocks ever!
Pulsars are slowly slowing down?
• Observations show that the pulsar rotation period (= time it takes to spin once around its axis) slightly increases with time è pulsars loose their energy
• An isolated pulsar slows down as it ages, so its pulse period increases. è faster pulsars are younger.
• Sometimes the pulsar’s diameter shrinks slightly, causing a momentary increase in the pulsar’s rotation
• These “glitches” are short lived, and the spin rate begins to decrease again.
The Crab Nebula
Remnant of 1054 AD SN
The Crab pulsar: periodic flashes of radiation detected at radio, visible, and X-ray wavelengths
Variety of Pulsars • Millisecond pulsars: have period of 1 to
10 milli (10-3) seconds; majority are in close binary systems. – Origin: a slow, aging pulsar is spun up by
mass transfer from its companion.
• Magnetars: highly magnetized neutron stars
• Most pulsars emit both visible and radio photons in their beams, older neutron stars just emit radio waves.
• Some pulsars emit very high energy radiation, such as X-rays and/or gamma-rays
What is the velocity on the surface of a milliseond pulsar?
What is the velocity on the surface of a milliseond pulsar?
Answer: v = 2πR / P For P = 0.001 seconds, v = 0.2c at the equator
Black Hole in 1987A?
• No pulsar 1987A
How do you make a SN?
• A few different ways: – Core Collapse – Type Ia (degenerate collapse?) – Pair Instability – Jet Powered?
Types of SN
• Alex Heger No Core Collapse
Type II
Core Collapse
Type Ia
Type Ib/c
Type Ia
• Start with a White Dwarf • Somehow get above Chandrasekhar limit
– Accretion from companion? – Merger?
• Start a little nuclear burning in the core
Type Ia Supernova
movie
Type Ia Supernova
• Burn most of the star up to Fe/Ni
• No compact remnant left, no pulsar
Tycho’s SN (1572) remnant
Type Ia
• All have (about) the same mass and power
• Light curve decline rate and brightness correlated
• Standard Candle – used to determine distance