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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