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Astronomy 114
Lecture 22: Neutron Stars
Martin D. Weinberg
UMass/Astronomy Department
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—1/20
Announcements
PS#5 due Wednesday
Questionnaires?
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—2/20
Announcements
PS#5 due Wednesday
Questionnaires?
Today:
Supernovae: two types!Standard candles
Neutron stars
Pulsars
Neutron Stars & Black Holes, Chaps. 23 & 24
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—2/20
Mass transfer in binary stars⇒Supernova
What happens when binary star evolves?
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—3/20
Mass transfer in binary stars⇒Supernova
What happens when binary star evolves?Suppose one star is a white dwarf
Material from evolving star is transferred to whitedwarf
Mass on surface of white dwarf builds up
Exceeds Chandrasekhar limits
Begins to collapse . . .
Supernova
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—3/20
Supernova as standard candles
Standard candle: an object whose luminosity is knownand reveals its distance by a brightness measurement
Two kinds of supernova:
Type I No hydrogen lines Little hydrogen in progenitor
Type II Hydrogen lines Hydrogen in star’s envelope
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—4/20
Supernova as standard candles
Standard candle: an object whose luminosity is knownand reveals its distance by a brightness measurement
Two kinds of supernova:
Type I No hydrogen lines Little hydrogen in progenitor
Type II Hydrogen lines Hydrogen in star’s envelope
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—4/20
Supernova as standard candles
Standard candle: an object whose luminosity is knownand reveals its distance by a brightness measurement
Two kinds of supernova:
Type I No hydrogen lines Little hydrogen in progenitor
Type II Hydrogen lines Hydrogen in star’s envelope
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—4/20
Supernova as standard candles
Standard candle: an object whose luminosity is knownand reveals its distance by a brightness measurement
Two kinds of supernova:
Type I No hydrogen lines Little hydrogen in progenitor
Type II Hydrogen lines Hydrogen in star’s envelope
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—4/20
Supernova as standard candles
Standard candle: an object whose luminosity is knownand reveals its distance by a brightness measurement
Two kinds of supernova:
Type I No hydrogen lines Little hydrogen in progenitor
Type II Hydrogen lines Hydrogen in star’s envelope
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—4/20
Supernova lightcurve
Data suggests that Type Ia curves are always similar
Good standard candles
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—5/20
The latest results
Even more violent events
Hypernovae
100 times more energy than supernovae
Remnants in distant galaxies, may be related toGRBs
Gamma ray bursts (GRBs)
γ rays (and light, and neutrinos)
Discovered by satellites designed to verify theconditions of the Nuclear Test Ban Treaty (1970s)
Compton Gamma Ray Observatory: observesone per day
Length: seconds or minutes, longer afterglows atlower energy
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—6/20
Type II Supernova ⇒ Neutron star
During collapse: e− + p → n+ neutrino
Roughly 1057 neutrinos created in the iron core asprotons are converted to neutrons
After the “bounce”, becomes a sphere of tightlypacked neutrons, known as: neutron star
Neutron star: single humongous atomic nucleus
Mass between 1 and 3 solar masses
Radius: 5 to 20 kilometers
Strong magnetic fields
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—7/20
Magnetic fields
Magnetic fields are produced by electric currentsExamples:
Currents in wires
Currents caused by electrons in atomic orbits
A permanent magnet is a collection of atoms whoseinteral currents are lined up in a single direction onaverage
A magnet will tend to try to align itself with amagnetic field
We call that direction a field line
[Demo]
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—8/20
How magnetic fields work
Current in a straight wire
The magnetic field lines are concentric circles
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—9/20
How magnetic fields work
Electric current in a circular loop
Magnetic field concentrated in the center of the loop
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—9/20
How magnetic fields work
Electric current in a circular loop
Magnetic field concentrated in the center of the loop
Stacking multiple loops concentrates the field;solenoid or electromagnet
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—9/20
How magnetic fields work
Electric current in a circular loop
Magnetic field concentrated in the center of the loop
Stacking multiple loops concentrates the field;solenoid or electromagnet
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—9/20
How magnetic fields work
Circulating electic currents in the Earth’s moltenmetalic core generate a magnetic field
Current loop
Similar in Sun
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—9/20
Magnetic fields in neutron stars
Rapidly rotating:
Frequency: 1000 rotations/second(1 rotation/month for the Sun)
Due to conservation of ang mom (recall skater)
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—10/20
Magnetic fields in neutron stars
Rapidly rotating:
Frequency: 1000 rotations/second(1 rotation/month for the Sun)
Due to conservation of ang mom (recall skater)
Strongly magnetized:
1012 Gauss (1 and 0.5 Gauss for the Sun & Earth)
Why? Magnetic field lines are conserved andcompressed as star collapses.
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—10/20
Magnetic fields in neutron stars
Rapidly rotating:
Frequency: 1000 rotations/second(1 rotation/month for the Sun)
Due to conservation of ang mom (recall skater)
Strongly magnetized:
1012 Gauss (1 and 0.5 Gauss for the Sun & Earth)
Why? Magnetic field lines are conserved andcompressed as star collapses.
Very hot:
106 K at surface after collapse (5800 K for Sun)
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—10/20
NS supported by degeneracy pressure
Core is compressed to nuclear density
Neutrons also must obey the Pauli exclusion principle
Density of 1 ton/cm3, electrons are degenerate
Density of 4 × 108 tons/cm3 neutrons aredegenerate
Upper limit to mass: M > 2 − 3M⊙
Above this mass, the degenerate-neutronpressure is insufficient to prevent collapse
Larger mass: black hole (?)
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—11/20
NS structure
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—12/20
NS structure
Neutron star “quake”
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—12/20
Beamed radiation (1/2)
The strong magnetic field and rapid rotation of aneutron star generate strong electromagnetic fields
Like electric power generators: rotating magnetsinside a coil of wires
Field generated by rotating magnetized neutron starpull electrons from neutron star surface
Charged particles confined to magnetic field lines
Acceleration: causes synchrotron radiation
We see narrow beams of radiation along themagnetic poles
Only observe if beam is pointing toward observer
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—13/20
Beamed radiation (1/2)
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—14/20
Beamed radiation (2/2)
Magnetic pole does not coincide with rotation axis
Beams of radiation are at an angle to the rotationaxis of the neutron star
As the neutron star rotates, the beams swing aroundin a cone
If a beam happens to sweep across our location inspace, we see a brief flash of light (“Lighthouseeffect”)
Neutron stars whose beams of electromagneticradiation happen to sweep across us are calledpulsars
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—15/20
Pulsars
Discovered in 1967: origin of pulses was initiallyunknown
Strong radio sources
Emit their bursts of radio emission at very regularintervals: first pulsar had period of 1.34 seconds
Periods are typically less than a second. Some asshort as millisecond
Time between pulses corresponds to one stellarrotation
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—16/20
Crab pulsar
P = 0.033 seconds
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—17/20
Crab pulsar
Observable in X-rayR = 10 km = 0.000014 R⊙
T = 106K = 170T⊙
Ln/L⊙ =
(Rn/R⊙)2(Tn/T⊙)4 = 0.2
Wien’s law: x-ray
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—18/20
Pulsars slow with time
Crab Nebula: P = 0.033 seconds
P increasing by 0.001 seconds per century
Rotational energy is converted to kinetic energy ofelectrons and other charged particles
Accelerated charges radiate (photons)
Energy of motion converted to photons
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—19/20
Binary systems with Pulsars
Pulsar in a close binary system with an ordinary starcan increase its rotation speed
The force exerted by the falling gas can make thepulsar spin faster (angular momentum)
Pulsars with millisecond periods have all been spunup by mass tranfer
Powerful energy sources
A114: Lecture 22—02 Apr 2007 Read: Ch. 23,24 Astronomy 114—20/20
