Neutron Stars

1
Neutron Stars

• Chapter Twenty-One

2
Scientists first proposed the existence of
neutronstars in the 1930s observed in 1960s
supernovae remnants

• A neutron star is the former core of a giant star
  that exploded.
• Consists primarily of closely packed degenerate
  neutrons
• A neutron star typically has a diameter of about
  20 km, a mass less than 3 M_, a magnetic field
  1012 times stronger than that of the Sun, and a
  rotation period of roughly 1 second
• Zwicky and Baade proposed that a highly compact
  ball of neutrons would produce a degenerate
  neutron pressure in star remnants too large to
  become white dwarfs

3
Guiding Questions

• What led scientists to the idea of a neutron
  star?
• What are pulsars, and how were they discovered?
• How did astronomers determine the connection
  between pulsars and neutron stars?
• How can a neutron star supply energy to a
  surrounding nebula?
• What are conditions like inside a neutron star?
• How are some neutron stars able to spin several
  hundred times per second?
• Why do some pulsars emit fantastic amounts of X
  rays?
• Are X-ray bursters and novae similar to
  supernovae?
• How massive can a neutron star be?

4
13.2 Neutron Stars

• What is a neutron star?
• How were neutron stars discovered?
• What can happen to a neutron star in a close
  binary system?

5
Neutron Star

• Neutron star - sphere of degenerate neutrons
• Produced during core collapse in high-mass star
  followed by supernova outburst
• Neutron star predicted by Robert Oppenheimer in
  1930s
• Weight of overlying layers supported by pressure
  from degenerate neutrons rather than degenerate
  electrons as in white dwarf (Pauli exclusion
  principle)
• Predicted properties
• Mass 1.4-3 MSun
• Radius
• Density 1014-1016 g/cm3
• Central temperature 1010 K
• Predicted object clearly so small that little
  chance of observing
• Prediction forgotten until discovery of pulsar in
  1968

6
PulsarPulsating Radio Star

• Pulsar - rapidly spinning neutron star, emits
  high-intensity bursts of radio radiation
• 1967 object found in Vulpecula emitting pulses of
  radio radiation with period of 1.337 seconds
• Within months more found with period for radio
  pulses between 0.001 second and a few seconds
• Constancy of time interval between pulses can be
  one part in 10 million
• Equivalent to gain or loss of 1 second/year for
  clock

7
The discovery of pulsars in the 1960s(1967
Jocelyn Bell with a radio telescope)
stimulatedinterest in neutron stars
8
Pulsar Emitting Region Size

• Emitting region cannot be larger than distance
  light can travel in time interval for pulses
• Pulse duration of 0.001 s, emitting region km
• White dwarfs have radii of 5000 to 10,000 km,
  thus smaller than white dwarf
• Neutron star is only star small enough to be
  pulsar

Assume pulsar turns on everywhere at same time
Earth
9
Pulsar Emitting Region Size

• Emitting region cannot be larger than distance
  light can travel in time interval for pulses
• Pulse duration of 0.001 s, emitting region km
• White dwarfs have radii of 5000 to 10,000 km,
  thus smaller than white dwarf
• Neutron star is only star small enough to be
  pulsar

Assume pulsar turns on everywhere at same time
Earth
10
Pulsars are rapidly rotating neutron starswith
intense magnetic fields

• A pulsar is a source of periodic pulses of radio
  radiation
• These pulses are produced as beams of radio waves
  from a neutron stars magnetic poles sweep past
  the Earth

11
Crab Nebula - SN1054
Radio pulses from first pulsar in 1967
Crab Pulsar
Inner smooth amorphous structure has continuous
spectrum
Outer filamentary structure has emission line
spectrum
Optical pulses from Crab Nebula pulsar
12
Crab pulsar is remnant of supernova observed by
Yang Wei Te in 1054

• Intense beams of radiation emanate from regions
  near the north and south magnetic poles of a
  neutron star
• These beams are produced by streams of charged
  particles moving in the stars intense magnetic
  field
• On state when beam is aimed at earth.

13
(No Transcript)
14
(No Transcript)
15
Radio telescope image of supernova remnant and
pulsar
16
Superfluidity and superconductivity are amongthe
strange properties of neutron stars

• A neutron star consists of a superfluid,
  superconducting core surrounded by a superfluid
  mantle and a thin, brittle crust
• There is evidence for an atmosphere

17
(No Transcript)
18
Evidence for an atmosphere of a neutron star
19
Pulsars gradually slow down as they radiate
energy into space

• The pulse rate of many pulsars is slowing
  steadily
• This reflects the gradual slowing of the neutron
  stars rotation as it radiates energy into space
• Sudden speedups of the pulse rate, called
  glitches, may be caused by interactions between
  the neutron stars crust and its superfluid
  interior

20
(No Transcript)
21
The fastest pulsars were probably created by mass
transfer in close binary systems

• If a neutron star is in a close binary system
  with an ordinary star, tidal forces will draw gas
  from the ordinary star onto the neutron star
• The transfer of material onto the neutron star
  can make it rotate extremely rapidly, giving rise
  to a millisecond pulsar

22
(No Transcript)
23
(No Transcript)
24
Neutron stars in close binary systems are also
pulsating X-ray sources

• Magnetic forces can funnel the gas onto the
  neutron stars magnetic poles, producing hot
  spots
• These hot spots then radiate intense beams of X
  rays
• As the neutron star rotates, the X-ray beams
  appear to flash on and off
• Such a system is called a pulsating X-ray variable

25
(No Transcript)
26
(No Transcript)
27
Novas Mild stellar explosion

• Occur in close binary systems stars with orbital
  periods of about 4 hours.
• One member is a white dwarf
• Hydrogen gas from companion star forms accretion
  disk on the white dwarf component.
• When temperature reaches 5 106 K,
  thermonuclear explosion begins
• This causes a runaway fusion with temperatures
  about 108 K
• Envelope expands to red giant size with enhanced
  carbon, nitrogen, oxygen and neon.

28
Explosive thermonuclear processes on white dwarfs
and neutron stars produce novae and bursters

• Material from an ordinary star in a close binary
  can fall onto the surface of the companion white
  dwarf or neutron star to produce a surface layer
  in which thermonuclear reactions can explosively
  ignite
• Explosive hydrogen fusion may occur in the
  surface layer of a companion white dwarf,
  producing the sudden increase in luminosity that
  we call a nova
• The peak luminosity of a nova is only 104 of
  that observed in a supernova
• Explosive helium fusion may occur in the surface
  layer of a companion neutron star
• This produces a sudden increase in X-ray
  radiation, which we call a burster

29
(No Transcript)
30
White Dwarf Accretion Disk
White dwarfs gravity pulls matter from red giant
companion, but the matters angular momentum
prevents it from falling straight on to the white
dwarfs surface. Infalling matter forms an
accretion disk around the white dwarf.
31
Novae - Observations

• Rapid rise to brightness followed by slow decline
• Increase is few 10,000s
• Decline may be up to year
• Spectra show large Doppler shifts
• Velocities from few 100s to several 1000s of km/s
• Mass ejected in explosion
• Few 0.01 MSun to few 0.1 MSun
• Expanding shell of gas often seen years later
• Recurrence few repeat outburst every 20-30
  years
• Frequency about 30/year in our Galaxy

32
Nova Outburst

• Novae are red giant/white dwarf binary system
• Outburst caused by mass transfer to white dwarf
• Layer of hydrogen-rich matter forms on surface of
  white dwarf
• Hot gas becomes degenerate
• Hydrogen burning begins in run away thermonuclear
  process (hydrogen flash)

33
Light emission versus time from a nova
34
X-ray emission versus time for an X-ray burster
35
Like a white dwarf, a neutron star has an
upperlimit on its mass

• The pressure within a neutron star comes from two
  sources
• One is the degenerate nature of the neutrons, and
  the other is the strong nuclear force that acts
  between the neutrons themselves
• The discovery of neutron stars inspired
  astrophysicists to examine seriously one of the
  most bizarre and fantastic objects ever predicted
  by modern science, the black hole

36
Key Words

• degenerate neutron pressure
• glitch
• millisecond pulsar
• neutron star
• nova (plural novae)
• pair production
• pulsar
• pulsating X-ray source
• superconductivity
• superfluidity
• synchrotron radiation
• X-ray burster