Lecture 15: Black Holes and Neutron Stars- Cosmological Snark Hunts

"In the Midst of the Word he was trying to say, In the Midst of his laughter and glee, He suddenly and silently vanished away, For the Snark was Boojum, you see?"

L. Carroll, Hunting of the Snark

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    • Date: March 23, 1995
    Reading Assignment: pp. 495-519

    Description : formation and characteristics of Neutron Stars and Black Holes

    Objectives

  • be able to describe how neutron stars form and they fit into the models of stellar evolution
  • be able to describe the composition, size, rotation, and magnetic field of typical neutron stars
  • be able to describe why neutron stars are pulsars and how pulsars appear from Earth
  • be able to describe how neutron stars appear in binary systems when there is mass transfer
  • be able to describe how pulsars can be used to test theories of relativity and to detect planets
  • be able to describe why black holes form
  • be able to describe the conditions near a black hole
  • be able to describe how black holes might be detected

    • Lecture Outline

    Slide # 1: Black Holes and Neutron Stars

  • The Cosmological Snark Hunt
    • Slide # 2: The Steps
  • gas cloud
  • fragmentation
  • protostar
  • Helmholtz contraction
  • Hayashi track
  • ignition
  • adjustment to the Main Sequence
    • Slide # 3: The HR Diagram (GRAPHICS)
  • pre-main sequence
    • Slide # 4: Post Main Sequence- Internal Changes
  • core depletion of hydrogen
  • hydrogen shell burning
  • helium flash and helium core burning
  • helium depletion
  • helium shell burning
  • helium shell flashes
  • planetary nebula - white dwarf
    • Slide # 5: Post-Main Sequence
  • subgiant branch
  • giant branch
  • horizontal branch
  • asymptotic giant branch
  • planetary nebula
  • white dwarf
    • Slide # 6: The HR Diagram (GRAPHICS)
  • one solar mass star over 8 billion years
    • Slide # 7: Helium Shell Flashes
  • the density of the Helium shells increase
  • contraction of the core increases the density
  • Helium burning becomes unstable
  • helium fusion rate depends strongly on temperature
  • Helium shell flashes begin
  • the star begins to pulse
    • Slide # 8: Helium Shell Burning (GRAPHICS)
  • 4 layers in the star
    • Slide # 9: Stellar Envelope Separates
  • extra energy pushes stellar envelope away from the core
  • hydrogen rich material escapes
  • nuclear burning ends in and around core
  • no pressure from the envelope
  • planetary nebula forms
    • Slide # 10: Planetary Nebula (GRAPHICS)
  • M57 - the Ring Nebula
    • Slide # 11: White Dwarfs
  • very high density
  • 107 gm per cubic cm
  • 1 ton = 1 teaspoon
  • composition mostly carbon
  • some oxygen from more massive stars
  • very common
  • 9% of stars are white dwarfs
    • Slide # 12: The HR Diagram (GRAPHICS)
  • Why?
    • Slide # 13: Constellation Corner (GRAPHICS)
  • Constellation De Jour
    • Slide # 14: Lyra and Cygnus (GRAPHICS)
  • Fairfax - May 1 - West - 1am - 4.0
    • Slide # 15: A Rule of Thumb
  • constellations and stars rise and set about 2 hours earlier every month
  • if Orion sets at 10pm tonight, it will set at 8pm in one month
  • 12 months = 24 hours
  • 1 month = 2 hours
    • Slide # 16: Lyra and Cygnus (GRAPHICS)
  • Fairfax - May 1 - East - 1am - 4.0
    • Slide # 17: Lyra and Cygnus (GRAPHICS)
  • Fairfax - May 1 - East - 1am - 4.0
    • Slide # 18: Evolution of Massive Stars
  • massive stars have the same internal changes as low mass stars
  • same types of compositional changes occur
  • massive stars evolve more rapidly
  • extra gravity causes rapid nuclear burning
  • much higher luminosity
  • massive stars can burn heavier elements
  • extra gravity increases temperature and pressure
    • Slide # 19: Type II Supernova
  • lots of hydrogen in the spectral lines
  • bump in light curve
    • Slide # 20: Type II Supernova
  • core collapse of SINGLE MASSIVE STAR
  • core made of degenerate iron
  • mass of core exceeds Chandrasekhar mass
  • electrons absorbed into nuclei
  • no pressure from electrons, so core collapses
  • very luminous
    • Slide # 21: What happens to the core?
  • original core size = 10,000 km
  • size of Earth
  • supported by degenerate electron pressure
  • new core size = 20 km
  • size of Fairfax County
  • supported by degenerate neutron pressure
    • Slide # 22: Core Composition
  • before collapse
  • composition - iron & electrons
  • density > 107 gm per cubic cm
  • after collapse
  • composition - neutrons
  • density > 1014 gm per cubic cm
    • Slide # 23: Neutron Stars (GRAPHICS)
  • size comparison
    • Slide # 24: Neutron Star
  • surface temperature > 1 million K
  • most energy in X-ray according to Wien's law
  • magnetic field causes more radio energy to be emitted
  • size = 10 km radius
  • L = R2 T4
  • L = (10/400,000)^2 x (1,000,000/6000)^4
  • L = 0.5 times the luminosity of the Sun
  • MUCH more luminosity from magnetic field and rotation
    • Slide # 25: Neutron Stars
  • few normal elements
  • almost no hydrogen, helium, iron...
  • mostly nuclear particles
  • neutrons
  • some heavy elements in the crust
    • Slide # 26: Neutron Stars (GRAPHICS)
  • high gravity creates unusual conditions
    • Slide # 27: Neutron Stars
  • rapid rotation
  • angular momentum is conserved
  • rotation rate increases with small size
  • rotation rate approximate once per second
  • very high magnetic field
  • compression increases magnetic field
  • trillions of times stronger than Earth's
    • Slide # 28: Pulsars
  • magnetic fields only allow EM energy to escape in certain directions
  • charged particles are trapped by magnetic fields
  • the rotation causes the magnetic field to rotate
  • the magnetic field is not aligned with the rotation axis
  • EM energy appears to pulse because of the rotation
  • 1 pulse approximately every second
    • Slide # 29: Pulsars (GRAPHICS)
  • the lighthouse effect
    • Slide # 30: Pulsars (GRAPHICS)
  • the lighthouse effect
    • Slide # 31: Pulses
  • most pulsars emit radio energy
  • hundreds detected in radio
  • some pulsars are seen at all wavelengths
  • visible, infrared, ultraviolet, gamma-ray, x-ray
    • Slide # 32: Pulsar Periods
  • normally between 0.03 and 0.3 seconds
  • some are longer - every few seconds
  • some are very short - every 0.002 seconds
    • Slide # 33: Discovery of Pulsars
  • Jocelyn Bell 1967
  • graduate student in UK
  • discovered accidentally
  • Anthony Hewish 1974
  • Nobel Prize in Physics
  • explained what caused the pulses
  • Bell's thesis advisor
    • Slide # 34: LGM and BEM's
  • at first, the source was unidentified
  • very regular pulses
  • faster than any known star could vary
  • sources were thought to be from intelligent origin
  • too short and too regular to be natural
    • Slide # 35: Pulsar Clocks
  • the timing of pulses is very accurate
  • as good as atomic clocks
  • timing depends on the rotation of a very large neutron star
  • not easy to speed up or slow down
    • Slide # 36: Observational Evidence
  • Supernova was observed in Taurus in 1054
  • recorded by American Indian and Asians
  • Crab Nebula now appears in Taurus
  • located approximately where Supernova was observed
  • Crab Pulsar observed in the center of the nebula
  • seen at all wavelengths
  • approximately 0.03 second rotation periond
    • Slide # 37: Observational Evidence (GRAPHICS)
  • the Crab Nebula
    • Slide # 38: Expansion of the Crab
  • we can measure the expansion speed
  • radial velocity measurements using Doppler shift
  • proper motion observations and distance measurements
  • few thousands of km per second
    • Slide # 39: Expansion Measurements (GRAPHICS)
  • we can calculate when it occurred
  • about 1054 AD
    • Slide # 40: Neutron Star Glitches
  • occasionally, the rotation period of a neutron star will change suddenly
  • very small, but measureable changes
  • change occurs very rapidly- few seconds
  • star quake causes readjustment in star
  • neutron star rotation period changes
    • Slide # 41: Gravity vs Pressure
  • gas pressure
  • gas cloud until the formation of a helium core
  • most "normal stars"
  • degenerate electron pressure
  • helium cores until helium flash
  • white dwarfs
  • degenerate neutron pressure
  • neutron stars
    • Slide # 42: What happens if you add more mass?
  • density increases from the extra gravity
  • more gravity causes more compression
  • eventually, nothing will stop the collapse
    • Slide # 43: Black Holes
  • eventually degenerate neutron pressure cannot stop the collapse of the star
  • star becomes smaller than its event horizon
  • A Black Hole forms
  • no light can escape
    • Slide # 44: Schwarzschild Radius
  • size of an object when escape velocity equals the speed of light
  • light cannot escape from the object
  • nothing can escape from the object
  • examples
  • Earth = 1 cm
  • Jupiter = 3 meters
  • 3 solar mass star = 9 km
  • also called the Event Horizon
    • Slide # 45: Theory of Relativity
  • gravitational energy
  • warped space
  • slowed time
    • Slide # 46: Gravitational Energy
  • it takes energy to overcome gravity
  • everyday experience
  • photon energy is related to its frequence
  • photoelectric effect
  • photon energies will change due to gravitational fields
    • Slide # 47: Gravitational Redshift (GRAPHICS)
  • photon energy is reduced due to gravity
  • object appears redder
    • Slide # 48: Warped Space (GRAPHICS)
  • gravity always bends light
    • Slide # 49: Warped Space (GRAPHICS)
  • strong gravity bends light more
  • black holes have very strong gravity
  • very large bending of light
    • Slide # 50: Slowed Time
  • gravity slows down time
  • observed on Earth
  • at the event horizon of a black hole, time stops
  • slowing time is called time dilation
    • Slide # 51: Singularity
  • nothing will stop the collapse of a black hole
  • no force will be more powerful than gravity
  • inside the event horizon, the black hole will become a point
  • when the star has no size, it is called a singularity
  • the size of the event horizon will be determined by the mass of the black hole
  • you cannot see inside the event horizon
    • Slide # 52: Detecting Black Holes
  • binary star systems
  • if one star is a black hole and the other is a normal star, you might be able
  • Cygnus X-1 is a good candidate for a black hole
    • Slide # 53: Spectroscopy
  • binary star system with an invisible companion
  • star #1 = Blue supergiant = 30 solar masses
  • orbital period = 5.6 days
  • orbital velocity measured by Doppler shift
  • gas apparently is flowing to the invisible companion
    • Slide # 54: Orbital Information
  • invisible companion must have a mass between 5 and 10 solar masses
  • too large for a neutron star
    • Slide # 55: X-ray Observations
  • X-ray observations require several million degree gas to be near Cygnus X-1
  • hot gas is creates the X-rays
  • X-ray variations indicated that size of the object must be less than 300 km
  • too small to be a normal star
    • Slide # 56: Are there really black holes?
  • Yes...
  • Wallin's odds of black holes being real = 98%