Grades - Physics & Astronomyhomepage.physics.uiowa.edu/~pkaaret/s09/L16_compactobjects.pdf · •...
Transcript of Grades - Physics & Astronomyhomepage.physics.uiowa.edu/~pkaaret/s09/L16_compactobjects.pdf · •...
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Grades
• Scores and curve for exam #2 on ICON, answers and key will come soon
• Extra credit by observing. Two observing lists each worth 10 points. Deadline for completing first list is March 26.
• No class on March 13.
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White Dwarfs and Neutron Stars
• White dwarfs– Degenerate gases– Mass versus radius relation
• Neutron stars– Mass versus radius relation– Pulsars, magnetars, X-ray pulsars, X-ray
bursters
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White dwarf• Core of solar mass star
• Degenerate gas of oxygen and carbon
• No energy from fusion or gravitational contraction
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Thermal Pressure:
Depends on heat content
The main form of pressure in most stars
Degeneracy Pressure:
Particles can’t be in same state in same place
Doesn’t depend on heat content
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Fermi exclusion principle
• No two electrons can occupy the same quantum state
• Quantum state = energy level + spin• Electron spin = up or down
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Electron orbits
Only two electrons (one up, one down) can go into each energy level
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Electron energy levels
• Only two electrons (one up, one down) can go into each energy level.
• In a degenerate gas, all low energy levels are filled.
• Electrons have energy, and therefore are in motion and exert pressure even if temperature is zero.
• White dwarfs are supported by electron degeneracy.
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Mass versus radius relation
• For objects made of normal matter, radius tends to increase with mass
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Mass versus radius relation
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Maximum white dwarf mass
• Electron degeneracy cannot support a white dwarf heavier than 1.4 solar masses
• This is the “Chandrasekhar limit”
• Won Chandrasekhar the 1983 Nobel prize in Physics
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What happens to a star more massive than 1.4 solar masses?
A) There aren’t anyB) They shrink to zero sizeC) They explodeD) They become something else
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Neutron Stars
• Degenerate stars heavier than 1.4 solar masses collapse to become neutron stars
• Formed in supernova explosions• Electrons are not separate
– Combine with nuclei to form neutrons
• Neutron stars are degenerate gas of neutrons
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Neutron energy levels
• Only two neutrons (one up, one down) can go into each energy level.
• In a degenerate gas, all low energy levels are filled.
• Neutrons have energy, and therefore are in motion and exert pressure even if temperature is zero.
• Neutron star are supported by neutron degeneracy.
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Neutron Star Mass vs Radius
20
15
10
5
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Neutron Stars
• Very compact – about 10 km radius• Very dense – one teaspoon of neutron star
material weighs as much as all the buildings in Manhattan
• Spin rapidly – as fast as 600 times per second• High magnetic fields – compressed from
magnetic field of progenitor star
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Spin up of neutron star
Collapse of star increases both spin and magnetic field
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Pulsars
Discovered by Jocelyn Bell in 1967.
Her advisor, Anthony Hewish, won the Nobel Prize in Physics for the discovery in 1974.
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Pulsars
Energy source is spin down of neutron star.
Must lie along pulsar beam to see pulsed signals.
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Crab Pulsar
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MagnetarsMagnetic fields so strong that they produce starquakes on the neutron star surface.
These quakes produce huge flashes of X-rays and Gamma-rays.
Energy source is magnetic field.
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X-Ray Pulsars
Neutron star in binary system with a normal star
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X-Ray Pulsars
High magnetic field neutron stars make regular pulsations. Energy source is gravitational energy of infalling matter.
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X-ray Bursters
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X-ray Burst
Low magnetic field neutron stars make X-ray bursts.
Source of energy is nuclear burning.
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Review Questions
• What is the Fermi exclusion principle?• Does a more massive white dwarf have a
larger or smaller radius than a less massive one?
• What is the maximum mass of a white dwarf?• What are some of the properties of neutron
stars?• Why do many neutron stars spin rapidly?• In what different forms does one find neutron
stars?