AST112 EXAM 2 White Dwarfs and Neutron Stars
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Review – Compact objectsform from the deathof stars
Low Mass Stars•Red Dwarfs•Sunlike Stars•Red giants•White Dwarfs
Massive Stars• Iron core collapse• Neutron Stars and Black Holes
- •mass similar to the Sun’s
- •diameter about that of the Earth
- •high surface temperature (typically 25,000 K)
- •but very dim (small size!)
- •mainly carbon and oxygen
- •thin H or He surface layer
- •no fuel to burn
- •residual heat inside
- initially hot (150,000K) >gradually cool > finally, a black dwarf
- White dwarfs are in hydrostatic equilibrium– Gravity electron degeneracy
- Density much greater than Earth
- •Earth = 5.5 grams/cc
- •WD = tons/cc!!
White Dwarfsin Binaries
• In a binary system, a white dwarf maycapture gas from its companion– accretion disk– hydrogen is sucked off larger star andheated on the white dwarf’s surface– gas eventually ignites – a fusion reaction in adegenerate gas results in an explosion: anova
What Causes aNova?
Main Ideas•Novae are white dwarfs inbinaries•Mass is transferred towhite dwarf•Mass builds up•Ignites nuclear fusion•Surface of white dwarfexplodes
Novae Can Become Supernovae
- Novae may finally become supernovae:
- • Mass of white dwarf continues to grow
- • Eventually the mass becomes too greatwhite dwarf collapsesthe core ignites, blowing the whitedwarf apart!
- A giant ball of neutrons.
- • Mass : at least 1.4 x mass ofthe Sun.• Diameter: 20 km!
- • Density: 1018 kg/m3– A thimble weighs as much as amountain• Rotation period: 1 – 0.001 seconds!
- • Magnetic fields as strong as the Sun, but in thespace of a city.
But What IS aNeutron Star?
- Dense ball of collapsedmatter
- • Atoms so compressed thatelectrons and protons areforced together to becomeneutrons
- • Basically, a giant atomicnucleus
Neutron starshave threelayers:
- – a mm thickatmosphere
- – an iron crust100s of mthick
- – a superfluidneutron corewith The coreand crust spinindependently
Pulsars – RotatingNeutron Stars
- – Powerful magnetic field beams radiation energy intwo opposing directions
- – The beamed radiation together with the rapid spinmakes the pulsar pulse
Pulsars emit beams of radio light. As the pulsarrotates, the beams sweep across the sky. When thebeam "sweeps" over Earth, we detect the radiation, asa ‘pulse.’
• InterstellarLighthouses.• See periodicbursts ofradiation.• Perfect clocks.
Pulsar rotation can change overtime
- • Electric/magnetic fields drag with surrounding gas– neutron stars generally slow down withage
- • Eventually spin so slowly that their pulses becometoo faint to detect - the pulsar “dies”
- • But neutron stars can sometimes speed up
- – Crust may shrink and suddenly crack making the star’sradius smaller
- – Conservation of angular momentum then requires theneutron star to speed up
- – Occasionally the slowing crust will reach a critical valueand the core will shed some of its rotational energy to thecrust, speeding it up
- – Intense X-rays from neutron stars inbinary systems There are several typesof X-ray binaries
- • X-ray bursters from gas falling on theneutron star
- • X-ray pulsars from hot-spots on the neutronstar
- • infalling gas can “spin up” an old neutron star
- • The planets makes thepulsar move in space
- • Radio pulses arrive slightlyearly or slightly late
- • Changes in arrival timeeasy to detect with an atomicclock
- 3 pulsar planets
- • 0.02, 4.3, and 3.9Earth masses
- • orbits fit within theorbit of Mercury
- • orbital periods of 25, 66and 98 days
- • the pulsar may have anasteroid belt
- White Dwarfs
- •Formed from the cores of stars lessthan 8 times the mass of the Sun
- •up to 1.4 solar masses
- •made of compressed He, C, or Fe
- •about the size of the Earth
- •One cc would weigh about 3 tons
- Neutron Stars
- •Formed in supernova explosions
- •from 1.4 to about 3 solar masses
- •Made of pure neutrons – a giant atomic nucleus
- •About 20 km in diameter
- •One cc would weigh about a million tons
- Black Holes
- •Formed in Supernova explosions
- •Usually a few times the mass of the Sun
- •A solar mass black hole is about 3 km indiameter
- •Density is infinite
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