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Stellar RemnantsWhite Dwarfs, Neutron Stars & Black Holes

• These objects normally emit light only due to their very high temperatures.

• Normally nuclear fusion has completely stopped.

• These are very small, dense objects.

• They exist in states of matter not seen anywhere on Earth. They do not behave like normal solids, liquids or gases.

• They often have very strong magnetic fields and very rapid spin rates.

Sirius & Sirius B - a White Dwarf Star

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White Dwarfs• composed mainly of Carbon

& Oxygen• formed from stars that are no

more than 8 Solar masses• White Dwarfs can be no

more than 1.4 Solar masses and have diameters about the size of the Earth (1/100 the diameter of the Sun).

• If a White Dwarf is in a binary system and close enough to its companion star it may draw material off this star. This material can then build up on the surface of the White Dwarf.

A White Dwarf pulling materialoff of another star in a binary system

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White Dwarfs in Binary Systems• This material pulled off the

companion star is mostly Hydrogen.

• As it accumulates on the star it may become hot enough for nuclear fusion to occur.

• The Hydrogen begins to fuse and the White Dwarf emits a bright burst of light briefly.

• We see this on Earth as a nova.

• This process can repeat as new material accumulates.

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Another Kind of Supernova

• If too much material accumulates the White Dwarf may collapse.

• Rapid fusion reactions of Carbon & Oxygen begin. Carbon & Oxygen fuse into Silicon and Silicon into Nickel.

• The energy from this event may cause the entire White Dwarf to explode leaving nothing behind.

• This is called a supernova but it is a different process from that which occurs for massive stars.

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Stellar Remnants and the Chandrasekhar Limit • Stellar remnants

greater than 1.4 Solar masses cannot form White Dwarfs.

• Objects this massive cannot support their own weight but collapse to form either Neutron Stars or Black Holes.

• This maximum mass is called the Chandrasekhar Limit.

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Neutron Stars• Except for a thin crust of

iron atoms a neutron star is composed entirely of neutrons.

• The gravitational forces inside a neutron star are too strong for atoms to exist.

• Instead electrons get crushed into the protons in the atomic nucleus forming neutrons.

• Neutron stars have very intense magnetic fields and very rapid rotation.

Neutron Stars weigh more thanthe Sun and are as large a city.

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Pulsars• Neutron stars can

sometimes be directly observed.

• Astronomers have discovered rapidly spinning stars emitting strong, very regularly timed bursts of radio waves.

• These types of neutron stars are called pulsars.

• Pulsar bursts are as regular as some of the best clocks on Earth.

As the beam from a pulsar sweeps pastEarth we see a brief pulse.

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The Discovery of Pulsars

• In 1967 in Cambridge England, Jocelyn Bell, a graduate student in astronomy, discovered very regularly spaced bursts of radio noise in data from the radio telescope at Cambridge University.

• After eliminating any possible man-made sources she realized this emission must be coming from space.

• The regularity of these pulses at first made her and her co-workers think they had discovered alien life.

• Later they realized these must be due to rapidly spinning neutron stars.

Jocelyn Bell Burnell in frontof the radio telescope used to discover pulsars.

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Black Holes

• For Main Sequence stars of mass greater than about 20 Solar masses the remnant of the star left behind after a supernova explosion is too large (more than 3 Solar masses) to be a white dwarf or even a neutron star.

• These remnants collapse to form Black Holes.• No light can escape from a Black Hole which is why it’s black.• We can only “see” Black Holes due to their effects on other

objects.

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Escape Velocity & Curved Space• There is a minimum

velocity that an object needs to escape the gravitational pull of any asteroid, planet, star, etc.

• This is the escape velocity and depends on the mass and radius of the object

• For the Earth the escape velocity is about 11 km/sec.

• Since a Black Hole has so much mass in so small a space its escape velocity is the speed of light 300,000 km/sec.

All objects exert a gravitationalpull on all other objects in the Universe.

One way to picture gravity’s effectis by imagining space as a rubber sheet.Heavy objects bend this sheet more thanlight objects. Black Holes are like tearsin this sheet.

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Schwarzschild Radius: The Radius of the Event Horizon

• The Event Horizon is the spherical region of space surrounding the Black Hole from which no light may escape– once matter or light crosses the event horizon it

can never return– tidal forces are extreme at the event horizon

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Two dimensional representation of the Event Horizon

•Consider a 2-D universe (graph paper) instead of a 3-D universe.•The massive Black Hole bends space (the graph paper).•Light paths near the Black Hole are bent.•Light paths that intersect the Event Horizon terminate at the Black Hole

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Escape Velocity and Event Horizon

• Compare the escape velocities and event horizons for the following:– A 200 pound person – The Sun – A 1.4 Msol white dwarf the size of the Earth– A 3 Msol neutron star the size of a city (10 km

radius)– A 15 Msol Black Hole, nominally city-sized

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Escape Velocity and Event Horizon

col 1.: labelcol 2.: escape velocitycol 3.: light speedcol 4.: escape velocity / light-speed

Human 0.010957189 3.0000000e+10 3.6523965e-13Sun 61736768. 3.0000000e+10 0.0020578923WhiteDwarf 7.6575036e+08 3.0000000e+10 0.025525012NeutronStar 2.8291341e+10 3.0000000e+10 0.94304471BlackHole 6.3261363e+10 3.0000000e+10 2.1087121

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col 1.: labelcol 2.: schwarzschild radiuscol 3.: actual radiuscol 4.: schwarzschild radius/ actual radius

Human 1.3340000e-23 100.00000 1.3340000e-25Sun 296444.44 7.0000000e+10 4.2349206e-06WhiteDwarf 415022.22 6.3700000e+08 0.00065152624NeutronStar 889333.33 1000000.0 0.88933333BlackHole 4446666.7 1000000.0 4.4466667

•Look at the last column in each table•Table I: the escape velocity for the neutron star is near light speed•Table II: the event horizon radius for the BH is 4.44 times the radius of its matter

•The “event horizons” of the other objects are less than their actual sizes – they effectively have no event horizon.

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• General approach to “observing” black holes is an indirect approach – look for an effect on an object that can be uniquely attributed to an interaction with a black hole

Observing a Black Hole

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Observing a Black Hole

• A black hole in a close binary system– An accretion disk may form around the black hole as it draws

in material from its companion– Material swirling around at or near the speed of light at the

black hole’s event horizon will emit X-rays due to the extreme temperatures

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• If the black hole is eclipsed by the companion, an x-ray telescope will observe the periodic disappearance of the x-ray signal

• From the periodicity of the X-rays and the known mass of the companion, the mass of the invisible black hole can be found

Observing a Black Hole

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Observing a Black Hole

• If this mass exceeds the maximum allowed for a neutron star (Cygnus X-1 and A0620-00 are two examples), a black hole is currently the only known object that can have high mass and not be visible (and yet its companion is)