1 Stellar Remnants White Dwarfs, Neutron Stars & Black Holes These objects normally emit light only...
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Transcript of 1 Stellar Remnants White Dwarfs, Neutron Stars & Black Holes These objects normally emit light only...
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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 pulsating 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 10 Solar masses the remnant of the star left behind after a supernova explosion is too large (about 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.