Black%20Holes

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Black Holes

• Michael Beattie

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Life Cycle of a Star

• Formed when a large amount of gas (mostly
  hydrogen) starts to collapse in on itself due to
  is gravitational attraction
• Eventually, the gas will be so hot that hydrogen
  atoms will coalesce to form helium
• Eventually the star will run out of its nuclear
  fuels

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When a Star Runs Out of Fuel

• In 1928 Indian graduate student Subrahmanyan
  Chandrasekhar worked out how big a star could be
  and still support itself against its own gravity
• The idea a star can maintain itself at a
  constant radius by a balance between the
  attraction of gravity and the repulsion from the
  Pauli exclusion principle

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The Chandrasekhar Limit

• Mass of more than one and a half times the mass
  of the sun
• White dwarf radius of about 5000 kilometers,
  density of 1 ton per cubic centimeter, supported
  by exclusion principle repulsion between
  electrons in its matter
• Neutron star radius of about 10 kilometers,
  density of 100 million tons per cubic centimeter,
  supported by exclusion principle repulsion
  between neutrons and protons in its matter

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Beyond the Limit

• Chandrasekhar showed that the exclusion principle
  could not halt the collapse of a star more
  massive than the Chandrasekhar limit
• Robert Oppenheimer solved the main problem of
  understanding what would happen to such a star
  and his work was later extended by a number of
  people
• As the star contracts more and more, the
  gravitational field becomes increasingly
  stronger, to the point where light can no longer
  escape

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The Event Horizon

• Imagine an object with such an enormous
  concentration of mass in such a small radius that
  its escape velocity was greater than the velocity
  of light. Then, since nothing can go faster than
  light, nothing can escape the object's
  gravitational field
• Think of the event horizon as the place where the
  escape velocity equals the velocity of light, a
  one-way membrane around the black hole where
  anything can fall in, but nothing can come out

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Size of a Black Hole

• There is no limit in principle to how much or how
  little mass a black hole can have. Any amount of
  mass at all can in principle be made to form a
  black hole if you compress it to a high enough
  density.
• The Schwarzschild radius (the radius of the
  horizon) and the mass are directly proportional
  to one another if one black hole weighs ten
  times as much as another, its radius is ten times
  as large.

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Evidence of Black Holes

• John Mitchell pointed out that a black hole still
  exerts a gravitational force on nearby objects
• Astronomers have observed systems in which one
  visible star orbits around an unseen companion,
  attracted to it by gravity
• Some of these systems, such as one called Cygnus
  X-1, are also strong sources of X rays

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