Exotic Stars

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Exotic Stars
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White Dwarfs

• White dwarfs form after the helium flash, where
  the helium ash at the core of the star ignites.
  This usually leads to enough increase in energy
  to further fuse atoms into Carbon, Nitrogen, and
  Oxygen.
• The white dwarf is the exposed core of the star,
  which continues to burn its nuclear fuel for a
  short time after the outer shell of the star has
  been pushed away (Planetary nebula)

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White Dwarfs

• After the nuclear furnace dies out, the white
  dwarf will have a surface temperature of 25,000
  K.
• In normal stars the outer mass is supported by
  the heat of the core
• In white dwarfs, the heat of the core disappears
  causing the star to contract into degenerate
  matter with an incredible density

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White Dwarfs

• The collapse of the star leads to densities of
  about 3 million g/cm3
• Water is 1 g/cm3 and lead is about 11 g/cm3
• A beach ball worth of white dwarf material would
  weigh as much as the Titanic

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White Dwarfs

• Without a heat source, white dwarfs simply
  radiate their heat into space.
• This causes them to move down to the right on the
  H-R diagram getting dimmer and cooler
• This cooling period is extremely long
• It takes more than 10 billion years to cool down
  to 3,000 k

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White Dwarfs

• However, white dwarfs can have a more exciting
  life if they have a partner
• If mass is transferred to a white dwarf from one
  of its partners several things could happen
• Nova explosion, re-ignition, or even becoming a
  neutron star

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White Dwarfs

• Nova Explosion
• When material accumulates on the surface it is
  squeezed by gravity that is 100,000 times
  stronger
• This material is super heated
• When the temperature reaches 10 million degrees,
  the hydrogen explodes on the surface (similar to
  our bombs)
• This is called a nova (meaning new star)

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White Dwarfs

• Re-ignition
• Under rare conditions, a white dwarf may
  accumulate so much material that it may re-ignite
  its nuclear core.
• This may extend the stars life by millions of
  years and create a second planetary nebula or
  lead to a neutron star if the core collapses
  further

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White Dwarfs

• Neutron star
• If the white dwarf is large enough and gains a
  little more material, it may be crushed under its
  own weight, turning into a neutron star.
• A white dwarf with more than 1.4 solar masses
  will contract into a neutron star

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Neutron Star

• A neutron star results from a supernova explosion
  or the collapse of a white dwarf
• The material that makes up a neutron star is not
  made of atoms like most stars
• While the neutron star is made entirely of
  neutrons, it is often called a quark soup because
  neutrons are made of Up and Down Quarks

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Neutron Star

• Similar to a white dwarf, a neutron star is the
  exposed core of the original star
• The outer surface is actually solid but the
  inside is a fluid of neutrons which creates a
  tremendous magnetic field
• Neutron stars have the most powerful magnetic
  field of any individual star.
• 1 trillion times stronger than Earths magnetic
  field

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Neutron Star

• Neutron stars are also called Pulsars
• The incredible magnetic field of neutron stars
  causes particles to be accelerated along the
  north and south magnetic field.
• When these particles smash into the gases
  surrounding the star it releases intense light.
• This light is pointed along the direction of the
  north and south magnetic pole NOT the rotation
  axis

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Neutron Star

• Since the beams of light are not in the same
  direction as the axis of rotation, the beams of
  light sweep through space like a light house
• The beam from most pulsars doesnt point towards
  us so we dont see the beam of light but when the
  beam points our way we see a pulse at regular
  intervals

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Neutron Star

• The rapid collapse of star cores that produce
  neutron stars also causes them to spin very
  rapidly
• Some pulsars spin more than a few hundred times
  per second (millisecond pulsars)
• This speed steadily slows during the lifetime of
  the neutron star

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Neutron Star

• Although, the rate of spin slows during the
  lifetime of the neutron star, they have
  occasionally sped up for an instant, then return
  to a steady slowdown
• Most astronomers believe that star quakes are the
  cause of the speed up
• When the surface makes a rapid collapse, it
  causes the star to speed up like a figure skater
  pulling her arms

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Neutron Star

• Neutron stars are also capable of producing nova
  explosions when material accumulates on the
  surface
• The ultra-hot surface means that the peak region
  of the blackbody radiation is in X-rays
• X-rays are also produced in the accretion disk of
  material drawn from its partner
• More X-rays are produced by the accretion disk
  around a neutron star than a white dwarf or a
  black hole.

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Neutron Star

• The diameter of a neutron star is about 5 to 10
  miles and has a density of 1014 g/cm3
• The mass range for a neutron star is between 1.4
  and 2.5 solar masses but there is some
  uncertainty in the upper range, it may be closer
  to 3.2 solar masses
• If a star exceeds the upper limit, it is further
  crushed and becomes a Black Hole

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Black Hole

• Black Holes are produced in a supernova explosion
  or by the collapse of a neutron star
• At this point the entire mass of the star has
  been crushed into the size of a particle
• It is unlike any other object in the universe
• It is not composed of material found in any
  normal matter

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Black Hole

• The density of a black hole is immeasurable
  (approaching infinite density)
• There is no limit to how large a black hole can
  grow
• There are two basic classifications of black
  holes Stellar and Super Massive
• Stellar about 3 to 10 solar masses
• Super massive exceeding millions of solar masses

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Black Hole

• Unlike any other object in space, the gravity of
  a black holes is so great that no object can
  escape its surface or even the region very close
  to it.
• The escape velocity is greater than the speed of
  light so even light cannot escape
• However, if an object remains far enough away it
  can orbit the black hole safely

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Black Hole

• Even though the gravity is stronger and black
  holes certainly have an accretion disk if they
  have a partner, they tend to produce less X-rays
  than neutron stars because some of the x-rays are
  too close to escape
• The boundary that marks the point of no escape is
  called the Event Horizon

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Black Hole

• To truly understand the nature
• of a black hole, we have to
• understand Einstein's Theory
• of General Relativity
• Einstein changed the way we think about space
  itself
• People tend to believe that space is empty and
  nothing
• But that is completely wrong!!

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Black Hole
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Black Hole

• Even the vacuum of space is a physical object
• There is energy is space and space has properties
  similar to fabric
• It can be stretched, twisted bent, or even warped
• Einstein realized that gravity is actually a
  depression in the fabric of space
• The more massive the object, the deeper the
  depression in the fabric of space, but nothing
  does what a black hole does

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Black Hole
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Black Hole
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Black Hole
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Black Hole