Black Holes And other collapsed stars

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Black Holes! And other collapsed stars

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The Hertzsprung-Russell Diagram
What will happen to the Sun, once its fuel
(hydrogen) ends?
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Evolution phases of a star like the Sun
Sun today age 4.5 Gyrs will last another 5
Gyr (hydrogen burning)
Yellow Giant (helium burning)
Red giant
Second red giant
Planetary nebula
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Example of a Planetary Nebula

• The Helix
• Nebula

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White Dwarfs

• A White Dwarf is a dying star, which has
  terminated its nuclear fuel, and has contracted
  to roughly the size of the Earth.
• WDs are prevented to collapse further by electron
  degeneracy (electrons, because of quantum
  properties, cannot be crowded more then certain
  limits)
• This fate is shared by all stars with masses
  below 8 MSun, and they end up with masses below
  1.4 MSun the Chandrasekar limit. Most WDs have
  masses around 0.6 MSun
• The core of a WD is commonly a mixture of Carbon
  and Oxygen, and is releasing as light the
  contraction heat.
• When cold (6,000-8,000 K) they may crystallize
  into giant diamonds (first confirmed
  observationally from WD oscillations in 2004).
• As the heat is releases, the WD cools down and
  will end up a Brown, and then Black, Dwarf

This is the fate of our Sun!
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White Dwarfs can Flare back to life
If they have a younger companion
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Novae are nuclear explosions on the surface of
white dwarf and neutron stars
Brightness changes by a factor of 4000!
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White Dwarf Supernova (SN-Ia)

• If a White Dwarf accretes enough matter from a
  companion star, it will eventually nova.
• If, after the nova, it does not shed all the mass
  it gained, it will continue to accrete mass until
  it novas again.
• If this process continues (accretion, nova,
  accretion, nova, etc.) such that the WD continues
  to gain mass, once it has a mass of 1.4Msun, the
  core will collapse, carbon fusion will occur
  simultaneously throughout the core, and the WD
  will supernova.

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Another distance indicator White Dwarf (Type
Ia) Supernovae in distant galaxies.
L4?D2 l
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Evolution Phases of a Star Much More Massive than
the Sun (gt8 Msun)
Massive star, main sequence (H burning)
Massive star, He burning
Neutron star or black hole
Red giant/supergiant
Supernova explosion (SN Type II)
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Example of a Type II Supernova

• The Crab
• Nebula
• The supernova explosion that created the Crab was
  seen on about July 4, 1054 AD.

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Lifetimes of Stars

• On the Main Sequence, stars appear to obey a
  Mass-Luminosity relation
• L ? M3.5
• For example, if the mass of a star is doubled,
  its luminosity increases by a factor 23.5 11.
• Thus, stars like Sirius that are about twice as
  massive as the Sun are about 11 times as
  luminous.
• The more massive a Main Sequence star is, the
  hotter (bluer), and more luminous, the star, and
  the shorter its life.
• For instance, Sirius is only twice the mass of
  the Sun, but is 11 times more luminous, implying
  its life will be about 5.5 times shorter than
  that of the Sun

T(Sirius) 2/11 T(Sun))
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Neutron star

• A neutron star --- a giant nucleus --- is formed
  from the collapse of a massive star, with
• Mcore gt 1.44 Msun.
• Supported by neutron degeneracy pressure.
• Only about 10 km in radius.
• A teaspoon full would contain 108 tons!
• Typically with very strong magnetic field

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SNR N157B in the LMC

• 16ms period
• The fastest young pulsar known

pulsar
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Pulsar

• A fast rotating, magnetized neutron star.
• Emits both strong radiation and high-energy
  particles.

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The Limit of Neutron Degeneracy

• The upper limit on the mass of stars supported by
  neutron degeneracy pressure is about 3.0-3.2
  MSun.
• If the remaining core contains more mass, neutron
  degeneracy pressure is insufficient to stop the
  collapse.
• In fact, nothing can stop the collapse, and the
  star becomes a black hole.

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What is a black hole?

• Well mostly nothing.
• When the ball of neutrons collapses, it forms a
  singularity a point in space with infinitely
  small volume and the mass of the parent material.
  

A singularity has infinite density!
The most interesting aspects of a black hole are
not what its made of, but what effect is has on
the space and time around it.
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A collapsed star can be sufficiently dense to
trap light in its gravity.
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The Size of a Black Hole

• The extent of a black hole is called its event
  horizon. Nothing escapes the event horizon!
• The radius of the event horizon is the
  Schwarzschild radius given by
• Rs 2GM/c2

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Some Examples of Black Hole Sizes

• A 3MSun black hole would have a Schwarzschild
  radius of 10km. It would fit in Amherst.
• A 3 billion MSun black hole would have a radius
  of 60 AU just twice the radius of our solar
  system.
• Some primordial black holes may have been created
  with a mass equal to that of Mount Everest. They
  would have a radius of just 1.5x10-15 m smaller
  than a hydrogen atom!

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What would a Black Hole look like?
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Gravitational lensing
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Some Odd Properties of Space Around a Black Hole

• Light emitted near the surface of a black hole is
  redshifted as it leaves the intense gravitational
  field.
• For someone far away, time seems to runs more
  slowly near the surface of a black hole. An
  astronaut falling into a black hole would seem to
  take forever to fall in.

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Gravitational Redshifts
A photon will give up energywhile climbing away
from a mass. It is trading its own energy
forgravitational potential energy.
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Black Holes Dont Suck You In!

• Many people are under the impression that the
  gravity of black holes is so strong that they
  suck in everything around them.
• Imagine what would happen if the Sun were to
  instantly turn into a black hole. What would
  happen to the Earth?

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Black Holes Dont Suck You In!

• Since the mass of the Sun and Earth dont change,
  and the Earth is no further from the Sun than it
  was before, the force on the Earth would remain
  exactly the same. The Earth would continue to
  orbit the black hole at a distance of 1 AU!

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Black Holes Dont Suck You In!

• So why are black holes so infamous?
• The reason is that the mass is so compact that
  you can get within a few kilometers of a full
  solar mass of material. Today, if you stood on
  the surface of the Sun, much of the material is
  hundreds of thousands of kilometers away. With a
  black hole, the mass is so concentrated that you
  can get very close to the full mass.

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The tidal forcesnear a moderatesized black
holeare lethal!
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How Do We See A Black Hole?

• Short answer we dont. But we can see
  radiation from the material falling into one.

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Evidence for Black Holes

• If black holes are black, how do we know that
  they exist?
• The star HD 226868 is an excellent example. It
  is a B supergiant.
• The spectral lines in the star clearly show that
  it is in a binary system with a period of 5.6
  days, however, we see no companion star.

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The companion is one of the brightest X-ray
sources in the sky and is called Cygnus X-1
HD 226868
Cygnus X-1
The blue supergiant is so large, that its outer
atmosphere can be drawn into the black hole. As
the material spirals into the black hole, it
heats up to millions of degrees and emits X-ray
radiation.
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Stellar Evolution on the Hertzsprung-Russel
Diagram
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Stellar Evolution in a Nutshell