Neutron Stars, Black Holes, and Relativity

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Neutron Stars, Black Holes, and Relativity
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Low Mass (M lt 8 M?) Stellar Evolution

• Main Sequence (core hydrogen fusion)
• Red Giant Star (core contraction, shell hydrogen
  fusion)
• Helium Burner (helium fusion in core)
• 2nd Giant Branch (core contraction, shell
  hydrogen and helium fusion, mass loss)
• Due to mass loss, the star is now less than 1.4
  M? (the Chandrasekhar limit)
• Planetary Nebula (ionization of mass lost as a
  giant star)
• White Dwarf star (inert carbon/oxygen core)

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Planetary Nebulae
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The Endpoint A White Dwarf
Note Electron Degeneracy only works if the star
is less than 1.4 M?. This is the Chandrasekhar
Limit. If the star is more massive than 1.4 M?,
something else must happen.
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The Death of a High Mass Star

• In stars with final masses over the
  Chandrasekhar limit, the gravity becomes so great
  that even carbon and oxygen can fuse. The result
  is a host of products, including neon, sodium,
  magnesium.

Since 24Mg weighs less than two 12C atoms, the
result is energy!
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The Death of a High Mass Star
The products of fusion are getting heavier!
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The Death of a High Mass Star

• Carbon-burning (temporarily) supplies energy to
  core. The core expands, shell-burning stops, and
  the star contracts.
• It doesnt take long to burn all the
  carbon/oxygen. When the C/O is gone, the core
  again contracts, and C/O fusing is forced into a
  shell around the core.

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The Death of a High Mass Stars

• Eventually, magnesium, etc., will begin to fuse.
  When it does, the result is

Aluminum, Silicon, Phosphorus, Sulfur , and
Energy!
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The Death of a High Mass Star

• Magnesium-burning (temporarily) supplies energy
  to core. The core expands, shell-burning stops,
  and the star contracts.
• The magnesium, etc., fuses very quickly, and when
  its gone, the core again collapses, and shell
  burning begins.

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The Death of a High Mass Star

• Soon, the core fuses silicon. When it does, the
  main products are

Iron, Cobalt, Nickel, and Energy!
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The Death of a High Mass Star

• This time silicon-burning (temporarily) supplies
  the energy. The core expands, shell-burning
  stops, and the star contracts.
• Silicon fuses extremely quickly, and when its
  gone, the core again collapses, and shell burning
  begins.

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The Death of a High Mass Star

• When the stars core turns to iron, it again
  collapses. The increased pressure and
  temperature then causes iron to fuse. However
• The products of iron fusion weigh more than the
  initial iron nucleus. According to E m c2,
  this means that iron fusion does not make energy,
  it absorbs energy.

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Fission and Fusion

• Up to iron, the products are lighter than the
  ingredients m c2
• After iron, the products are heavier than the
  ingredients - m c2
• For heavy elements, you make energy by fission.

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The Death of a High Mass Star

• When the stars core turns to iron, it again
  collapses. The increased pressure and
  temperature then causes iron to fuse. However
• The products of iron fusion weigh more than the
  initial iron nucleus. According to E m c2,
  this means that iron fusion does not make energy,
  it absorbs energy.

The more iron that fuses, the more energy is
taken out of the core. The temperature
decreases, the gas pressure decreases, the core
collapses faster, more iron fuses and
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Supernova

• The star explodes! In that explosion, every
  element heavier than iron is created. This is
  the only way these heavier elements (such as
  silver, gold, etc.) can be created in a
  supernova explosion.

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The Products of Supernovae
In a supernova, all the elements previously made
in a star are thrown out into space. In
addition, every element heavier than iron is made
and ejected as well.
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The Supernovae
For about a month, a supernova will outshine an
entire galaxy of 100,000,000,000 stars!
Many of the elements made in a supernova
explosion are radioactive, i.e., they make energy
by nuclear fission. This is keeps the material
bright for some time.
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Supernova Remnants
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Galactic Supernovae

• In a galaxy such as the Milky Way, a supernova
  should occur once every 50 to 100 years. The
  last few were

Crab Supernova (1054 A.D.)
Tychos Supernova (1572 A.D.)
SN 1006 (1006 A.D.)
Keplers Supernova (1604 A.D.)
Casseopia A (1680 A.D.?)
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Neutron Stars

• In addition to ejecting a large amount of
  (nuclear processed) matter into space, a
  supernova explosions will leave behind a stellar
  remnant. In the remnant, the electrons of atoms
  are crushed into their nucleus. The star becomes
  one gigantic atomic nucleus made up only of
  neutrons a neutron star.

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Neutron Stars

• Neutron stars have masses that are similar to
  that of the Sun, but they are extremely small
  only a few miles across!

And because neutron stars are so small, they spin
very rapidly, due to conservation of angular
momentum. Neutron stars rotate about once a
second!
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Pulsars

• Neutron stars are extremely small, so, by L 4 ?
  R2 ? T4 , their blackbody emission is minimal.
  However, they can beam light out from their
  magnetic poles via synchrotron emission.

If the searchlight points towards earth, we see
a pulsar.
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Pulsars
Pulsar light comes out at all wavelengths, but is
especially bright in the radio and the x-ray.
The Crab pulsar is detectable in the optical.
(When first detected, these objects were dubbed
LGMs for Little Green Men)
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What Supports a Star Against Gravity?
Type of Star What Holds it up? Limitation
Normal Stars Gas Pressure Must continually generate energy
White Dwarfs Electron Degeneracy Mass must be less than 1.4 M?
Neutron Stars Neutron Degeneracy Mass must be less than 3 M?
What if a neutron star is greater than 3 M??
The neutrons will get crushed! There is nothing
left to hold up the star. You get a Black Hole!
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The Speed of Light

• Imagine yourself in a river. The time it takes
  for you to swim upstream is longer than it takes
  for you to swim downstream.

The equivalent should be true for light. The
time it takes for light to move upstream (against
the motion of the Earth) should be longer than
the time it takes to go downstream.
But it isnt! The speed of light is always the
same!
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Special Relativity

• Premise constant velocity motion is relative
  (i.e., are you moving, or is the entire world
  moving past you?)
• Since the speed of light is always the same, this
  has some weird implications.

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Implication A Real Speed Limit

• Imagine holding a flashlight. You turn the
  flashlight on, and the light illuminates your
  path ahead.
• Now perform the same experiment while running,
  i.e., while racing a beam of light. Can you win?
  

ANSWER NO! For you are not running you are
standing still, and the whole world is running
past you. And the speed of light as you measure
it is always the same!
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Wacky Addition of Velocities

• Imagine running at ¾ the speed of light in one
  direction, while another person runs at ¾ the
  speed of light in the other direction.

0.75 c
0.75 c
0.94 c
You do not observe the other person going away at
1.5 times the speed of light. The addition of
velocities always add to lt 1.0 c .
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Implication Time Dilation

• Imagine yourself in a large stationary spaceship.
  It takes light 1 second to get from the back of
  the spaceship to the front.

1 second
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Implication Time Dilation

• Imagine yourself in a large stationary spaceship.
  It takes light 1 second to get from the back of
  the spaceship to the front.

1 second
1.5 seconds
Light is traveling 1.5 rocket-ship lengths
Pinky you are a little slow.
Now the spaceship is moving. To you, the ship is
standing still, and light still takes 1 second to
go the length of the ship. But to someone
outside, the light has traveled more than one
rocket ship length. Therefore, more than 1
second has elapsed.