Transcript Document

Neutron Stars, Black
Holes, and Relativity
Low Mass (M < 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)
Planetary Nebulae
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.
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!
The Death of a High Mass Star
The products of fusion are getting heavier!
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 doesn’t 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.
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!
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 it’s gone,
the core again collapses, and shell burning begins.
The Death of a High Mass Star
Soon, the core fuses silicon. When it does, the main products are
Iron, Cobalt, Nickel, and Energy!
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 it’s gone, the core
again collapses, and shell burning begins.
The Death of a High Mass Star
When the star’s 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.
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.
The Death of a High Mass Star
When the star’s 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 …
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.
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.
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.
Supernova Remnants
Galactic Supernovae
In a galaxy such as the Milky Way, a supernova should
occur once every 50 to 100 years. The last few were
SN 1006
(1006 A.D.)
Kepler’s
Supernova
(1604 A.D.)
Crab Supernova
(1054 A.D.)
Tycho’s Supernova
(1572 A.D.)
Casseopia A
(1680 A.D.?)
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.
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!
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.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
If the “searchlight” points towards earth, we see a pulsar.
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.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
(When first detected, these objects were dubbed
“LGMs” for Little Green Men)
What Supports a Star Against Gravity?
Type of Star
Normal Stars
What Holds it up?
Limitation
Gas Pressure
Must continually
generate energy
Mass must be less
White Dwarfs Electron Degeneracy
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!
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 isn’t! The speed of light is always the same!
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.
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!
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 < 1.0 c .
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
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
“Pinky … you are
a little slow.”
Light is traveling 1.5
rocket-ship lengths
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.