Download lecture 27 nuclar fusion in stars

LECTURE 27
NUCLAR FUSION IN STARS
Instructor: Kazumi Tolich
Lecture 27
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Life cycle of stars
Star formation
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Stars are formed by the gravitational collapse of huge
interstellar clouds of gas (mostly hydrogen) and dust.
As it collapsed, the speed of the new gas molecules joining
the ball of gas increased, raising the temperature of the
gas cloud.
Hubble telescope image of
young bright blue stars in
Small Magellanic Cloud.
Start of nuclear fusion in the sun
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The sun became hot enough to glow, and the radiation pressure of the light swept
away the dust and gas in the solar system.
The core kept collapsing, and the temperature of the center of the cloud kept
increasing eventually reaching millions of degrees, hot enough for nuclear fusion of
hydrogen to take place about 5 billion years ago.
The radiation pressure from nuclear fusion is balanced with the gravitational force,
reaching the stability.
Dying sun
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In about another 5 billion years, the sun will exhaust the hydrogen in its core.
The core will begin to collapse and become even hotter.
Fusion of hydrogen will take place in a shell surrounding the core.
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The sun will brighten and expand to three times its present size.
The increased heat output will evaporate ocean water on Earth.
For the following a few hundred million years, the sun will become even brighter and 100 times
larger.
Earth will reach around 1000 ºC, inhabitable for any life.
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The core becomes hot enough to fuse helium, created in hydrogen fusion.
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The sun will spend ~100 million years as a helium-fusing star.
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Death of the sun
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Finally, the sun exhausts all its fuel.
The sun will again expand and brighten.
The outer layers of glowing gas blow away, which engulf all the
planets.
Only the core is left, no longer generating energy.
Without pressure from the heat from nuclear fusion, gravity collapses
the core.
The core becomes as small as quantum mechanics will allow it to be,
about 10-6 of the current volume, the size of Earth, and incredibly
dense (~105 times denser than Earth).
¤ The atoms will be squashed so much that the sun becomes a ball of
bare nuclei and unattached electrons.
White dwarfs
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During the final collapse, the sun will warm
up, and glow brightly.
A star in this stage of its existence is called
a white dwarf.
It will gradually cool off and stop emitting
light.
Sirius
B
Hubble telescope image of Sirius A
and Sirius B (white dwarf)
Other stars’ lives
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A star must have at least 10% of the sun’s mass in order for fusion to
start.
If the mass is much more than 100 times the sun’s mass, it will blow
apart very quickly.
In between, a star’s life cycle is determined primarily by its mass.
¤ Stars
up to about ten times the sun’s mass have a life cycle similar to the
sun’s, ending up as white dwarfs.
Death of massive stars
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Massive stars go through multiple cycles of core collapse and heating, fusing in turn
helium, carbon, and all the way up to iron.
Iron continues to form, but does not fuse.
The core is no longer generating energy, and it collapses due to gravity.
(M - Zmp - Nmn)/A vs. A
Supernovae
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If the star is so massive, the iron core collapses in about 1 second, causing a
supernova explosion that blasts the rest of the star into space.
The supernova explosions are as bright as ~4 billion suns!
The last supernova that occurred in our galaxy happened in 1604; it was bright
enough to be seen in the daytime.
SN 1604 remnant nebula
Crab nebula (SN 1054)
Supernova 1987A
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In 1987, a supernova occurred in a galaxy nearby (1.5 × 105 ly
away).
Even at that great distance, it was visible to the naked eye.
After the supernova explosion
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About 10% to 20% of the star remains after the explosion, which
collapses quickly.
The gravitational force is too strong for nuclei to exist.
Protons and electrons are squashed to form neutrons.
The entire star becomes a solidly packed ball of neutrons, called a
neutron star.
Neutron star
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Neutron stars are incredibly dense – a neutron star with the mass of
the sun would be about 10 km in diameter!
The star’s rotation also speeds up as it collapses. Neutron stars
typically rotate between 1 and 1000 times per second.
Spinning neutron star
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This spinning, along with very large magnetic fields, produces very
intense radiation (visible and radio), which appears to blink on and
off as the star rotates.
Neutron star in Crab nebula producing visible 0.05 second flashes.
Death of even more massive stars
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After a supernova explosion, the material that remains collapses due to its own
gravitational pull.
No other forces can balance the gravity, and the material collapses indefinitely until
all the material is confined in a single point, a black hole.
Nothing can travel faster than speed of light, c, but the escape velocities near black
holes are greater than c, making nothing, even light unable to escape (therefore it’s
black).
An imaginary surface around a black hole where the escape velocity is c is called
the event horizon.
Detection of black holes
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We detect black holes by their gravitational influence on objects around them.
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Cygnus X-1 (and several similar objects):
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Centers of galaxies
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One visible giant star orbiting around a compact invisible object of 10 solar masses.
Gasses from the visible star are drawn to the invisible object, emitting x-rays.
Near the center of galaxy, stars and gas are orbiting so fast about an invisible object of several billion
solar masses.
Right before the black hole swallows nearby objects, they radiates x-rays and light.
~300 billion giant black holes are estimated to exit in the observable universe!