Transcript Life Cycle of a Star

Life Cycle of a Star
Star Life Cycle
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We do not see this activity when
observing the night sky
Shortest-lived O type stars survive for
millions of years
Birth of a Star
Where are stars born?
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Stages 1-3
Our galaxy, along with many others,
contains many large clouds of gas and
dust, mostly made up of hydrogen.
These clouds are called "nebulae."
If the cloud becomes large enough, then
its own gravity begins to overcome the
gas pressure, and the cloud can begin to
collapse.
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As the cloud collapses, gravity, temperature,
and pressure increase, until the cloud has
collapsed enough to raise the temperature to
that required to fuse (burn) the hydrogen.
Once that fusion begins, the energy released
halts the contraction, and the outer layers of
gas are blown away. What's left is an
incandescent ball of mostly hydrogen, set
aglow by the fusion reactions in its core: a
star.
How many particles must accumulate?
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Nearly 1057 atoms are required
 Much more than the 1027 grains of sand on
all the beaches in the world
 Even more than the 1051 particles that
constitute all atomic nuclei on our planet
Protostar
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Stage 4
A forming star is known as a protostar.
A protostar has two possible fates.
1. If a critical temperature in the core of a
protostar is not reached, it ends up a brown
dwarf. This mass never makes “star status.”
Brown Dwarf
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A brown dwarf is a protostar that never
had enough dust and gas accreted
(roughly 0.05 solar mass) to achieve a
temperature hot enough to ignite fusion.
It has low mass (it’s not very big), and
low luminosity (it doesn’t shine much).
It's bigger than a planet, but smaller
than a regular star.
Protostar cont.
2. If a critical temperature ( 10,000,000 K)
in the core of a protostar is reached, then
nuclear fusion begins (stages 6). We
identify the birth of a star as the moment
that it begins fusing hydrogen in the core
into helium.
Main Sequence
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Stage 7
Stars live out the majority of their lives in a
phase termed as the Main Sequence.
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Because interstellar medium is 97% hydrogen and
3% helium, with trace amounts of dust, etc., a star
primarily burns hydrogen during its lifetime.
A medium-size star will live in the hydrogen phase,
called the main sequence phase, for about 50
million years. Once hydrogen fuel is gone, the star
has entered “old age.”
Death of a Star
After Main Sequence
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What happens to a star after the main sequence
phase? Old age and death! How long it takes for a
star to die depends upon its initial mass.
Larger stars have more fuel, but they have to burn
(fuse) it faster in order to maintain equilibrium.
Because thermonuclear fusion occurs at a faster rate
in massive stars, large stars use all of their fuel in a
shorter length of time. This means that bigger is not
better with respect to how long a star will live.
A smaller star has less fuel, but its rate of fusion is
not as fast. Therefore, smaller stars live longer than
larger stars because their rate of fuel consumption is
not as rapid.
Red Giant
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Stages 8-11
Eventually, the hydrogen in the core whose
fusion supports the star begins to run out.
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The core becomes mostly helium (the product of
hydrogen fusion), and hydrogen burning moves out
away from the core, forming a burning shell around
the core.
When this happens, the core begins to
collapse again, but the outer regions of the
star are pushed outwards.
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The star becomes brighter and cooler. This is the
Red Giant stage. When the sun reaches the Red
Giant stage, 5 billion years from now, it will likely
grow to engulf Mercury, Venus, and the Earth.
Low Mass Star
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If the star has little mass, it may end its life
here, throwing off its outer layers, creating a
planetary nebula out of its atmosphere, and a
hot, dense "white dwarf" out of its core.
White dwarfs are extremely small stars with
huge densities.
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Although some white dwarfs are no larger than the
Earth, the mass of such a dwarf can equal 1.4
times that of our sun. A spoonful of white dwarf
matter would weigh several tons.
Planetary Nebula
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The white dwarf is surrounded by an
expanding shell of gas in an object known as
planetary nebula (stage 12).
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They are called this because early observers
thought they looked like the planets Uranus and
Neptune.
Planetary nebulae seem to mark the transition of a
medium mass star from red giant to white dwarf
(stage 13).
Eventually, the star will cool down, radiating heat
into space, fading into black lumps of carbon. This
is when the star becomes a black dwarf (stage 14) .
Nova
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Latin for “new”
White dwarf star undergoing an explosion on
its surface
May increase brightness 10,000 times in a
matter of days
If the white dwarf is part of a binary system,
its gravitation may pull in hydrogen from
neighboring star.
The stolen gas will begin fusing causing a
sudden flare of luminosity until the fuel is
exhausted
High Mass Star
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Fate has something very different, and very
dramatic, in store for stars which are some 5
or more times as massive as our Sun.
Following the red giant phase, the outer
layers of the star swell into a red supergiant
(i.e., a very big red giant), and the core begins
to yield to gravity and starts to shrink. As it
shrinks, it grows hotter and denser, and a new
series of nuclear reactions begin to occur,
temporarily halting the collapse of the core.
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However, when the core becomes
essentially just iron, it has nothing left
to fuse (because of iron's nuclear
structure, it does not permit its atoms
to fuse into heavier elements) and
fusion ceases. In less than a second, the
star begins the final phase of its
gravitational collapse.
High Mass Star cont.
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The core temperature rises to over 100
billion degrees as the iron atoms are
crushed together.
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The repulsive force between the nuclei
overcomes the force of gravity, and the
core recoils out from the heart of the star
in an explosive shock wave.
As the shock encounters material in the
star's outer layers, the material is
heated, fusing to form new elements
and radioactive isotopes.
Supernova
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In one of the most spectacular events in the
Universe, the shock propels the material away
from the star in a tremendous explosion
called a supernova.
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The material spews off into interstellar space -perhaps to collide with other cosmic debris and
form new stars, perhaps to form planets and
moons, perhaps to act as the seeds for an infinite
variety of living things.
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Unlike in smaller stars, where the core becomes
essentially all carbon and stable, the intense pressure
inside the supergiant causes the electrons to be
forced inside of (or combined with) the protons,
forming neutrons.
In fact, the whole core of the star becomes nothing
but a dense ball of neutrons. It is possible that this
core will remain intact after the supernova, and be
called a neutron star.
However, if the original star was very massive (say 15
or more times the mass of our Sun), even the
neutrons will not be able to survive the core collapse
and a black hole will form!
Neutron Star
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Neutron stars are typically about ten
miles in diameter and spin very rapidly
(one revolution takes mere seconds!).
Neutron stars are fascinating because
they are the densest objects known.
Due to its small size and high density, a
neutron star possesses a surface
gravitational field about 300,000 times
that of Earth.
Black Holes
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Black holes are objects so dense that not
even light can escape their gravity and, since
nothing can travel faster than light, nothing
can escape from inside a black hole.
Nevertheless, there is now a great deal of
observational evidence for the existence of
two types of black holes:
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those with masses of a typical star (4-15 times the
mass of our Sun), and those with masses of a
typical galaxy.
This evidence comes not from seeing the black
holes directly, but by observing the behavior of
stars and other material near them.
Discovery Streaming
Stars of the Universe: Grand Tour