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STARS & THEIR
EVOLUTION
Star Dust & Elements
• About 4 and a half billions years ago
something interesting happened in an outof-the-way part of the galaxy. The sun
ignited at the center of a cloud of gas and
dust. The rest, as they say, is history.
• We now know that stars (the sun included)
are born in dust clouds, redden and swell
to many times their original size as they
age, then end their lives in incredible
explosions that leave the most bizarre
objects in the universe behind.
• They are the major engines for pumping
out atoms heavier than hydrogen -- the
stuff that makes up the Earth, its rocks, its
oceans and its life forms -- even the twolegged variety reading this page.
• Gravity pulls the outer layers of
the dust and gas cloud toward
the center of the cloud. The
center of the cloud is not very
hot yet. There is nothing at the
center strong enough to hold up
the weight of the outer layers
So the cloud continues to
collapse. This is similar to the
egg in the left-hand panel. The
weight of the foot is like the
weight of the outer layers of the
star. The pressure in the egg is
like the pressure in the core of
the star. What do you think will
happen?
• When the temperature at the
center gets very hot, nuclear
reactions begin, converting
hydrogen to helium with the
release of energy. The
temperature increases even
further and the gas at the
center presses outward
holding up the outer layers
of the star. Like the bowling
ball on the left, high internal
pressure means that the
weight of the outer layers
can no longer crush the star.
The star enters the main
sequence and is stable for a
large part of its life.
Gravity Wins
• The end, however, is
inevitable. When the
star's fuel supply is used
up and nuclear reactions
stop, the star will
collapse. Gravity always
wins. What's left at the
end depends on how
massive the star was to
begin with.
Stellar Evolution
• Nebula - Stars begin as a nebula of gas and dust.
• Protostar- gravity pulls it together, temperature rises.
• Main Sequence- Temperature, luminosity & mass
directly proportional.
• Red Giant or Supergiant- large, luminous, low
temperature.
• Nova or Supernova- outer layers of star blown off.
As the star expands, the outer layers blow off at an
incredible rate. A star can lose more than half of its
mass during this stage.
– The gas cloud surrounding the star is called a planetary
nebula.
• Ending is based on initial mass and can be either:
What Happens to Stars Much
Heavier than the Sun?
• Main Sequence
• Stars that are much heavier than the
sun start off as hot white, bluewhite or blue stars, rather than
yellow stars like our sun.
• They last only a short time
compared to the sun. A star 20
times heavier than the sun will use
up its fuel in about 8 million years.
The sun takes 1000 times longer.
• Very big stars that were born when
dinosaurs lived on Earth have
already used up their fuel and
exploded. The sun has been around
since long before the dinosaurs and
its still here today.
Red Supergiant Phase
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After the star depletes the hydrogen in its core
and moves off the main sequence, the core
contracts and heats causing the outer layers to
expand. The expansion of the outer layers
results in a cooling of the surface temperature.
The star gets redder.
Meanwhile the core is getting hotter reaching
150 million K. At this temperature, helium (He)
fuses explosively into carbon (C) and oxygen
(O).
Gravitational collapse continues to raise the
temperature of the core. When the core reaches
an amazing 1 billion K, carbon fuses to produce
neon (Ne), magnesium (Mg) and oxgyen (O).
These elements fuse to produce even heavier
elements.
At each stage less total energy is released and
thus the stages get progressively more shortlived.
Finally an iron core is produced. Since no
energy can be gained by fusing iron to make
higher mass elements, the core collapses at
blinding speeds.
Supernova Stage
• In the final moments of the star's
life, nuclear fusion produces an iron
core. No energy can be gained by
fusing iron into heavier elements.
There is now no energy source to
sustain the intense internal pressures
holding off gravitational collapse.
The core collapses catastrophically
inward at 1/4 of the speed of light.
• In less than 1/10 of a second the
core of the star is crushed into a
sphere only 100 km across. The
gravitational energy released in this
fraction of a second is 100 times
greater than the sun releases in its
entire 10 billion-year lifetime.
Neutron Star
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After the supernova blast blows off
the outer layers of the star, all that
is left is the central core. The core
now contains a mass between 1.4
and 3.0 times the sun's mass but
condensed into a volume 10- to 20km across - roughly the size of a
small town on Earth.
The matter in a neutron star would
be incredible to behold. It is
thought to be no longer gaseous.
The surface may be crystalline.
Densities are a trillion times greater
than those in a white dwarf. In fact,
it is so dense that one teaspoonfull
would outweigh the Empire State
Building.
Black Hole
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If the central core, that remains after the
supernova blast, is greater than 3 times the
sun's mass, the internal pressure cannot
halt the gravitational collapse. The core
will continue to collapse to form a black
hole.
In a black hole, there is so much mass
compressed into such a small volume that
gravity prevents even light from escaping.
Since no light can ever reach your eyes
from the collapsed core, it appears black,
hence the name.
The apparent surface of the black hole is
the radius at which light just manages to
escape. It is called the event horizon.
For example, a black hole with 3 times the
solar mass would have a circumference of
55.5 kilometers (the size of a town). The
Earth as a black hole would only be a
couple of centimeters in diameter (about
the size of a marble)!
What happens to stars around the
size of the sun?
• Main Sequence
• A star's life begins when nuclear
reactions start deep in the core.
Hydrogen nuclei are fused to form
a helium nucleus. Each helium
nucleus has slightly less mass than
the hydrogen nuclei that formed it.
The missing mass is converted into
energy. This energy released in the
core creates high temperatures and
pressures. The high internal
pressure would blow the star apart
if not for the weight of the outer
layers pressing down upon the core.
• The star spends almost all of its life
on the main sequence as a very
average star.
Red Giant Phase
• When the star uses up its hydrogen
fuel in the core, it can no longer
hold up its outer layers. They fall
inward. The center gets hotter. The
hot temperatures cause the outer
layers of the star to expand. As a
result, the star gets bigger and the
surface cools. The star is now a red
giant.
• While the outer atmosphere is
expanding, the core inside collapses
to about the size of the Earth.
• The temperature again increases,
reaching about 100 million degrees.
Suddenly helium in the core begins
to fuse to carbon and oxygen at a
very rapid rate. Within a short time
the helium is gone, and
gravitational collapse continues.
The Outer Layers Blow Off
• As the star expands,the outer
layers blow off at an
incredible rate. A star can
lose more than half of its
mass during this stage.
• The gas cloud surrounding
the star is called a planetary
nebula.
• The exposed inner part of the
star remains as the central
star in the planetary nebula.
These central stars are the
hottest stars known. They are
not very luminous because
they are extremely small -about the size of the Earth.
White Dwarf
• When the fuel is used up, there is
nothing to hold the star's layers up
against gravity and the star begins
to collapse. Gravity squeezes the
star down into a very small size.
• As you might imagine, the matter
that makes up a white dwarf is like
nothing you've ever seen before.
It's as if the mass of an elephant
were compressed into the space of
a marble. A 70 kg (154 lbs) person
would weigh about 600,000 kg
(600 metric tons) standing on the
surface of a white dwarf.
• As bizarre as they are, white
dwarfs are very common objects in
the Universe.
•
Black Dwarf
• White dwarfs are some of
the hottest objects in the
known universe with
temperatures ranging from
30,000 K to 200,000 K.
• They continue to radiate
their immense internal
energy into space as they
continue on their journey to
become black dwarfs.
• When they reach 4000 K
these bizarre stellar objects
must start to crystallize. The
time this requires is very
long. In fact, no white dwarf
generated has yet had
enough time to cool to a
black dwarf.
End of a Star’s Life
• Small Mass
– White Dwarf- small, white star with low luminosity
and high temperature. Fuel is used up,gravity squeezes
the star down to a very small size.
– Black Dwarf-cool, star that has crystallized
• Large Mass
– Neutron Star (Pulsar is one type)- only has neutrons
– Black Hole- so small and dense that even light cannot
escape it
RED GIANT & SUPERGIANTS
• Hydrogen fuel is being
used up.
• Outward pressure
overcomes gravity.
• Star swells.
Novas and Supernovas
• Nova- Outer layer of
star swells and leaves
• Supernova- outer
layers explode
White Dwarfs
• White dwarfs are the
hot cores of stars that
have blown away their
outer layers.
• They are about the
same size as the Earth.
• http://www.windows.uc
ar.edu/
Neutron Stars
• Neutron stars are the
cores of stars that have
been compressed to a
very small size by
gravity and are cooling
off slowly.
• They are made of
neutrons.
• They are barely as large
as a small town.
Black Hole
• Black holes are the
strangest object in
space. They are the
core of stars that
gravity has compressed
down so small that
nothing can escape not
even light.
• They are even smaller
than neutron stars.
H-R Diagram
by Hertsprung-Russell
led to understanding of
stellar evolution.
Enjar Hertzsprung (Danish)&
Henry Norris Russell(American)
The H-R Diagram
• http://www.windows.ucar.edu/tour/link=/co
ol_stuff/tours_main.html
• http://www.enchantedlearning.com/subjects
/astronomy/stars/lifecycle/