25.2 stellar evolution...the birth: nebula •a nebula is a cloud of gas & dust. •this cloud...
TRANSCRIPT
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25.2 Stellar Evolution
Textbook pages 707 - 714
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What is a star, anyway?
• A star is an object in space that produces radiant energy through nuclear fusion.
• The size of a star does not matter.
• Star size ranges from over 1000 times the mass of the sun to smaller than the moon.
• A planet reflects light; a star produceslight.
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•Stars have a “life cycle” that lasts for
billions of years.
•Scientists learn about stellar
evolution from observing stars in
different stages of their cycles and
piecing together their observations.
WHAT? STARS have LIVES?????
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The Birth: Nebula
• A nebula is a cloud of gas & dust.
• This cloud has just enough gravitational
attraction between its molecules for it to
stay together.
• Most nebulae are about 70% hydrogen.
• This is the First Stage of Stellar Evolution.
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• This is an
example of a
nebula.
• This photo was
taken by the
Hubble
Telescope.
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What happens to the nebula?
• A force, such as an explosion of a
nearby star, or another nebula,
“bumps” into these particles.
• The nebula begins to CONTRACT.
• A region of denser material builds up.
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Protostar: The Second Stage
• As this denser
material shrinks, it
spins more & more
rapidly, and flattens
into a disc.
Think of how a pizza
baker spins a ball of
pizza dough in the air
pulls it into shape…
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This Hubble telescope photo shows the edges of
the forming protostar starting to flatten out, while
the center pulls in, or contracts.
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• The center of the protostar gets hotter
and hotter, while its internal pressure
increases.
• After several million years, the
protostar’s gas becomes so hot it
forms PLASMA, or a state of matter in
which electrons are torn from their
parent atoms.
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• At 10,000,000 C, nuclear fusion begins.
• Hydrogen atoms will fuse together to form heavier, helium atoms. Enormous amounts of radiant energy are released.
• Once nuclear fusion begins in a star, it continues for billions of years!
• Now we have a star!
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Our sun is
producing
radiant energy
RIGHT NOW
by nuclear
fusion.
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Other facts about nebulae…
• A nebula may produce more than one
star at a time. (Binary System.)
• A contracting nebula may also produce
planets at the same time it is creating
a star.
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This “eagle nebula”
may be forming at
least three stars.
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Stage Four: Main Sequence Stars
• The main sequence is the longest stagein the life of a star.
• Stars spend most of their existence within the Main Sequence.
• A star is in this part of its existence as
long as it is producing radiant energy through nuclear fusion.
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Nuclear Fusion
• Radiant energy is generated in the core
of the star via nuclear fusion.
In Main Sequence stars,
hydrogen atoms fuse
to become helium
atoms.
When one kilo of hydrogen
is converted to helium,
enough energy is released
to keep a 100-watt light bulb
burning for 3,000 years!
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Gravity vs. Energy
• Gravity keeps
the star from
expanding, as
long as the star
has enough
hydrogen to
fuse into
helium.
ENERGY
GRAVITY
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Stage Five:
Giants and Supergiants
• When all the hydrogen with a star’s
core has fused into HELIUM atoms,
• The core contracts from GRAVITY, and
• The contraction RAISES the core’s
temperature.
• The higher temperature makes the
helium atoms FUSE into a shell of
CARBON.
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Nuclear fusion is still going on
inside the shell, but the inside is
mostly HELIUM.
The carbon shell expands as energy is
released. It grows cooler, too.
This star is now a:
Red Giant (10x larger than the sun) or a
Red Super Giant (at least 100x larger
than the sun.
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Our sun has converted
about 5% of its hydrogen in
5,000,000,000 ( or 5.0 x 109) years
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Final Stages: White Dwarfs
• When helium fusion ends there is no
more energy.
• The red giant enters its final stages…
• The star loses its outer gases, and
reveals its core. This is called a
Planetary Nebula, and the hot, dense
core is a WHITE DWARF.
• It will shine for billions of years.
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Planetary
Nebula
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Rose Nebula
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Spider Nebula
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Ant Nebula
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Southern Crab
Nebula
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Black Dwarfs, Novas and Beyond
A Black Dwarf is a
completely dead star.
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When a star is part of a binary system AND
it is a high-mass star, It may explode while
cooling. These are called NOVAS.
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Novas can be up to one million times
brighter than the sun.
A nova lasts only a few days
before fading.
A white dwarf can explode
several times.
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Supernovas
• A star of HUGE mass may produce such a HUGE explosion that it blows itself apart.
The amount of energy
radiated at that time
equals the amount of
energy radiated during
the star’s entire lifetime.
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Neutron Star
• The core of a dead supernova.
• Incredibly DENSE
• Incredibly SMALL
A sugar cube’s worth of neutron matter
= 1,000,000,000 TONS on earth
20 km (12.5 miles) in diameter
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Black Holes
• Sometimes a core is simply too
massive to become a neutron star.
• These stars contract with HUGE force,
leaving what appears to be a hole in
space.
• Not even light can escape from it.
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NASA artist’s image of a black hole.
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Image of largest black hole yet discovered.
Weighs about 18 billion Solar masses.
13.5 billion light years from sun.
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