Chapter 26
Part 1 of Section 2:
Evolution of Stars
How Do Stars Form?
• Formation begins with condensation of a large cloud
of cold gas, ice, and dust called a nebula.
• The nebula contracts, breaks into fragments, and
increases in temperature.
• When the center of the cloud reaches 2 million
degrees F (1 million K), it becomes a protostar.
• When the temperature reaches 18 million degrees F
(10 million K), the hydrogen fuses to become helium
and a star is born.
H-R Diagram:
Hertzsprung and Russell
• Early 1900’s chart to
compare the temperature
of a star to its luminosity
(magnitude).
• Higher temperature stars
have higher magnitudes
(radiate more energy)
• 90% of all stars are found
in the main sequence of
the diagram.
How do Stars Change?
• A star like our Sun was probably
100 x’s bigger as a protostar.
• As it continues to form it shrinks,
becomes more dense, and
increases in temperature.
• The protostar has a large amount of
hydrogen in it.
• The hydrogen begins to fuse
together to form helium.
• The energy of hydrogen fusion
is that of a bomb.
Stellar Equilibrium
• Equilibrium (balance) is attained when the outward
force of fusion is = to the inward pull of gravity.
• When a star reaches equilibrium it will be found in
the main sequence of the H-R diagram.
• Stars spend most of their lives in equilibrium (main
sequence).
• Our Sun has been in equilibrium for approximately
5 billion years.
Losing Equilibrium
• When a star has fused all of its
hydrogen into helium it begins to
lose equilibrium because the
outward force from fusion becomes
less than the inward force of
gravity.
• What a star becomes next depends
on its mass.
• The bigger the mass of a star, the
shorter its life and more dramatic
its death will be.
Stars Smaller than our Sun
•
•
•
•
•
80% of stars in the universe are smaller than our Sun.
These are called red dwarfs.
Proxima Centauri is a red dwarf.
Could remain on main sequence for 16 trillion years.
Slowly fade away
Average Size Stars Like Our Sun
• Our sun is considered a medium sized yellow dwarf.
• As the Sun fuses hydrogen the outer layers will expand
and our sun becomes a red giant.
• When the Sun runs out of hydrogen in approx. 5 billion
years, gravity will begin to take over and crush the star
inward.
• This makes the star hotter. Hot enough to fuse helium
into heavier elements like carbon and stop the crushing
force of gravity.
• The fusion of helium causes the outer layer to swell so
much that it escapes the gravity of the star and is
released into space.
• What’s left behind is a white dwarf.
The White Dwarf
• Is it the final stage for medium size stars?
• For our Sun- YES.
• For stars that are part of a binary system or
star cluster- NO (on next slide)
• The white dwarf is the dense core left behind
from the previous red giant.
• 1 tsp. of white dwarf matter would have a
mass of several tons.
• This will continue to shine for billions of years.
If the Star is Part of a
Binary System or Star Cluster…
• It’s not over for the dying white dwarf.
• More than ½ of the stars are part of a binary system.
• The dying white dwarf can steal hydrogen from its binary
partner and grow in mass to an unstable limit 40%
higher than our Sun.
• This leads to Type 1 A
Supernova.
Stars 8-10 x’s More Massive
Than Our Sun
•
•
•
•
These are called supergiants.
Example of a supergiant is Betelgeuse.
Hydrogen fuses to become helium
Higher temperatures allow helium to fuse to become
carbon or oxygen.
• Carbon and Oxygen can further fuse to become even
heavier elements which leads to elements like iron and
nickel.
• It cannot fuse iron any further so gravity takes over and
the star collapses leaving a core the size of the Earth.
• This causes a Type 2 Supernova releasing heavy elements
into space.
After a Type 2 Supernova:
The Neutron Star and Pulsar
• The remaining dense core is made up almost entirely
of neutrons.
• This is called a neutron star.
• These are as small as 10 miles across.
• They are extremely dense.
• 1 tsp. of neutron star matter = 1 billion tons
• The Neutron star begins to spin rapidly and create a
large magnetic field that begins to glow.
• This is called a pulsar. (Like a lighthouse)
Stars that are 25-40 x’s
the Mass of the Sun
• Also called
Supergiants
• So massive that even
the neutron star
cannot hold up under
the pressure of
gravity.
• When the neutron
star collapses it
becomes a black hole.
Black Holes
• Ultimate death of a
VERY massive
supergiant.
• A region of space
with such high
density and gravity
that nothing can
escape it.