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Stellar
Evolution
The Birth & Death of Stars
Chapter 33
Section 33.2 and 33.3
Star Formation: Interstellar Medium &
Protostars.
Stars & Their Properties.
Stellar Death: Supernovas, Neutron Stars &
Black Holes.
Star Formation
 The Interstellar Medium is the space between
stars and is made up of trace amounts of gas(90%
hydrogen, 10% helium) and dust.
 Regions of this medium are denser than normal
and are known as nebula for their cloud like
appearance.
 It is in these interstellar clouds that stars are
born.
Star Formation
A shockwave from a supernova can hit one
of these clouds.
This triggers a gravitational collapse, which
pulls the gas and dust particles together.
As the cloud condenses, smaller regions
break off into “globules”
Star Formation
 These globules eventually form hot gaseous spheres known as
protostars.
 A protostar is not a true star in the sense that it has not
started “burning hydrogen” through nuclear processes.
 Gas and dust continue to accrete on the protostar and the
temperature in the core rises.
 Once the core reaches 10 million Kelvin, nuclear fusion
begins.
A STAR IS BORN!!!
Nuclear Fusion
As gravity is pushing inward on the core,
nuclear fusion is creating energy that
pushes outward. These forces create an
equilibrium that allow the star to sustain a
definite size and shape.
The star is now in a state of hydrostatic
equilibrium.
Nuclear Fusion
 In stars with a core temperature of less than 15
million K, nuclear fusion occurs via the Protonproton cycle.
 First step - 2 protons (H nuclei) fuse together
forming a deuterium nucleus, a neutrino, and a
positron.
 Next, the deuterium nucleus fuses with a proton
and forms an isotope of He.
 Finally, 2 He isotopes fuse and form a normal He
atom and 2 protons (H nuclei).
 Overall, 4 H+ nuclei are fused to form 1 He nucleus.
 E=MC2 demonstrates that mass and energy are
interchangeable. This missing mass is released as
energy.
Nuclear Fusion
 Stars with a core temperature ranging from 15
million to 100 million K undergo the carbon cycle.
The overall result is the same as the proton-proton
cycle, but with different intermediates.
 In stars with a core temperature above 100 million
K, the dominant nuclear process is the triple alpha
process. 2 alpha particles fuse to form Be, and a
third alpha particle combines with the Be to
produce 1 carbon nucleus.
Stellar Properties:
Luminosity and Brightness
Luminosity (L) - total power radiated in
watts.
Apparent Brightness (l) - the power crossing
unit area at the Earth perpendicular to the
path of the light.
l = L/4πd2
Stellar Properties:
Parallax
How do we measure the distance to stars
outside of our solar system?
The method of parallax is used to measure
the distance to nearby stars.
 One parsec (pc) is the distance to a star
whose parallax angle is one second of
arc(1″), where 1″=1/3600°
 1 pc=3.26 light-years
 Distance to Star (in pc)=1/parallax(″)
Stellar Properties:
H-R Diagram
 A graph of temperature vs. luminosity with stars
plotted as single dots.
 When thousands of stars are plotted, they fall into
definite regions, suggesting a relationship between
a stars temperature and luminosity.
 90% of stars fall into a band called the main
sequence which runs from the upper left to lower
right corners.
Stellar Death:
Small Stars
Some models predict that the smallest red
dwarf stars may stay on the main sequence
for a few trillion years.
All of these small stars that have ever been
born are still on the main sequence. We do
not yet know what happens to them at the
end of their lives.
Stellar Death:
Medium Sized Stars
 Medium sized stars, like our sun, begin to show
their age as helium builds up in the core.
 The helium core does not provide any energy and
gravity causes the core to contract while hydrogen
continues to fuse in a shell around the helium.
 This gravitational collapse of the core causes
releases tremendous amounts of heat that causes
the outer hydrogen shell to expand.
 At this point, the surface temperature drops and
the star appears red. It is now known as a red
giant.
Stellar Death:
Medium Sized Stars
 The core will continue to collapse due to gravity
until the temperature reaches 100 million K.
 At this point, helium begins to fuse into carbon.
This signals the last stage in a medium stars life as
there is not energy to fuse carbon into heavier
elements.
 The outer shell of the star has such a low density
that it can drift off into space and form a
planetary nebula.
Stellar Death:
Medium Sized Stars
 After the outer shell is gone, the helium envelope
continues to burn around the carbon core.
 Eventually the helium fuel will run out and gravity
will once again compress the core.
 With no source of energy to stop the gravitation
collapse, the star will shrink until it reaches a
point called electron degeneracy.
 The star is now know as a white dwarf and will
continue to radiate energy until all that remains is
dead core of ash.
Stellar Death:
Massive Stars
 Massive stars follow the same evolutionary track
as medium sized stars up to the point of the
carbon core.
 The core of these massive stars will eventually
reach 600 million K and carbon will begin to fuse
and produce oxygen, neon and magnesium.
 Once the core reaches 1 billion K, oxygen ignites
and produces silicon.
Stellar Death:
Massive Stars
 At 2 billion K the silicon ignites.
 This process of producing new elements is known
as nucleosynthesis and is the source of all the
elements heavier than hydrogen and helium.
 The human body is made up of 10% hydrogen mass
with the remaining 90% made up of heavy
elements. In other words, most of our body is
made up of materials that were once inside the
core of very massive stars.
Stellar Death:
Massive Stars
This process of nucleosynthesis will
continue until the core is made of iron.
Iron does not release energy in the fusion
process but instead requires energy. As the
core continues to heat up, the iron atoms
will simply absorb this energy but will not
fuse with each other.
Stellar Death:
Super Nova
 Once the iron core has absorbed the energy from
the fusion taking place in the outer shells, gravity
contracts the core together.
 Eventually the individual particles will be packed
so tightly they touch each other, at which point
the collapse is stopped.
 At this point the star explodes in what is called a
type II supernova.
 During these explosions, free neutrons may be
captured by atoms to produce elements heavier
than iron.
 The debris from a supernova can create a nebula.
Stellar Death:
Neutron Stars
 After a supernova, an extremely dense core of neutrons may
be left in what is called a neutron star.
 These neutron stars are so dense that one teaspoon of
material from a neutron star would weigh billions of tons.
 All stars rotate and thus have angular momentum. When a
star loses most of its mass in a supernova, the remaining
neutron star rotates very quickly.
 The fastest observed neutron star rotates at 716 revolutions
per second.
Stellar Death:
Black Holes
 A supernova may explode so violently that the
remaining core is compressed into an infinitely
small, infinitely dense black hole.
 Black hole’s have such a strong gravitational pull
that even light can not escape if it gets any closer
than the event horizon.
 The radius, R, at which a body of mass M must be
contracted to in order to form a black hole is
given by R=2GM/c2.
 This radius is given a special name, the
Schwarzchild radius.