Stellar Evolution

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Stellar Evolution

• The Birth Death of Stars

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Chapter 33Section 33.2 and 33.3

• Star Formation Interstellar Medium
  Protostars.
• Stars Their Properties.
• Stellar Death Supernovas, Neutron Stars Black
  Holes.

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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.

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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

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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.

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A STAR IS BORN!!!
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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.

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Nuclear Fusion

• In stars with a core temperature of less than 15
  million K, nuclear fusion occurs via the
  Proton-proton 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).

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• Overall, 4 H nuclei are fused to form 1 He
  nucleus.
• EMC2 demonstrates that mass and energy are
  interchangeable. This missing mass is released as
  energy.

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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.

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Stellar PropertiesLuminosity 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/4pd2

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Stellar PropertiesParallax

• 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.

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• One parsec (pc) is the distance to a star whose
  parallax angle is one second of arc(1?), where
  1?1/3600
• 1 pc3.26 light-years
• Distance to Star (in pc)1/parallax(?)

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Stellar PropertiesH-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.

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Stellar DeathSmall 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.

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Stellar DeathMedium 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.

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Stellar DeathMedium 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.

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Stellar DeathMedium 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.

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Stellar DeathMassive 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.

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Stellar DeathMassive 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.

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Stellar DeathMassive 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.

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Stellar DeathSuper 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.

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Stellar DeathNeutron 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.

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Stellar DeathBlack Holes

• A supernova may explode so violently that the
  remaining core is compressed into an infinitely
  small, infinitely dense black hole.
• Black holes 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 R2GM/c2.
• This radius is given a special name, the
  Schwarzchild radius.