Astrophysics

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Astrophysics Cosmology

• Interestingly enough, we make a connection
  between theories of quarks and leptons and the
  structure of the universe
• Need to deal with Einsteins General Theory of
  Relativity with is basically a theory of gravity

2
Astrophysics

• The application of physics as we have learned it
  so far, to the structure of stars, galaxies,
  planets, comets, etc.

3
Cosmology

• Study of the structure and origins of the
  universe using the laws of physics as we
  understand them.

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Questions

• Is there a beginning of time?
• Does the universe extend forever in space?
• Will there be a definable end of time?
• Is there more than one universe?

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Stars and Galaxies

• From the time of Galileo, we have had telescopes
  to observe the universe
• Prior to that, we just thought the stars were
  fixed in the heavens (whatever that word means)
  and there were some wandering orbs called planets
  that moved around against the background of the
  fixed stars

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Distances Involved

• We now know that distances between objects in the
  universe are huge.
• Talk about light-years as opposed to meters
• 1 ly is about 1013 km
• Earth-Moon distance is 1.28 light-seconds
• Earth-Sun distance is 8.3 light-minutes
• Nearest star is 4.3 light-years away

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Galaxies

• If you look at the night sky, you will see an
  intense band of stars across the sky
• This is the Milky Way, and is in fact looking
  edge-on toward the center of our galaxy at a huge
  number of stars
• The diameter of our galaxy is about 100,000 light
  years and it is about 2000 light years thick and
  it is a rotating disk containing 1011stars

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Galaxies

• Our sun is about 28,000 light years from the
  center of the galaxy and orbits the center of the
  galaxy every 200 million years.
• See the text for a calculation of the mass of the
  galaxy and thereby the number of stars it must
  contain

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Milky Way
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Galaxies

• Looking with powerful telescopes, you can see
  other elliptical objects and in some of them we
  can discern that there are individual stars
• These are externals galaxies much like the Milky
  Way
• Nearest galaxy is Andromeda with is 2 x 106light
  years away

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Andromeda
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Galaxies

• Using very large telescopes we can see about 1011
  galaxies!!
• These galaxies tend to occur in clusters
• Clusters organize into superclusters
• When we look at very distant galaxies (up to 1010
  ly away), we are looking back that far in time!

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Other Stuff Out There

• There are lots of interesting star types
• Regular stars like our sun
• Red giants
• White dwarfs
• Neutron stars
• Exploding stars (nova, supernova)

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Other Stuff Out There

• Quasars
• Black Holes
• Background Radiation
• Cosmic Rays

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Measuring Distances

• Nearby stars can be measured by using parallax
• This is the apparent motion of a star due to the
  earths movement around the sun
• Some stars appear to move slightly during the
  course of the year and appear at a slightly
  different angle six months apart

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Parallax
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Parallax Angles

• The angle phi is a useful guide to distance and
  gives rise to a new unit of distance
• Divide angles into degrees, minutes and seconds
  with 60 minutes to a degree and 60 seconds to a
  minute.
• We specify the distance in parsecs (parallel
  seconds of arc)

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Parallax Angle

• So to convert seconds of arc into a distance we
  need to do a quick approximation
• If phi is 1 second of arc (parsec) its tangent is
  the radius of the earths orbit divided by
  distance to the star
• For this tiny angle the tangent is equal to the
  angle in radians

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Parallax Angle

• One second of arc is 1/60 minute of arc or 1/3600
  degree of arc
• Converted to radians, this is 1.74 x 10-3
• The radius of the earths orbit is 1.5 x 1011
  meters
• So a parsec (distance to the star) is 8.62 x
  1013 meters or 3.26 light-years
• We can measure distances to 30 parsecs (1/30 of a
  second of arc)

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Other Distance Estimates

• Assume brightest stars in a galaxy have the same
  intrinsic brightness (same type of star)
• Then comparing brightness of stars in two
  galaxies, we can use inverse square law to
  compute a distance
• Cephid variables are stars whose brightness
  correlates to the period of brightness
  oscillations

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Stellar Evolution

• First some background stuff
• Absolute Luminosity, L
• Apparent brightness, l, is the power crossing
  unit area

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Describing Stars

• More massive stars have greater luminosity
• Radiation is blackbody spectrum
• Deduce temperature from known blackbody spectra
  here on earth
• Turns out star color (temperature) is related to
  luminosity and thus to mass
• Summarized on H-R Diagram

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Hertzsprung-Russell
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Interior of a Star
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Evolution of a Star
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Helium Fusion
Helium burning moves star to horizontal branch.
For massive stars nucleosynthesis ends at iron
and nickel (most stable nuclei with highest
binding energy per nucleon)
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Whats Next?

• Depends on stars mass
• Less than 1.4 solar masses, star collapses as
  fusion stops and star becomes a white dwarf
• Our sun will be a white dwarf about the size of
  the earth
• Radiates away energy and becomes a black dwarf
  that is dead and lifeless

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Massive Stars

• Continued collapse enables further fusion even
  though it is endothermic
• Reverse beta decay combines electrons and protons
  into neutrons
• Star can become a massive nucleus called a
  neutron star
• Much energy must be released

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Supernovae

• Final collapse to a neutron star is accompanied
  by a huge explosion which forms all the elements
  of the periodic table and blows away the outer
  envelope of the star
• The neutron star may become a black hole