Stars

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StarsGiants, Supergiants, Dwarfs.

• How we can tell a lot about stars from starlight.
  (You need a lot of physics)

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Here are moreevery star tells a story
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Last weeks thrilling episode you can learn a
lot about stars from their spectra
The spectrum of the Sun
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(No Transcript)
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Starlightapplication of spectroscopy to stars

• Continuous spectrum gives surface temperature
  (Wiens Law)
• Spectral lines give chemical composition,
  temperature (also), speed of rotation (How?) and
  other properties
• Examples of stellar spectrawhat can we say?

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Spectral classes of stars O,B,A,F,G,K,M
What can you say about the temperatures of these
stars?
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So what have we learned about stars from their
spectra?

• Temperatures (real hot)
• Recipes (what elements they are made of)
• Speed of rotation
• More neat things

We can also tell which ones are big and which
ones are not
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The physics of pressure broadening

• What can we learn from observing whether a
  spectral line is narrow or broad?

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A tale of three stars (a black one, a pink one,
and a red one)

• What does it mean?

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How does pressure modify the shape of a spectral
line?

• Think first of an atom emitting (or absorbing)
  light in isolation

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When the pressure goes up, atoms feel their
neighbors and have identity crises. The atomic
energy levels, instead of being crisp and unique,
get fuzzed out. Fuzzed out is technical
terminology for a change in the energy which
depends on how close the neighbors are, how many
of them there are, what direction they are,
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Broad lines from pressure-broadened atoms
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What determine the pressure in a stars
atmosphere?

• The weight of the gas on top of you
• Weight determined by two things
• How much gas (density and height of column)
• The acceleration due to gravity
• g GM/r2

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Pressure in a stellar atmosphere

• The larger the star is, the bigger its radius r
• If the mass is the same as a smaller star, the
  acceleration of gravity will be smaller
• The atmospheric pressure will be less, as well as
  pressure broadening.

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Just from the spectra (no idea of distance) we
can tell big stars (giants) from little stars
(dwarfs)
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A real question if two stars have the same
color, and pressure broadening of the spectral
lines tells you that one is much larger than the
other, what can you say about the relative
brightnesses?
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Summary

• From spectra, we can determine that some stars
  are much larger, and thus much more luminous
  than others of the same color (temperature).
• There is a wide range in luminosity (and size)
  for stars of the same temperature.
• Leads to the classification of dwarfs, giants,
  and supergiants

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Disclaimer

• Historically, this conclusion was reached by
  measurement of distances to stars. It is also
  easier to show that stars must have different
  luminosities (given their distances). However,
  you can conclude this just from the spectra.

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With information provided by spectroscopy, we can
search for correlations between stellar properties
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With information provided by spectroscopy, we can
search for correlations between stellar properties
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What the data show the Hertzsprung-Russell
Diagram

• Highest quality data from the Hipparchus
  spacecraft

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The Hertzsprung-Russell Diagram and the Types of
Stars

• See Figure 16.20
• Types of stars, important terms
• Main Sequence
• Giants
• Supergiants
• White dwarfs

What does it all mean?
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Hertzsprung-Russell Diagram