Title Slide

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Title Slide
Astronomy 204 Lecture 19 Spectral Classification
the HR Diagram
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Lecture Summary

• Spectral Classification
• Balmer Thermometer
• Spectral Type
• HR Diagram
• Special Problems
• Why are bright stars bright?
• What are the most common stars?
• Distance Determination Techniques
• Main Sequence Fitting
• Spectroscopic Parallax

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Stellar Spectrum
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Balmer Thermometer
- Balmer absorption lines are produced by
hydrogen atoms whose electrons are in the second
energy level.

• Cool stars ? weak Balmer lines
• Hot stars ? weak Balmer lines
• Medium Temperature stars ? strong Balmer lines

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Beyond Balmer Lines
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Beyond Balmer Lines
You Try It Estimate the surface temperature of
a star that has very strong ionized calcium
absorption lines?
Answer 4800 K
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Spectral Classification
The original classification was done by Annie
Cannon at Harvard Observatory 1920.
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Example of Spectral Types
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More Examples of Spectral Types
O and B stars have wavelength peaks in the
Ultraviolet.
A,F,G,and K stars have wavelength peaks in the
visual.
M stars have wavelength peaks in the infrared.
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Spectral Class Chart
You Try It What is the approximate surface
temperature of a star with spectral type B5?
Answer 14,000 K
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Shapes of Spectral Lines

• Natural Width the thin, but finite width owing
  to quantum uncertainty
• Rotational Broadening spinning blue and
  redshifts the lines
• Doppler Broadening the Doppler shift from the
  thermal motion of the molecules
• Collisional Broadening collisions spread out
  the lines, highly dependent on density

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Rotational Broadening
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UBV Filters

• Spectral shape on the cheap

The V designation is the apparent brightness of
an object through the V filter and is sometimes
also written as mV
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Color Index

• The blackbody curve can be inferred from just 2
  points

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The Sizes of Stars(Hertzsprung-Russell Diagram)
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A Colorful HR Diagram
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Yet Another HR Diagram
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Lines of Equal Radius
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Possible stellar sizes
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Conceptual HR Diagram

• Luminosity increases upward
• Temperature increases to the left
• Radius increases to the upper right

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Main Sequence Fitting

• Plot observed HR diagram of a cluster of stars
  (in a cluster the stars are all approximately
  the same distance)
• Assume the stars will obey the same HR
  relationship as other stars
• Adjust distance to fit the cluster onto the
  main sequence

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Example Main Sequence Fitting
(m-M)
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Spectroscopic Parallax

• Ia Bright Supergiant
• Ib Supergiant
• II Bright Giant
• III Giant
• IV Subgiant
• V Main-sequence star

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Luminosity Classes
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Spectroscopic Parallax

• Determine the spectral class of the star from the
  types of spectral lines present
• Determine the luminosity class of the star from
  the thickness of the spectral lines
• Plot the point on the HR Diagram at the
  intersection of the spectral type and luminosity
  class
• Read off M on the HR Diagram for this star
• Observe m
• Use the distance modulus m M to determine the
  distance to the star.