Announcements

1
Announcements

• Observing on the roof of Van Allen Hall has
  started and will run Tuesday to Thursday from
  9-11 pm this week.
• The first hour exam will be on Friday, September
  17.

2
Temperature and Color, Classifying Stars

• Spectrum of light
• How the eye sees color
• Temperature and color/spectrum
• Colors/spectra of stars
• Classifying stars
• Reading sections 16.5-16.6, 6.2

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Electromagnetic spectrum
The spectrum of a particular star is how much
light it produces at each wavelength.
4
How your eye sees light and color
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Rods and cones on the retina sense light
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Rods and cones

• Cones are color sensors
• There are cones for red, green, and blue
• The color ones perceives depends on the firing
  rates of the red vs. green vs. blue cones
• Cones need relatively bright light to work
• Rods give finer, more detailed vision
• Rods can work with less light
• At night, color vision is less effective because
  only the rods function

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Sensitivity of cones
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A star will produce light overlapping the
response of all three cones. The color of the
star depends on how strong its spectrum is in the
ranges covered by the different cones.
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A star will produce light overlapping the
response of all three cones. The color of the
star depends on how strong its spectrum is in the
ranges covered by the different cones.
10
A star will produce light overlapping the
response of all three cones. The color of the
star depends on how strong its spectrum is in the
ranges covered by the different cones.
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What can we learn from a stars color?

• The color indicates the temperature of the
  surface of the star.
• The same is true for the filament in a light bulb
  or any other hot object. In general, we call
  radiation from a hot body black body radiation
  (do demonstration 6B40.10).

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Wiens law

• Cooler objects produce radiation which peaks at
  longer wavelengths (redder colors), hotter
  objects produce radiation which peaks at shorter
  wavelengths (bluer colors).

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A stars color depends on its surface temperature
Spectrum demonstration 6B40.55
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Stars are assigned a spectral type based on
their spectra

• The spectral classification essentially sorts
  stars according to their surface temperature
• The spectral classification also uses spectral
  lines, which will discuss on Friday

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Spectral type

• Sequence is O B A F G K M
• O type is hottest (25,000K), M type is coolest
  (2500K)
• Star Colors O blue to M red
• Sequence subdivided by attaching one numerical
  digit, for example F0, F1, F2, F3 F9 where F1
  is hotter than F3 . Sequence is O O9, B0, B1,
  , B9, A0, A1, A9, F0,
• Useful mnemonics to remember OBAFGKM
• Our Best Astronomers Feel Good Knowing More
• Oh Boy, An F Grade Kills Me
• (Traditional) Oh, Be a Fine Girl (or Guy), Kiss Me

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The spectrum of a star is most determined by

• The temperature of the stars surface
• The stars distance from Earth
• The density of the stars core
• The luminosity of the star

17
Classifying stars

• We now have two properties of stars that we can
  measure
• Luminosity
• Color/surface temperature
• Using these two characteristics has proved
  extraordinarily effective in understanding the
  properties of stars the Hertzsprung-Russell
  (HR) diagram

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HR diagram
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HR diagram

• Originally, the HR diagram was made by plotting
  absolute magnitude versus spectral type
• Now, its better to think of the HR diagram in
  terms of physical quantities luminosity and
  surface temperature

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If we plot lots of stars on the HR diagram, they
fall into groups
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These groups indicate types of stars, or stages
in the evolution of stars
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Luminosity classes

• Class Ia,b Supergiant
• Class II Bright giant
• Class III Giant
• Class IV Sub-giant
• Class V Dwarf
• The Sun is a G2 V star