Atoms

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Atoms Light (Spectroscopy)
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Blackbody Radiation A. Blackbody a
hot solid, hot liquid, or hot high density gas
that emits light over a range of
frequencies - stars are almost blackbodiesB.
Radiation emitted by a blackbody 1. graph of
intensity emitted vs. wavelength
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I. Blackbody Radiation A. Blackbody a
hot solid, hot liquid, or hot high density gas
that emits light over a range of
frequencies - stars are almost blackbodiesB.
Radiation emitted by a blackbody 1. graph of
intensity emitted vs. wavelength 2. ?max
wavelength of maximum intensity emitted
by a blackbody
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II. A Brief History of Spectroscopy A. Issac
Newton (1666) - passes sunlight through a slit
and a prism gt full rainbow of colors
(continuous spectrum) B. Joseph Fraunhofer
(1814) - passes sunlight through slit a
diffraction grating - finds 100s of dark lines
in suns spectrum - labels darkest lines A, B,
C, D, E, F, G, H, K C. Robert Bunsen Gustav
Kirchhoff (1859) - Vaporize chemical elements
take the spectrum of
the light that is emitted
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Robert Bunsen Gustav Kirchhoff (1859)1.
Vaporize chemical elements take the spectrum of
the light that is emitted gt spectrum is a
series of bright lines - unique set of lines
for each chemical element2. Identify unknown
samples by bright line patterns3. Recognize
that sodium's two bright lines have the same
wavelength as Fraunhofer dark D lines
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4. Kirchhoffs 3 Laws of Spectral Analysis a.
Hot solids, hot liquids, and hot high density
gases gt Continuous Spectrum b. Hot low
density gases gt Bright (Emission) Line
Spectrum c. Light from a continuous spectrum
source passing through a cooler low
density gas gt Dark (Absorption) Line
Spectrum
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Three types of Spectra Continuous from
glowing solids or very compressed gases, such as
the photosphere of the Sun Emission from hot,
glowing gases that are rarefied (not very
compressed, such as an emission nebula or
features in the solar atmosphere Absorption a
combination spectrum produced by a continuous
light source passing through cool gases. The
gases take what they want from the
spectrum. Examples planetary atmospheres,
stellar spectra
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Continuous Spectrum
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Emission Spectrum
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Absorption Spectrum
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D. Niels Bohr (1913) 1. Spectral lines (both
bright dark) are due to electrons in
atoms changing energy gt electron allowed only
certain energies 2. Structure of the
hydrogen atom - proton () at nucleus
electron (-) outside - atom diameter 10-10
m - proton diameter 10-15 m 3. Energy
level diagram for the electron of a hydrogen
atom a. Electron absorbs a photon - goes
to higher energy level - photon must have
correct energy gt dark (absorption) line
spectrum b. Electron emits a photon - goes
to lower energy level gt bright (emission)
line spectrum
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D. Niels Bohr (1913) 1. Spectral lines
(both bright dark) are due to
electrons in atoms changing energy
gt electrons allowed only certain
energies
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Star Temperatures

• Spectral lines can be used as a sensitive star
  thermometer.

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Spectral Lines and Temperature

• From the study of blackbody radiation, you know
  that temperatures of stars can be estimated from
  their colorred stars are cool, and blue stars
  are hot.
• However, the relative strengths of various
  spectral lines give much greater accuracy in
  measuring star temperatures.

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Spectral Lines and Temperature

• The strength of the hydrogen Balmer lines depends
  on the temperature of the stars surface layers.
• Both hot and cool stars have weak Balmer lines.
• Medium-temperature stars have strong Balmer lines.

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Spectral Lines and Temperature

• Each type of atom or molecule produces spectral
  lines that are weak at high and low temperatures
  and strong at some intermediate temperature.
• The temperature at which the lines reach maximum
  strength is different for each type of atom or
  molecule.

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Temperature Spectral Classification

• Astronomers classify stars by the lines and bands
  in their spectra.
• For example, if it has weak Balmer lines and
  lines of ionized helium, it must be an O star.

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Temperature Spectral Classification

• The star classification system now used by
  astronomers was devised at Harvard during the
  1890s and 1900s.
• One of the astronomers there, Annie J. Cannon,
  personally inspected and classified the spectra
  of over
• 250,000 stars.

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Temperature Spectral Classification

• The final classification includes seven main
  spectral classes or types that are still used
  today
• O, B, A, F, G, K, and M
• Oh, Be A Fine Guy/Girl, Kiss Me!

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Temperature Spectral Classification

• This set of star typescalled the spectral
  sequenceis important because it is a temperature
  sequence.
• The O stars are the hottest.
• The temperature continues to decrease down to
  the M stars, the coolest.
• For further precision, astronomers divide each
  spectral class into 10 subclasses.
• For example, spectral class A consists of the
  subclasses A0, A1, A2, . . . A8, and A9.
• Next come F0, F1, F2, and so on.

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Temperature Spectral Classification

• These finer divisions define a stars temperature
  to a precision of about 5 percent.
• Thus, the sun is not just a G star.
• It is a G2 star, with a temperature of 5,800 K.

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Temperature Spectral Classification

• The figure shows color images of 13 stellar
  spectraranging from the hottest at the top to
  the coolest at the bottom.

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Temperature Spectral Classification

• Color spectra as converted to graphs of intensity
  versus wavelength with dark absorption lines as
  dips in the graph.
• Such graphs show more detail than photos and
  allow astronomers to quantitate data..

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Temperature Spectral Classification

• Notice also that the overall curves are similar
  to blackbody curves.
• The wavelength of maximum is in the infrared for
  the coolest stars and in the ultraviolet for the
  hottest stars.

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Temperature Spectral Classification

• Compare the figures and notice how the strength
  of spectral lines depends on temperature.

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The Doppler Effect A. Doppler Effect for
sound - source of sound moving away gt hear
longer ? - source of sound moving toward gt
hear shorter ? - amount of shift in
wavelength gt speed toward or away B.
Doppler Effect for light - star's spectral
lines shifted - shift to longer ? (Red
Shift) gt star moving away - shift to
shorter ? (Blue Shift) gt star moving toward
- amount of shift gt stars speed toward or
away
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