Atoms & Light
(Spectroscopy)
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 blackbodies
B. Radiation emitted by a blackbody
1. graph of intensity emitted vs. wavelength
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 blackbodies
B. Radiation emitted by a blackbody
1. graph of intensity emitted vs. wavelength
2. max = wavelength of maximum intensity
emitted by a blackbody
II. A Brief History of Spectroscopy
A. Issac Newton (1666)
- passes sunlight through a slit and a prism
=> full rainbow of colors (continuous spectrum)
B. Joseph Fraunhofer (1814)
- passes sunlight through slit & a diffraction grating
- finds 100s of dark lines in sun’s 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
C.
Robert Bunsen & Gustav Kirchhoff (1859)
1. Vaporize chemical elements & take the spectrum of the
light that is emitted
=> spectrum is a series of bright lines
- unique set of lines for each chemical element
2. Identify unknown samples by bright line patterns
3. Recognize that sodium's two bright lines have the same
wavelength as Fraunhofer dark D lines
4. Kirchhoff’s 3 Laws of Spectral Analysis
a. Hot solids, hot liquids, and hot high density gases
=> Continuous Spectrum
b. Hot low density gases
=> Bright (Emission) Line Spectrum
c. Light from a continuous spectrum source passing
through a cooler low density gas
=> Dark (Absorption) Line Spectrum
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
Continuous Spectrum
Emission Spectrum
Absorption Spectrum
D. Niels Bohr (1913)
1. Spectral lines (both bright & dark) are due to
electrons in atoms changing energy
=> 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
=> dark (absorption) line spectrum
b. Electron emits a photon
- goes to lower energy level
=> bright (emission) line spectrum
D. Niels Bohr (1913)
1. Spectral lines (both
bright & dark) are due to
electrons in atoms
changing energy
=> electrons allowed
only certain energies
Star Temperatures
• Spectral lines can be used as a
sensitive star thermometer.
Spectral Lines and Temperature
• From the study of blackbody radiation, you
know that temperatures of stars can be
estimated from their color—red stars are
cool, and blue stars are hot.
• However, the relative strengths of various
spectral lines give much greater accuracy
in measuring star temperatures.
Spectral Lines and Temperature
• The strength of the hydrogen Balmer lines depends on
the temperature of the star’s surface layers.
– Both hot and cool stars have weak Balmer lines.
– Medium-temperature stars have strong Balmer lines.
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.
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.
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.
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!”
Temperature Spectral Classification
• This set of star types—called the
spectral sequence—is 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.
Temperature Spectral Classification
• These finer divisions define a
star’s 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.
Temperature Spectral Classification
• The figure shows color images of 13
stellar spectra—ranging from the hottest
at the top to the coolest at the bottom.
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..
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.
Temperature Spectral Classification
• Compare the figures
and notice how the
strength of spectral
lines depends on
temperature.
III.
The Doppler Effect
A. Doppler Effect for sound
- source of sound moving away
=> hear longer 
- source of sound moving toward
=> hear shorter 
- amount of shift in wavelength
=> speed toward or away
B. Doppler Effect for light
- star's spectral lines shifted
- shift to longer  (Red Shift)
=> star moving away
- shift to shorter  (Blue Shift)
=> star moving toward
- amount of shift
=> star’s speed toward or away