A stellar spectrum is one of the most fundamental and
important astronomical tools available. By examining the lines in
the spectrum of a star or galaxy, we can determine such important
properties as temperature, chemical composition, velocity of
approach or recession, pressure, and rate of rotation. A spectrum
is observed when light from an emitting source, such as a star,
passes through a dispersing element such as a prism or
diffraction grating and is broken up into its component colors,
or wavelengths. An instrument consisting of a dispersing element
and the other necessary components needed to produce a useful
spectrum is called a spectroscope.
Spectroscopy involves fundamental atomic physics and is based
upon three laws formulated in 1859 by Gustav Kirchhoff:
In this Exercise we will investigate these three laws of
spectroscopy and learn about their relevance to astronomy.
Although we will be working in the visible part of the spectrum,
it is important to remember that the electromagnetic spectrum
consists of a range of energies well beyond that of visible
light, and much insight into the nature of the stars and galaxies
can be obtained from analyses of gamma rays, X-rays, ultraviolet,
infrared, and radio radiation. Kirchhoff's laws hold for all
wavelengths of the electromagnetic spectrum.
The inner layers of a star's photosphere produce a continuous
spectrum, and as the radiation passes through the outer,
relatively cool layers, an absorption spectrum is produced. The
overall appearance of the spectrum and the intensity of the
absorption lines produced by various elements and molecules
depends largely on the temperature of the stellar atmosphere.
From the background continuum of a stellar spectrum, the
so-called blackbody temperature of the star can be deduced making
use of the observation that the color (wavelength) of maximum
intensity in the continuum shifts toward the blue, or shorter
wavelengths, as the temperature of the source increases. The
physical law governing this effect is named after its discoverer,
and is known as Wien's law. As a mathematical formula, Wien's law
can be expressed: lmax = 2.89x107 Å
where lmax is the
wavelength of the peak intensity in angstroms, Å, and T is the
temperature of the blackbody source in degrees Kelvin. The angstrom
is the most commonly used unit for measuring the wavelength of
visible light in astronomy, and it is equal to 10-10
meters. Another common unit of wavelength is known as a nanometer
(nm), where 1 nm = 10-9 m.
Wien's law informs us that hot stars display a peak intensity
in the ultraviolet or blue region of the spectrum, while cooler
stars have spectra which peak in the red. The total energy
radiated per second by each square centimeter of a radiation
source is known as the "flux" in the terminology
of physics. The flux produced by a blackbody source depends on
the temperature of the source according to the Stefan-Boltzmann
radiation law: E = sT4 W
where s, the Stefan-Boltzmann
constant, is equal to 5.56x10-8 W m-2
K-4. In words, this law simply states that, as the
temperature of a blackbody increases, the total flux (energy
emitted per unit surface area per second) increases with the
forth power of the temperature.
Diffraction Gratings and Spectroscopes
In this Exercise we will use a light dispersing element known
as a diffraction grating which is produced by carefully etching
thousands of parallel grooves per inch onto a transparent medium
like a glass plate. Like a prism, a diffraction grating breaks
light up into its component colors, but, unlike a prism which
does this because of the wavelength dependence of the refraction
index, the grating produces a spectrum by a process involving
constructive and destructive interference of light. The grating
replica you will receive in this lab was formed by rolling a
stiff plastic over an original metal etching, thus imprinting it
with grooves, much in the way that vinyl records are mass
produced from the original expensive cuttings. There are various
technical and practical advantages to the use of gratings over
glass prisms, and in fact many modern astronomical spectrographs
use a combination of the two elements known as a
"grism". In looking through a transmission grating at a
light source, the source itself is seen in the center, and
symmetrically on either side of it are seen the spectra of the
source. Further still from the center of the field of view are
higher order spectra with higher dispersion. That is, in these
high order spectra, the wavelengths are more widely separated,
but generally fainter.
A spectrum with more detail than seen in the simple hand-held
gratings is produced by a spectroscope which contains a slit to
select a thin section of the light source, a collimating lens or
mirror to make the light rays from the source parallel, a higher
quality dispersing element (prism or grating), and an eyepiece to
view the resulting spectrum against a calibrated wavelength
scale. See Figure 1.
Figure 1 -- Schematic Diagram of a Spectrograph
With this introduction and the assigned readings in your
textbook, you should now be able to investigate the properties of
different types of spectra following the format of the following
work sheets. Your instructor may choose to "walk" the
whole class through each section of the lab, or to let you work
your way around each station individually. When you are finished,
staple your work sheets together and hand them in to your
Principles of Spectroscopy -- Preview Questions
Why are objects that radiate with a Planck
distribution called "blackbodies"?
What is the characteristic, or peak, wavelength of
the blackbody radiation emitted by a human being? In what
part of the electromagnetic spectrum is it found? What
real-life applications does this have?
Look up the terms resolution (in your textbook) and
dispersion (in a dictionary).
Which has better resolution?
Which has better dispersion?
Principles of Spectroscopy -- Work Sheets
A continuous spectrum is emitted by an incandescent solid,
liquid, or opaque gas. All colors of the spectrum from red to
violet are represented in a continuous band of light. Such a
spectrum tells us nothing about the chemical composition of the
source; however, the relative intensities of the colors indicate
the temperature of the emitter. From Wien's law, you should know
how the color of a spectrum changes as the source gets hotter.
Look through the grating at the spectrum of the bulb.
- Obtain from your instructor some diffraction grating
material and find an incandescent lamp to examine the
spectra described earlier. You should notice the presence
of more than one spectrum as you look through the
grating. We refer to the first spectrum on each side of
the light source as the first order spectrum, and
the spectra located at greater angles from the line of
sight are known as higher order spectra.
How many higher order spectra can you see?
- Compare the overall appearance of these
higher order spectra to the first order spectrum.
What do you notice? You should be able to find
two major differences between these and the first
With the voltage set very low (about 30 volts
or less). At this low voltage, which is brighter,
the blue end of the spectrum or the red?
How does the overall brightness of the
spectrum change when the voltage is increased
from the low voltage to over 110 volts?
At the highest voltage, which has gotten
significantly brighter, the blue end of the
spectrum or the red?
Is the spectrum continuous, or is it discrete
(are there breaks in it)?
At what voltage is the bulb hottest?
How is this type of radiation actually
produced in the light bulb? Be specific. What
type of physical processes are going on?
Would you expect a red star to be hotter or cooler than a
blue star? Why? Do not use Wien's law and the
Stefan-Boltzmann law to answer this question. Base your
answer on your observations of the bulb.
If two stars, one blue and the other red, have the same
physical size, would you expect the red star or the blue
star to be intrinsically brighter? This time explain your
answer in terms of Wien's law and the Stefan-Boltzmann
An emission-line spectrum is emitted by a glowing,
low-density gas. The gas may be excited to the state
where it emits its characteristic wavelengths when
collisions between atoms at very high temperatures occur,
stimulating transitions between two specific energy
levels in the atoms of the gas, with the corresponding
release of photons of a particular wavelength (color).
Since the energy levels are different in different atoms
and molecules, each atom and molecule has its own unique
set of emission lines.
Using any of the available spectroscopes, examine
the spectra of various elements in the gas
discharge tubes set up around the room. In the
following set of grids, sketch the relative
positions of the bright lines of hydrogen and at
least three other elements as accurately as you
can. Use dark marks for the bright lines and
indicate the colors you perceive the lines to be
for future reference. Also, examine the hydrogen
spectrum carefully. From your textbook readings
and lectures you should know the characteristic
visible lines in the hydrogen spectrum. If you
see many more lines in addition to those that you
expect, indicate on your sketch that these are
not hydrogen lines. Label the bright hydrogen
lines clearly. What is the origin of these
other spectral lines?
- Use the black or gray laboratory spectroscopes to
view one of the discharge tubes. Note that you
can adjust the slit width with a screw on the
Open the slit as wide as possible.
How does the resolution (separation) of
crowded emission lines change as you
close the slit?
- Now open the slit again. How does the
overall intensity of the spectrum change
as you close the slit?
- Suppose you are observing a bright
star and a faint star. How would you
obtain high-resolution spectra of these
stars? What happens to the intensity? So,
would it be easier to obtain a
high-resolution spectrum of a bright star
or a faint star? Explain.
Now that you have examined the spectra of various
discharge tubes with a spectrograph, look at them
again with the spectrophotometer, which is
operated from one of the computers. In the second
set of grids on page 7, sketch the digital
spectrum of each of these elements, being sure to
note relative intensities. Label the exact
wavelength of the three strongest lines.
Observations from spectrographs:
A dark-line or absorption spectrum is observed when a
relatively cool gas a liquid, or a solid is interposed
between a source of continuous radiation and the
spectroscope. The absorption lines correspond exactly to
the emission-line pattern that would be observed if the
same interposing medium were sufficiently excited. The
medium is transparent to most colors, but opaque to those
colors in which it can emit its own characteristic
spectrum. If the medium is hot, but the background
continuum source is hotter still, the line radiation
emitted by the foreground medium will be dark by
comparison with the nearby bright continuum radiation.
Astronomers use the concept of absorption to their
advantage, especially when using small telescopes near
urban areas. Filters are pieces of glass or plastic that
are created such that certain energy photons will, or
will not, go through the filter. As an example of
absorption, we will use a filter that can be easily
attached to the end of a Celestron 8 or 11 inch
So far, we have applied our knowledge of spectra
only to laboratory sources. In stellar spectra we
find a continuous spectrum superimposed with many
- Look at the spectrum of an incandescent lamp with
a diffraction grating as in the Continuous
Spectra part of this Exercise. Now hold the
SkyGlow filter between the light source and the
Describe what you see.
- What elements have strong spectral
lines in the region that the SkyGlow
- What are these elements used for
(i.e. why is this filter called a SkyGlow
Keeping Kirchhoff's radiation laws in
mind, what part of the star does the
continuous radiation come from?
Which part produces the absorption
What can you say about the relative
temperatures of the outer layers of a
stellar atmosphere with respect to the
layers which lie further within the star?
Using the hand-held spectrograph, examine the
spectrum of a fluorescent light bulb.
What type of spectrum do you
How might you explain this spectrum?
You may need to get hints from your
instructor to answer this!
Principles of Spectroscopy -- Concluding
For part of this lab, we use our eye as the
detector and for another part we use a
spectrophotometer. Name three advantages the
spectrophotometer has over the eye as a detector.
(Hint: the detector in the spectrophotometer is
like a CCD.)
To which of the three types of spectra
does Wien's law apply? Notice that the mercury
discharge tube glows bluish-white while the neon
tube is bright red. Can we use Wien's law to tell
the relative temperatures of the gas within the
tubes? Explain clearly.
for the following stars and estimate which color
dominates the visual spectrum.