Information in Radio Waves
Background: Spectroscopy / 21-cm Line
Overall, spectroscopy is the study of matter and its interactions with radiant energy. Either
matter is going to absorb the radiant energy or it is going to emit it and the extent to which it
does so can tell the observer a significant amount about the material doing the
absorbance/emittance. Because of this, spectroscopy techniques are widely used throughout
many areas of science.
Historically, spectroscopy started with the study of visible
light but was later greatly expanded to all areas of radiation
as a function of both wavelength and frequency. Today,
spectroscopy is widely used in the field of physical and
analytical chemistry. In chemistry, spectroscopy is an
invaluable tool in determining the atomic composition and
functional groups of just about any compound. The
principle behind this is that every atom and molecule has a
specific resonance with certain frequencies or wavelengths
of radiant energy. This resonance is due to molecules and
atoms having unique energy states, and because of this,
these molecules will exhibit a unique spectral line that
corresponds to the transitions between energy states and
can therefore be used to identify the components of any
unknown compound.
These same principles are paramount in the investigations
in astronomy, as almost all research telescopes and
remote sensing equipment include a spectrograph of some sort. Spectroscopy in astronomy
began with the simple dispersion of sunlight using a prism. These early astronomers, such as
Isaac Newton, probably observed dark lines within the overall continuous spectrum. Those
distinct dark lines were eventually described in detail by Joseph von Fraunhofer as absorption
lines and correspond to the absorption and deficiency of photons at those specific wavelengths.
Each absorption line corresponds to a specific element which can therefore be used to identify
the chemical composition of many celestial bodies. Of particular importance is hydrogen.
Hydrogen is theorized to be by far the most abundant element in the universe. By knowing the
specific frequencies hydrogen resonates with, an astronomer can much more readily locate and
investigate new sources in space. The first of these frequencies was the Balmer series and lay
within the visible spectrum. This series corresponds to the transitions between individual
energy levels of Hydrogen. Of more significance though was the discovery of the “21-cm line.”
The 21-cm line (1420 MHz) is the resonant frequency of the spin-flip transition of neutral
hydrogen. This individual frequency lies within the radio spectrum and has been readily used
since its discovery as the benchmark line in astrochemistry. The presence of this line, due to its
prominence throughout space is used for a variety of information: direction, velocity,
temperature, pressure, presence of electric and magnetic fields, and more.
Information in Radio Waves
Reference:
"The HI 21 Cm Line." The HI 21 Cm Line. Web. 16 July 2014. <http://www.cv.nrao.edu/course/astr534/HILine.html>.
"The Hydrogen 21-cm Line." The Hydrogen 21-cm Line. Web. 16 July 2014.
<http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/h21.html>.
"Interpreting Astronomical Spectra." Google Books. Web. 16 July 2014.
<http://books.google.com/books?id=avns1CRdaCYC&pg=PA6#v=onepage&q&f=false>.
"NIST: Atomic Spectroscopy." NIST: Atomic Spectroscopy. Web. 16 July 2014.
<http://www.nist.gov/pml/pubs/atspec/index.cfm>.
Standards:
HS-PS1-5
Apply scientific principles and evidence to provide an explanation about the effects of changing
the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
HS-PS2-1
Analyze data to support the claim that Newton’s second law of motion describes the
mathematical relationship among the net force on a macroscopic object, its mass, and its
acceleration.
HS-PS2-2
Use mathematical representations to support the claim that the total momentum of a system of
objects is conserved when there is no net force on the system.
HS-PS2-4
Use mathematical representations of Newton’s Law of Gravitation and Coulomb’s Law to
describe and predict the gravitational and electrostatic forces between objects.
HS-PS3-2
Develop and use models to illustrate that energy at the macroscopic scale can be accounted for
as a combination of energy associated with the motions of particles (objects) and energy
associated with the relative positions of particles (objects).
HS-PS4-1
Use mathematical representations to support a claim regarding relationships among the
frequency, wavelength, and speed of waves traveling in various media.
HS-PS4-4
Evaluate the validity and reliability of claims in published materials of the effects that different
frequencies of electromagnetic radiation have when absorbed by matter.
HS-ESS1-2
Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra,
motion of distant galaxies, and composition of matter in the universe.
Enrichment:
Online course materials:
-http://www.astronomynotes.com/ismnotes/s2.htm
○ Decent section in an online astronomy course
Variety of tools, lessons and activities:
-http://lasp.colorado.edu/home/education/k-12/project-spectra/
○ Good Website with a variety of resources and activities involving spectroscopy
Science Fair Idea -http://nfsiserver.yerkes.uchicago.edu/FTProot/spectroscope/Spectroscopy/complex_spectroscope.htm#step5
○ Idea for a classroom demonstration or for a science fair project
Presentation on a Hydrogen line investigation:
-http://egg.astro.cornell.edu/alfalfa/grads/introalfalfa_feb11.pdf
○ good slides and pictures/spectrums included
Podcasts on using 21-cm line to measure mass loss from stars:
-http://egg.astro.cornell.edu/alfalfa/grads/introalfalfa_feb11.pdf
○ 7 and 8 minutes long respectively
SRT project (measuring the rotation curve of Galaxy):
-http://www.haystack.mit.edu/edu/undergrad/srt/SRT%20Projects/rotation.html
○ can be done without SRT if you can get your hands on a set of data
Information in Radio Waves
Unit Breakdown:
This unit was designed to introduce the high school student to one of the staples of radio astronomy, spectroscopy
and specifically the 21-cm line. Included in this unit is a powerpoint presentation along with its “Teacher Notes”
section that directly aligns to it. An assessment for the students is also included that directly goes along with the
presentation. In addition, a secondary reading supplement has been included with its own associated questions and
Two activities have been provided. One, titled Identify Me, gives students basic practice with the ideas behind
spectroscopy and is simpler overall. The other, titled Identifying Life and its Origins, is much more complex and has
students investigate spectral data to identify organic molecules with the overall goal of showing students how every
molecule has its own unique fingerprint. The second activity also has a case study component where students will
read two articles about life’s origins and answer open-ended questions about them. A secondary goal of this activity
is to expand the students minds on the search for life outside of Earth and that it goes beyond just hoping to see
aliens somewhere. Hopefully it will spark interest and discussion among your students. Lastly, a small handout has
been included for the students that lists many of the chemicals that can and have been identified in space which goes
along well with the previous activity.