Information in Radio Waves
Using the “Spectroscopy and the 21-cm Line” Presentation:
Slide 1: Spectroscopy and the 21-cm Line
- Focus of this presentation is to introduce what the hydrogen line is and where it comes from
and go into detail about the types of information that can be gathered by observing the
hydrogen line.
Slide 2: To get you thinking...
- Have students answer these questions on their own or as a class. Make sure to cover the
correct answers together and clear up any misinformation. A good fifth question to ask would
be the difference between neutral atomic hydrogen and molecular hydrogen.
Slide 3: Neutral Hydrogen
- Atomic Hydrogen consists of a single proton and electron. The electron exists in the ‘1S’
orbital.
- The first motion of the electron is its orbit. Have students guess what the second motion of the
particles would be.
Slide 4: “2nd” Motion
- The second motion is the individual spins of the particles. Both the proton and the electron are
going to have an individual spin which can be in the same direction as one another (aligned) or
opposite (anti-aligned).
- Through reasons described by quantum mechanics, the electron when in the aligned state is
going to have slightly higher energy than the anti-aligned state.
Slide 5: Aligned vs. Anti-aligned
- Make note that despite the energy differences, the electron of hydrogen is still going to exist in
the lowest 1S orbital.
- The diagram included in this slide shows how each orbital can be broken down into ito
corresponding hyperfine structure.
Slide 6: Emission
- It is possible for atomic hydrogen to jump from the higher energy aligned state to the antialigned state.
- On its own, this is very unlikely to happen spontaneously.
○ If it weren’t for processes that include atomic collisions, the signal of this would
be too weak to detect.
○ Atomic Collisions increase the probability of this spin-flip and therefore increase
the signal strength of it
Information in Radio Waves
Slide 7: The 21-cm Line
- The energy difference between the two spin orientations corresponds to the frequency 1420.4
MHz.
- Was predicted in 1944 and discovered in 1951
Slide 8: Detecting the 21-cm line
- Picture in this slide is of the horn developed and used at Harvard by Edward Mills Purcell and
Harold Irving Ewen.
- The link is to an NRAO page that describes in length the work in developing the detecting
horn.
Slide 9: The 21-cm Line (2)
- First maps of the Galaxy were made based on the brightness of the signal in 1952.
- By combining this data with measurements of the doppler shift a better model of our Galaxy’s
structure could be made. This revealed the spiral structure of our galaxy.
○ A more extensive discussion on the doppler shift can be found in later slides
Slide 10: What Else?
- Be sure to point out to students that the hydrogen line is the energy of the spin-flip transition of
neutral atomic hydrogen. Atomic Hydrogen is not the same thing as Molecular hydrogen which
does not have the same corresponding spectral line(s). Molecular hydrogen, while present in
equal amounts to atomic hydrogen, does not have clearly defined spectral lines and is therefore
not as useful in astronomical observations.
- This slide describes the abundance of Atomic Hydrogen universe wide and how it therefore
can be used as a probe to find other far away sources.
- Also describes how using the doppler shift, the motions and velocities of these sources can be
measured.
Slide 11: Other Spectral Lines...
- Included with this presentation is a handout that lists many chemicals that have been identified
in space.
Slide 12: Other Spectral Lines (2)
- A few of the chemicals in space that have clearly defined spectral frequencies.
- These chemicals can therefore also be used to identify interesting cosmic sources for study
Slide 13: Ok we have found something... what else?
- List of details and information that can be gained via the spectral lines of a cosmic source
Slide 14: Emission / Absorption
- The presence or absence of certain spectral lines can signify whether energy is being
absorbed or emitted
○
presence of spectra in general has to do with atoms making energy transitions, either up
or down
Information in Radio Waves
Slide 15: Emission
- Emission spectra of a star or other thermal body is going to exhibit the black body radiation
curve of that temperature. Therefore, stars will exhibit a continuous spectra of emission.
- Other sources, like clouds of dust and gas do not exhibit this thermal emission and give off
specific wavelengths of energy that correspond directly to the atomic / molecular components
○ These spectra are the frequencies of energy that are given off when energy
transitions occur in the atoms or molecules (ex - the hydrogen line)
○ the figure shows the spectra of specific elements as measured in a lab
Slide 16: Absorption
- When waves of a continuous spectrum (from a star) pass through a cloud of interstellar matter,
certain frequencies of wavelengths are going to be absorbed. Again, the frequencies that are
absorbed directly correspond to the atoms or molecules that are absorbing the energy and
corresponds to the energy gap between the different energy states.
Slide 17: Direction and Velocity
- By measuring the doppler shift of a spectral line, you can tell the general direction it is
travelling. For example, if the hydrogen line is shifted to a higher frequency, then the source is
moving towards you (blue shift). If the hydrogen line is seen at a lower frequency, the source is
moving away (red shift). The extent to which it is shifted reveals how fast the source is moving.
- By measuring this shift over time, a pattern or path of movement can be determined.
○ When this was done with our milky way galaxy, its spiral structure was revealed.
- Using the equation given, if you know the frequencies of the source and whats observed, you
can determine the source’s velocity
- If students have difficulty with this, give them some examples.
Slide 18 and 19: Temp. and Pressure
- These slides explain that high temperatures and pressures will broaden a typically narrow
spectral line. This is because a spectral line includes local doppler shifts within the source itself.
- Matter likes to move in all directions, if matter is heated or pressurized it moves even faster in
all directions. This causes the doppler shift to increase in both the red and blue and therefore
increases the local doppler shifts of the source (broader spectral line).
Slide 20: Electric / Magnetic Fields
- Contains links that describe what happens to light when it travels through different scenarios of
electric / magnetic fields
- Fairly high level, might be easiest to show that it can cause the spectral line to undergo
splitting.
Slide 21: 21-cm line reveals extra...
- Measuring the amplitude of the hydrogen line can give a measurement of the overall source’s
mass. The relative abundance of atomic hydrogen is known and can therefore be used as a
Information in Radio Waves
measure against the strength of the source’s line. This gives a measure of that source’s mass.
- When this method is used to measure the mass of our Galaxy, something peculiar is
observed.
○ The galaxy’s rotation curves don’t match up
○ Means there must be some unaccounted for source of mass in our galaxy
○ One of the best pieces of evidence for the existence of dark matter