To get you thinking...
1.) What is an element? Give at least 4 examples of
elements.
2.) What is the atomic number of hydrogen? What does a
neutral hydrogen atom consist of? Describe its "motion".
3.) How does an atom become "excited"? When it becomes
"un-excited", what does it do?
4.) What is spectroscopy? How is it used on Earth? How
can it be used by an astronomer?
Neutral Hydrogen
- As you (should) know, neutral hydrogen
consists of a single proton and a single electron
orbiting the proton
●  This electron is found orbiting in the ‘1S’
orbital
- There is a second motion besides the orbit of
the electron around the proton
●  This is true for all atoms, but we will focus
on Hydrogen as it is much simpler to
explain
"2nd" Motion:
- Both the proton and the electron are going to
have an individual spin
●  The spin of both can therefore be in the
same direction (aligned) or in opposite
directions (anti-aligned)
●  Because of quantum mechanics, it turns
out when the spins are aligned, the
hydrogen is higher in energy
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/HydrogenLineParallel.svg/500px-HydrogenLineParallel.svg.png
Aligned vs. Anti-Aligned
- Even though the Aligned version is higher in
energy, its electron still exists in the 1S orbital
●  Instead, the aligned version compared to
the anti-aligned version has hyperfine
structure
Emission
- It is possible for hydrogen to jump from its higher
energy aligned state to the lower energy antialigned state
●  Very unlikely to happen spontaneously:
o
o
o
probability of 2.9×10−15 s−1
time it takes for a single isolated H atom to undergo
this transition is ~ 10,000,000 yrs
atomic collisions decrease the time interval greatly
allowing for the spectral line to be observable
●  When it does happen, it releases a specific
wavelength of light...
o
Care to guess what that wavelength is?
The 21-cm Line:
- The energy gap between the
hyperfine structures directly
corresponds to the 21-cm
wavelength (1420.405... MHz)
●  This wavelength was
predicted Hendrick C.
van de Hulst in 1944
●  Discovered by Edward
Mills Purcell and Harold
Irving Ewen in 1951
http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Green_Banks_-_EwenPurcell_Horn_Antenna.jpg/321px-Green_Banks_-_Ewen-Purcell_Horn_Antenna.jpg
Detecting the 21-cm line:
Full story/history can
be found here:
http://www.gb.nrao.edu/
fgdocs/HI21cm/
ephorn.html#thestory
- So what's the point? What can be done with
this information?
●  First use of this was in 1952 where the
first maps of neutral hydrogen in our
Galaxy were made
●  These maps, using the doppler shift of the
1420 MHz spectral line, revealed the
spiral structure of our Galaxy
http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/NGC_6384_HST.jpg/320pxNGC_6384_HST.jpg
http://upload.wikimedia.org/wikipedia/commons/thumb/4/43/ESO-VLT-Laser-phot-33a-07.jpg/320pxESO-VLT-Laser-phot-33a-07.jpg
The 21-cm Line:
What Else?
Note: Neutral hydrogen is not the same as
molecular hydrogen. Both are found in roughly
the same amount in our galaxy, but molecular
hydrogen does not have a clearly defined
spectral line for detection.
- Hydrogen is the most abundant
element in our universe
One of the first to form during the
aftermath of the big bang
o  Therefore the spectral line of
hydrogen can be used to identify
clouds or sources outside of our
own galaxy
- By using the doppler shift, the relative
motions of objects in our universe can be
tracked as well
o
http://upload.wikimedia.org/wikipedia/commons/1/14/Redshift.png
Other Spectral Lines...
- So if hydrogen can be identified by a specific
frequency, can other elements or compounds?
●  Yes! Radio astronomers have identified a
lot ranging from diatomic (2-atoms)
compounds all the way up to compounds
made of more than 10 atoms
o
Wikipedia lists nearly 200 of them!
Other spectral lines...
•Hydroxyl radical (OH)
1612.231 MHz
•Methyladyne (CH)
3263.794 MHz
•Formaldehyde (H2CO)
•Methanol (CH2OH)
•Helium Isotope (3HeII)
829.66 MHz
6668.518 MHz
8665.65 MHz
•Cyclopropenylidene (C3H2) 18.343 GHz
•Water Vapour (H2O)
22.235 GHz
•Ammonia (NH3)
23.694 GHz
On the left is a list of some
astronomical chemicals and
their corresponding
frequencies. Find radiation at
one of these frequencies, and
you have found a source
containing that molecule.
Ok, we have found
something... what else?
- There is more information embedded in the
spectral line other than the origin's identity:
●  Emitting/Absorbing
●  Abundance/Density
●  Direction and Velocity
o
Rotation
●  Temperature and Pressure
●  Electric / Magnetic Fields
Emission / Absorption
- The presence of absorption or emission
spectral lines depends on atoms or molecules
making an energy transition:
o
Emission - energy change to a lower state
o
Absorption - energy change to a higher state
Emission
- Results in the
actual emission of
light and a very
specific and
limited spectrum
of light. (exneutral Hydrogen
emits the
wavelength of 21cm)
Absorption
●  Happens when waves of
an electromagnetic wave
(shown in visible light)
passes through a cool gas.
The gas “robs” (absorbs)
photons at specific
wavelengths to energize
its electrons.
●  Result – Specific
wavelengths “drop out” of
the otherwise continuous
spectrum.
Direction and velocity
Velocity
- By measuring the doppler shift of spectral
lines in space you can tell how fast they are
moving.
- General Direction is measured based on which
direction the shift is occurring (red or blue)
o
o
Longer observations can yield a true path or orbit by
measuring changes in the doppler shift over time
Pattern of movement can also be determined in the
same way (ex - milky way’s spiral structure)
Temp. and Pressure
- High temperatures and pressures can broaden
spectral lines.
Typical
Broadened
- Due to high doppler shifts of individual molecules
Temp. and Pressure cont:
- The more uniform in direction the atoms are,
the sharper the spike.
o
Not a common occurance
- Matter tends to move in all directions, the
spectral spike is a measure of the average
motion of all atoms in the area
o
The higher the temperature, the faster molecules
move, the broader the spike is going to be
Electric / Magnetic fields
- Depending on the differing directions and
strengths of these fields, the observed light
waves will have undergone specific changes or
have certain characteristics
●  Ex Stark Effect – Plasma density (learn more)
o  Zeeman Effect – Magnetic field (learn more)
o  Paschen-Back Effect – Strong Magnetic Field (
learn more
o
21-cm line reveals extra...
- Mentioned already: the hydrogen line
revealed the spiral structure of our galaxy via
the doppler shift
o
spectral line also allows for a means to “weigh” areas
of the cosmos
- Interesting fact: the observable mass from
stars and gas cannot account for the Galaxy
rotation curves
o
o
the Laws of Physics demand there must be more
mass involved...
some of the best evidence for the existence of dark
matter!