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 anti-aligned state ●  Very unlikely to happen spontaneously:

o  probability of 2.9×10−15 s−1

o  time it takes for a single isolated H atom to undergo this transition is ~ 10,000,000 yrs

o  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_-_Ewen-Purcell_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

The 21-cm Line: - 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

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http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/NGC_6384_HST.jpg/320px-NGC_6384_HST.jpg

What Else? - Hydrogen is the most abundant element in our universe

o  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

http://upload.wikimedia.org/wikipedia/commons/1/14/Redshift.png

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.

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) 829.66 MHz •Methanol (CH2OH) 6668.518 MHz •Helium Isotope (3HeII) 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. (ex- neutral Hydrogen emits the wavelength of 21-cm)

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  Longer observations can yield a true path or orbit by measuring changes in the doppler shift over time

o  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 -

o  Stark Effect – Plasma density (learn more) o  Zeeman Effect – Magnetic field (learn more) o  Paschen-Back Effect – Strong Magnetic Field (

learn more

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  the Laws of Physics demand there must be more mass involved...

o  some of the best evidence for the existence of dark matter!