Being able to breathe a few steps away from the largest steerable thing on the surface of Earth is something an astronomy minor would not say no to, especially if that thing is a telescope which has the power to answer mankind's biggest questions. We are talking here about the Robert C. Bird Green Bank Telescope, the site of which (The Green Bank Observatory, or GBO) our astronomy minors had the chance to visit and explore. Eighteen of our students were accompanied by four faculty members for an overnight trip there, where they got the chance to observe with GBO's 40-foot teaching telescope and participate in quite a number of other educational programs.
Overall, the experience seemed to be overwhelmingly positive: one might not get the chance to see so many smiles and hear "wow"s from physics, computer science, or engineering majors, all while learning about the our host galaxy, Milky Way, through their own observations of Hydrogen emission.
Far away from hot O and B-type stars, the hydrogen in space is in the ground (i.e., lowest energy) state. However, when the spin of the electron flips from being parallel to anti-parallel with the spin of the proton, there is a tiny energy difference that is emitted at the wavelength of 21 cm (or a frequency of 1420.4 MHz). While a hydrogen atom can wait on average a few million years before it undergoes this transition (YES, it is this rare!), the large amount of hydrogen gas makes this particular emission one of the most prominent and easiest to detect with radio telescopes.
blocking (filtering) out all of the waves but the ones coming at this exact frequency. This 21-cm line radiation provides the best way to map the structure of the Galaxy (note: for astronomers, Milky Way is the only galaxy with capital "G").
Here is an example of detection of (the center of the) Milky Way's emission at 21 cm with the 40-foot telescope's spectrometer (sorry folks! no pic from there). The red line is a calibration measure, while the black line records the data, i.e., the intensity (in Jansky units) as a function of frequency. The two strong peaks at the left side of the spectrum show detection of HI at two different frequencies: the strongest peak is located at 1419.5 MHz, and the other at 1485 MHz, depicting two different clouds of Hydrogen emitting the 21-cm transition that is redshfited (smaller frequency, or longer wavelength), implying that the detected Hydrogen is moving away from us (while rotating in the disk of the Galaxy). The peak at the far right depicts an artificially created signal of 500 Jy, for calibration purposes.
Multiple data sets from eight groups of students, acquired during the night, were collectively analyzed the next morning. After not much debate, there was a pretty good agreement that the data shows strong evidence that Milky Way has the shape of a flat disk, that is rotating counterclockwise.
Here are some of the students' thoughts about this trip, with some cool, funny, or downright amazing things they have learnt:
* Mary Ogborn & Ebony Williams (physics majors): This trip really illustrated aspects of radio astronomy that I wasn’t aware of before. Although I was aware of radio interference, I was not aware of how sensitive these telescopes could be to various sources [...] The control room was also impressive, as it was copper-insulated, in order to keep in the radio waves coming from all the computers and machines. [...] it was interesting to step into the past and see how the original radio astronomers operated these huge telescopes. I can’t imagine having to manually dial in the declinations, change the frequency every second by pushing the mark button, and having the chart reader draw out the peaks without any other form of labelling. [...] I now understand how observatories in the past would hire people to be ‘computers’ before the advent of computers.*Ryan Ferrell (physics major): I did not expect to be able to extract this much information from the data which made me appreciate how much information can be brought out of even just a little data.[...] I learned was how many common things cause radio interference. I knew previously that most electronics caused radio interference, however I did not know that signals from modern electronics are a billion times stronger than the radio waves that the GBT was measuring from the Milky Way. [...] I never knew the GBT was the largest radio telescope in the world or that the Drake Equation had been thought of there. [...] I was very surprised to find the control center so close and surrounded in a giant Faraday Box.
* Tanna Walters (engineering major) & Brandon O'Neal (physics major): This trip to Greenbank Observatory has taught us a good bit about the actual ways in which the data is collected and how the hardware works. GBT actually has a clam-shaped dish and an arm that is off-center that collects the radio waves. The purpose for this is to allow for the collection of more radio waves as opposed to parabolic telescopes that have the receiver in the middle of the dish, blocking a significant portion of the incoming radio waves. [...] we learned just how sensitive the telescopes are to interference (RFI). Even taking a picture with our phones could interfere and ruin astronomers’ data all over the world.
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| In the control room of the GBT |
* Tom Gagne (computer science major) & Kris Pickens (physics major): We learned the remarkable fact that if a cell phone were placed as far away as Mars, it would still outshine the brightest distant radio sources by several orders of magnitude. So we had to turn them off when out among the telescopes. We also learned that a shipbuilding company built one of the telescopes and put an enormous ball-bearing in it. The ball-bearing was so large that the bridges had to be fortified along the path of the train that took it there.
* Cameron Kelahan (computer science major): For the first time, I was able to operate a radio telescope and perform radio astronomy. I learned how to operate a 40 foot radio telescope, the equipment that goes along with it, the meaning and importance behind the 21 cm line, and how the shape of the Milky Way was originally discovered. I also got an idea of what it may be like to work at an observatory from talking with [GBT operator]. His job seems very interesting and also challenging! 12 hour shifts are something one can get used to, but there is also a lot of responsibility that goes into operating a MASSIVE TELESCOPE! The overall experience [...] allowed me to strengthen friendships with classmates and possibly future colleagues while learning with them about something we all have a passion for.
