When a total solar eclipse cast shadows across the contiguous United States on Aug. 21, Whitman College was not far from the path of totality. Associate Professor of Astronomy Nathaniel Paust '98 took advantage of the rare sighting to conduct research on the appearance of the sun's outer layers, the chromosphere and the corona. On campus, a large group gathered on Ankeny Field to view the 96 percent eclipse through solar-safe telescopes; theatre major Lud Brito '18 also led a group of international students on an outing to Sacajawea State Park in Pasco, Washington.
Paust's Louis B. Perry Summer Research student, astronomy and physics major Lucas Napolitano '18, accompanied him to John Day, Oregon, to collect data during the eclipse. Paust's wife Kirsten Paust '98 and two daughters tagged along. So did astronaut Dottie Metcalf-Lindenburger '97, her husband, Jason Metcalf-Lindenburger '99, and their daughter, among others.
Nathaniel Paust answered questions about the once-in-a-lifetime experience; edited excerpts follow.
Why study an eclipse?
We know a lot about the sun. Based on how bright it is, along with observations of the number of neutrinos we detect, we have a good handle on exactly which nuclear reactions are happening and exactly how much energy they create. By measuring the spectrum of light from the sun, we can determine the exact elemental composition to extremely high precision. And by studying waves passing across the surface, we can tell how the density changes through the different layers inside the sun. All that information gives us a really good picture from the core up to the photosphere, the layer of the sun that we see in the daytime sky. However, there are two other layers that are normally hidden from us: the chromosphere (a circle of color around the sun) and the corona (a crown-like structure much larger than the visible surface of the sun). Both of them are thin and wispy and therefore inherently faint, so they're invisible in the glare from the photosphere.
Why did you want to learn more about them?
Besides their relative invisibility, these layers are interesting because we don't understand why they're so incredibly hot. We know the center of the sun is approximately 15 million degrees Kelvin and that the interior of the sun cools as one moves farther out until you find temperatures of approximately 5,600 Kelvin at the photosphere. The chromosphere, which was originally assumed to be cooler, is closer to 10,000 Kelvin. The corona, amazingly, has a temperature of around 3 million degrees Kelvin. This extreme temperature jump was a mystery until recently and we still have only a rough understanding of what is happening in these regions. The current theory is that the extreme heating is being done by magnetic fields using something called Alfvén waves. Now, the problem is that data to test the new theories is hard to take, because the corona and chromosphere are essentially invisible. We need some event that can block out all of the light from the photosphere but leave the chromosphere and corona untouched. That event is a total solar eclipse!
What was your reaction to seeing a solar eclipse for the first time?
I've been professionally studying astronomy for about 22 years. I've used really large telescopes and seen really cool stuff, so I went into the eclipse a little bit jaded. I'd seen pictures of the things that I would soon see in person and I assumed that it wouldn't really be a huge deal. I did find it pretty amazing when the eclipse started and the moon started eating a larger and larger bite out of the Sun, but it was tempered a little bit by the fact that that phase took over an hour. After a while, I was looking around and said, "Hmm, the sun is half covered, but it really doesn't look any different here." Which is really just a credit to how well our eyes can adjust to changing light. At the 75 percent coverage point, one person in our group went inside and said, "It's actually really dark out here." We hadn't noticed since it was so gradual. Then, as totality approached, things started changing fast. The light from the sun got redder and noticeably dimmer. Then, the moment when totality hit was a surreal experience.
Other thoughts while watching the eclipse?
We had been talking about the fact that eclipses would be extremely scary events if you didn't know one was coming. The moment where the moon completely covered the sun was somewhat terrifying for me, even with all my knowledge. The star that I'm used to seeing every day blinked out of existence and left my eyes madly adjusting until the full glory of the corona popped into view. It was such an unworldly sight that I immediately thought, "Something must be wrong." Then I relaxed and was able to pay attention to just how amazing of a sight it was. All the things that I've studied were laid bare. The stars popped out in the middle of the day. Venus was plainly visible high in the sky. And I took some glimpses at my camera and realized that I was getting truly amazing data.
What kinds of images were you hoping to capture?
During the eclipse, we can get amazing images of the chromosphere and the corona. Even more interestingly, we can directly see the magnetic fields. The corona is composed of a phase of matter called a plasma. Plasmas are similar to gasses, only the particles have an electric charge because electrons have been ripped off the atoms due to the high temperature. That electric charge prevents the particles from crossing the magnetic field lines. Instead, the plasma has to stream along them. So, when we look at images of the corona and see the fine lines stretching away from the sun, we're directly viewing the magnetic fields. There are other features on the sun caused by magnetic fields as well, things like sunspots, prominences and flares. In the upper right of some of the short exposure images that I took, you can see a looping prominence coming out of the chromosphere. In the longer exposure images, you can see similar looping lines farther out in the corona, showing how the surface and corona are connected by magnetic fields.
What other goals did you have for the experiment?
Besides understanding the behavior of the outer layers of the sun, there are two other scientific questions that can be answered by taking images of the eclipse. First, we have very high accuracy maps of the surface of the moon and know the heights of all the mountains. During the very beginning and at the end of totality, we can see something called Bailey's beads, in which light from the photosphere passes through valleys on the edge of the moon. By looking at the exact time that we see Bailey's beads and how long they're visible, it's possible to make extremely accurate measurement of the radius of the sun. Finally, we were also monitoring temperature and atmospheric pressure during the eclipse. Since the amount of sunlight goes dramatically down, and then goes out, during the eclipse, the temperature drops significantly. By measuring that drop, we can learn about the structure of the earth's atmosphere and potentially make measurements of the strength of the greenhouse effect.
What was Napolitano's role in this research?
Lucas did a large amount of the preparation work for our data collection as part of his Perry summer research project. Neither of us had ever seen an eclipse or photographed one and we had no experience with accurate weather measurements. He threw himself into several tasks, identifying exactly what equipment we would need to make our atmospheric measurements and then running commissioning tests to make sure it worked as expected. He also learned to use Geographical Information System software to find ideal locations to view the eclipse based on historical weather patterns, population areas and the path of the moon's shadow on the earth. He also was solely responsible for taking images from one of the two telescopes we were using to build up our data set. He accomplished all this while also working on a project identifying and categorizing variable stars in the M53 globular cluster.