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Student discovers eclipsing variable star using images from PA observatory

Josh Kim ’15 confirms breakthrough with help from alumnus astronomer

April 04, 2013 --The American Association of Variable Star Observers (AAVSO) has confirmed that Ji Seok “Joshua” Kim ’15 discovered an eclipsing variable star last month. Kim made the discovery in an advanced astronomy class led by science instructor Caroline Odden. By analyzing images taken from the 16-inch reflector telescope in the Phillips Academy Observatory, Kim made what is considered a find-of-a-lifetime to many avid stargazers, though his interest in astronomy only “sparked” last September.

“When I met Ms. Odden, my instructor in physics, she encouraged me to take astronomy and start doing research, and it turned out to be a great idea,” said the 10th grader from Seoul, South Korea. “But I still have a lot to learn in astronomy. My knowledge of astronomy is very small. I mean I was doing research, but that was just plain observing stuff from the observatory. And I still have more to learn, just in general, about the stars and what goes on in the universe.”

This is the first time Odden has offered the advanced astronomy course, in which students spent most class time, and many nights, in the PA observatory over the course of winter term. In addition to learning to how to operate the telescope and the 360-degree rotatable, retractable-roof dome, the group learned the art of visual observation and image analysis. Kim focused on a group of images that were originally taken to determine the rotation period of an asteroid named 4611 Vulkaneifel.

“Other students had made these images to target asteroids. But it’s not just asteroids that are in these images, it’s also hundreds of stars. So I ended up looking at all these stars that are in the midst of an asteroid and made light curves of them,” said Kim.

Variable stars are stars that vary in brightness. Often this variation is caused by an object passing between the terrestrial observer and the target star. In many cases, as in Kim’s, the object is another star, known as a binary companion star. Half of all stars in the universe are part of a binary star system in which two stars orbit each other.

After consulting a catalogue of all discovered variable stars only to find his missing, Kim contacted astronomer John Briggs of the HUT Observatory in Eagle, Colorado, at the suggestion of Odden. Briggs, a member of the Class of 1977 and a former visiting physicist at Andover, collaborates regularly with Odden.

“Kim’s preliminary information, complete with pictures, plots, and celestial coordinates, made it reasonably certain that the stars in question were indeed variables. Variable stars are relatively common in the sky. But it is fundamental astronomical exploration to discover and document additional examples,” says Briggs.

Kim’s request for assistance came at a moment when the sky above Eagle was clear for the first time in a long while. Briggs was able to open the 16-inch telescope and secure eighty 3-minute-long exposures of one of the suspects, in an automatic sequence taken by HUT’s computer-controlled camera as Kim followed the action over the Internet.

Briggs transferred the images and certain calibration frames electronically to Odden, who in turn relayed the pictures to Kim. Using specialized software, he measured the changing star’s brightness relative to other stars recorded in the same pictures. Combining the new data with his own from the Phillips Academy telescope, Kim saw a more complete pattern emerge.

Inspired by the emerging pattern, Kim, Odden and Briggs agreed that there was no choice but to observe the same object the following night in order to fill in Joshua’s plot of the changing brightness cycle. The Colorado sky remained clear. This time, the images were relayed to Massachusetts before sunrise. With the revised plot, Kim combined measurements from all three nights, where the light curves matched “perfectly where they overlap,” he said.

The “overlapping” suggested that Kim had discovered a binary star system, an interpretation made official by the AAVSO’s recent confirmation.

At about magnitude 13, as measured on the brightness scale used by astronomers, the combined light from Kim’s binary star is 630 times fainter than the dimmest star that can be seen by the human eye alone. Kim’s discoveries likely will be reported in the “Journal of the American Association of Variable Star Observers.”

Commenting on Kim’s discovery, Odden said, “I am thrilled for Joshua and for our astronomy program. This facility has developed into a place where motivated students have the opportunity to conduct scientific research. I expect Joshua’s discovery to be just one of many interesting results to come out of this observatory.

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