HUT Observatory

HUT Observatory Confirms Student Discovery of Eclipsing Variable Star

OLYMPUS DIGITAL CAMERA

Gelb Observatory, at Phillips Academy in Andover,
Massachusetts, houses a telescope that is twin to
the 16-inch at HUT Observatory.

Ji Seok Kim, a 10th grader taking advanced subjects at Phillips Academy in Andover, Massachusetts, has discovered two variable stars – stars that change brightness – in images originally taken to determine the rotation period of an asteroid.  Kim is participating in an astronomy research course with instructor Caroline E. Odden, who visited HUT Observatory last spring as part of a workshop in practical astronomy.  After an e-mail introduction by Mrs. Odden, young Mr. Kim, who is a native of South Korea and goes by Joshua with his American friends, contacted HUT Observatory on March 4 requesting follow-up observations to confirm the suspected variable nature of the stars.

Joshua’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.  For many years the General Catalogue of Variable Stars has been maintained by Russian astronomers at the Sternberg Astronomical Institute of Moscow University.  And since 1911, a leading organization in the Western world has been the American Association of Variable Star Observers, founded by Harvard College Observatory.

Joshua’s request came at a moment when the sky above Eagle was clear for the first time in a long while.  HUT astronomer John 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.  Joshua eagerly followed the action over the Internet, writing, “Cool. Thanks!  I’m excited to see what happens.”  The imaging stopped only when the target star, in the constellation Gemini, had set too low in the west.  Of Joshua’s two suspected variables, one hinted at an unusually short-period change – a repeating pattern with a cycle only five hours long.  The short-period change was especially compelling to understand.  It was thus the first of Joshua’s objects to be targeted.

After sunrise and about two hours’ sleep, Briggs transferred the images and certain calibration frames electronically to Mrs. Odden, who in turn relayed the pictures to Joshua.  Within just a couple hours, Joshua used specialized software developed by Colorado astronomer Brian D. Warner (also a guest at last year’s HUT workshop) to measure 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, Joshua saw a more complete pattern emerge.  It seemed that the star was not a single, pulsating object, as Briggs had first suggested.  There was a beautiful, reasonably smooth, doubled-up pattern, suggesting to Joshua that the “star” was actually a system of two stars in very close orbit around each other, aligned in such a way that one star blocks the other as seen from Earth.  This type of double star is known as an eclipsing binary.  The period of the repeating pattern proved to be close to eight hours.

Variable_Star_1

Joshua e-mailed to John, “THE POTENTIAL VARIABLE IS NOW CONFIRMED!  I just calibrated your images and ran the Variable Star Search.  Your images were superb.  Take a look at the comparison stars–simply perfect.  And guess what.  The lightcurve of the potential variable again shows up beautifully.”

Inspired by the success, Caroline, Joshua, and John agreed that there was no choice but to observe the same object the following night, to fully 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, and Joshua had a revised plot, combining measurements from all three nights,  by the next mid-morning:

Variable_Star_1_Code2

“This is really lookin’ awesome!,” he wrote.   “We have successfully covered the space that was left blank.  Now we can be absolutely certain that this is the lightcurve.  I’m amazed by how accurate our data are.  The data from 1/12 and 3/5 and 3/6 seem to match perfectly where they overlap.”

The “overlapping” mentioned by Joshua is the result when measurements from different nights are combined in one graph, called a phased plot, which is drawn using an assumed period for the star.  Brian Warner’s software automatically tests many different assumed cycle periods for a variable star.  It selects the period that results in the best match, and the result can be amazing.  As indicated in the plot above, the period of this eclipsing binary star is 8.3395763 hours, plus or minus a small fraction of a second.  A task now before the collaboration is to understand better the significance of this amazingly precise result.  Is it in fact accurate for the reality of the binary star system?

The Phillips-HUT team will confirm the eclipsing binary interpretation of the data with specialists at the American Association of Variable Star Observers.  At about magnitude 13 as measured on the brightness scale used by astronomers, the combined light from the binary star is 630 times fainter than the dimmest star that can be seen by human eye alone.  The fact that such objects can be studied so easily now testifies to the great advances that have been made with electronic cameras and image reduction software.  Ji Seok Kim’s discoveries will likely be reported in Journal of the American Association of Variable Star Observers.  Odden, Briggs, and Phillips Academy students are frequent coauthors in papers appearing in Minor Planet Bulletin, edited by Professor Richard P. Binzel of MIT.  Brian D. Warner of Palmer Divide Observatory was the 2006 recipient of the prestigious Chambliss Award of the American Astronomical Society.