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Janaury 14, 2000


Mary Ann Hill
(781) 283-2376



WELLESLEY, Mass. -- What started out as a routine observing project from a small campus telescope has turned into an exciting discovery of a possible black hole.

The black hole candidate is part of a binary star system in which the two components revolve around each other every 91 days. If confirmed, this will be the longest-known orbital period for a black hole binary by a factor of about 10 and the first system known to undergo eclipses from Earth's viewpoint. This fortuitous combination makes it an excellent laboratory for learning more about how black holes are fueled.

The findings were presented today by Dr. Priscilla Benson from Wellesley College at the meeting of the American Astronomical Society in Atlanta.

"When we started this project, I never dreamed that a black hole might be lurking in this star system," said Benson, a professor of astronomy. "What makes this discovery even more exciting is that much of the work was done by undergraduates using relatively small telescopes."

The investigation of this system, a variable star of unknown type and period named BG Geminorum (known as BG Gem), began in 1992 when Dr. Benson asked her undergraduate students to monitor its brightness changes with Wellesley College's 0.6-meter (24-inch) telescope at the school's Whitin Observatory. Five years' worth of data were then compiled by student observer Alceste Bonanos, now a Wellesley College senior, who determined BG Gem is a long period ellipsoidal binary system.

The resulting light curve piqued the interest of binary star expert Dr. Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics, who recognized this object as an interesting binary in which the lighter "secondary star" is being stretched out by the strong gravity of the more massive "primary star." The stretching is so extreme that material from the secondary is actually flowing towards the primary.

To probe this system further, Kenyon obtained visible spectra of BG Gem with the 1.5-meter (60-inch) telescope of the Fred L. Whipple Observatory on Mt. Hopkins, AZ. "I was very excited by our first spectrum showing emission from hot gas orbiting the primary star," explained Kenyon. "Then, I was sure we had an interesting and rare type of binary."

The spectra, analyzed by Swarthmore College junior Allyn Dullighan, during a summer at Wellesley, along with Drs. Benson, Kenyon, and Kim McLeod, assistant professor of astronomy at Wellesley College, yielded some surprises. Hot gas around the primary appears to be eclipsed by the cooler secondary once in each orbital period. Likewise, the secondary, estimated from its spectrum to contain half the mass of the Sun, is eclipsed half a cycle later. The eclipses imply that the binary system orbits in a plane tilted at nearly 90 degrees to the line of sight -- a rare arrangement. The eclipses allow estimation of the sizes of the star and the disk.

But the big surprise was yet to come. Dullighan determined that the secondary star races around the primary at 75 km/s (170,000 mph), but the hot gas shows that the primary itself stands nearly still. Like a small dancer being whipped around by a big, strong partner, the secondary in BG Gem is being pulled around at high speed by the gravity of a more massive primary. Using Kepler's Laws of Motion, the students estimated the mass of the primary to be about 4.5 times the mass of the Sun. Kenyon refined this basic result with detailed computer modeling and determined that such a massive primary should be easily visible. While the spectra do indicate hot gas swirling in an accretion disk around the primary, they fail to show the light of the primary itself.

"At this point, we wondered what could be hiding this massive object," noted Benson. "One possibility is that the primary is a normal hot star which is emitting most of its light in the ultraviolet part of the spectrum. The alternative is that the primary star is a black hole." To test the first possibility, the astronomers have applied for time to obtain ultraviolet spectra with NASA's Hubble Space Telescope. However, such a star would have a hard time heating the surrounding gas to the high observed temperature.

If the ultraviolet does not show a hot star spectrum, the best alternative is that the primary star is actually a black hole whose gravity fuels the hot gas. Final determination of the nature of this hidden object will require more observations by the authors and confirmation by other astronomers.

If BG Gem is confirmed to contain a black hole, it will open up an important new window on how black holes are fueled. Its orbital period, 91 days, is nearly 10 times longer than any other known black hole binary. This, along with its favorable inclination, will allow future observers to make slow, accurate measurements of the hot accretion disk as it is orbited and eclipsed by the secondary. By monitoring spectra of the disk as the secondary moves in front of it, astronomers hope to build up a detailed picture of the disk's structure. This can provide an important check on theories of how the black hole is fueled.

This work was supported by the W. M. Keck Foundation through its generous grants to the Keck Northeast Astronomy Consortium, a group of eight small liberal arts colleges in the Northeast U. S. The Keck Foundation grants helped to supply each school with the instrumentation and computers necessary to carry out this type of modern astronomical research, and supported Dulligan for her summer project at Wellesley College. Benson, Bonanos, and student observers were supported by National Science Foundation Grant AST-9417359 and Brachman Hoffman grants from Wellesley College.

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