In a Robert Frost poem, a farmer relies on a telescope called the "Star Splitter" to satisfy his curiosity about our place in the universe.* Two international teams of astronomers supported by the National Science Foundation (NSF) recently used another “star splitter,” the technique of gravitational lensing, to examine another curiosity -- isolated black holes drifting among the stars in our galaxy.
David Bennett of the University of Notre Dame and his collaborators presented these results today at the 195th meeting of the American Astronomical Society in Atlanta, Ga.
Albert Einstein, in his General Theory of Relativity, first showed that the gravitational field of a star distorts space so that light passing near is deflected, much like an optical lens or magnifying glass deflects light. In 1936 Einstein predicted what an observer would see if a very faint object with a strong gravitational field such as a black hole passed in front of a faint star. The black hole's gravity would act like a powerful lens, splitting the star's image into multiple images and making the star appear brighter. In some cases, the images would blend into a perfect ring of light. This phenomenon is known as gravitational lensing; in the case of stars, it is called microlensing.
Einstein thought that because the separation between images would be so small, earth-based instruments would never be able to observe it -- and in fact, no one has observed the split images of stars (though astronomers have seen split images of galaxies). But today's powerful instruments can detect the brightening associated with the stars and pinpoint their locations.
Astronomers supported by NSF through its Science and Technology Center for Particle Astrophysics at the University of California at Berkeley are using microlensing to look at millions of stars in a search for the changing brightness predicted by Einstein. Dubbed the MACHO project, for Massive Compact Halo Objects, the search seeks to learn more about the "dark matter" that dominates the mass of our galaxy, some of which might be in the form of massive compact objects. Already, the MACHO astronomers have noted 300 instances of gravitational microlensing near the central regions of our galaxy, each marking the passage of an unseen object in front of the star.
Bennett and his collaborators examined two microlensing events, seen in 1996 and 1998, and followed up with observations by the Mount Stromlo Observatory in Australia, NSF's Cerro Tololo Inter-American Observatory in Chile, and NASA's Hubble Space Telescope. Complex calculations of the location and brightness of the stars and the mass of the objects passing in front led the teams to conclude the objects were probably black holes drifting through space -- a new find for astronomers. All previously known black holes have been found in orbit around stars, with their presence detected by their effect on the companion star.
The microlensing technique holds promise for detecting other solitary black holes, dark matter and planets. “Robert Frost's Star Splitter" didn't do a thing but split a star in two or three,’” said Morris Aizenman of NSF’s Astronomical Sciences Division. “Microlensing, which is nature’s ‘star splitter,’ is revealing much more about the unseen matter in our galaxy.”
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* "The Star Splitter", by Robert Frost (1874-1963)
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