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The Star Splitter: Microlensing Technique Pioneered
by NSF Researchers Finds Black Holes
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."
Editors: Images are available on the Internet at:
For more information, see:
* "The Star Splitter", by Robert Frost (1874-1963)