Because light rays are bent when they pass close to a massive object, light from a distant source may be focussed by a closer object to produce a sudden brightening of the distant object known as "gravitational microlensing''. If there is a significant population of faint and small mass stars or Jupiters in the Milky Way, gravitational microlensing of distant stars might be common enough to be detectable. A microlensing event would be the explanation for an otherwise unpredicted increase in the brightness of one of the targets stars, followed by a similar decrease in brightness, returning the star to its original intensity. Hence, the monitoring of the brightness of distant stars looking for the characteristic brightening and subsequent fading might provide proof of the existence of unseen MAssive Compact Halo Objects (MACHOS)?
The availability of hardware and software designed to allow accurate monitoring of brightness variations has made the search for microlensing a very active pursuit for a number of research groups currently.
One research group, dubbed the MACHO project, involves the monitoring of the brightness of 1.8 million stars. Another group, called the Experience EROS (How is your French?) or the EROS Experiment (If you prefer English), based at the European Southern Observatory in La Silla Chile, has been looking for variations in the brightness of 8 million stars in the direction of the Large Magellanic Cloud. Both groups have detected brightness variations that could be microlensing events. The variations last on the order of a month.
Another group is conducting the OGLE - Optical Gravitational Lensing Experiment looking for microlensing events in the Milky Way's bulge. So far OGLE has detected 12 microlensing events.
As these different observing teams continue their observations, lots of new results should be reported in the next few years.
``OGLEing, MACHOs, and the Search for Dark Matter'', Taubes, G. 1993, Science 260, 493.
``Possible Gravitational Microlensing of a Star in the Large Magellanic Cloud'', Alcock, C. et al., 1994, Nature 365, 621.
``Evidence for Gravitational Microlensing by Dark Objects in the Galactic Halo'', Aubourg, E. et al., 1994, Nature 365, 623.
``Microlensing Events of the LMC are Better Explained by Stars within the LMC than by MACHOs'', Sahu, K.C. 1994, Publ. Astron. Soc. Pacific 106, 942.
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