When a mass, like a brown dwarf or another massive but dark object (a MACHO), passes in front of a background star, the light from the star is gravitationally lensed. The scale is not large enough for two or more separate images to form, like in strong lensing events, but the star light can be significantly magnified/amplified. This amplification can produce a sharp peak in the intensity of light from the background star.

The plot below is the peak magnification (or amplification) factor for the modeled microlensing event shown in the mpeg. Below it is a plot of amplifications from a microlensing candidate described by The MACHO Dark Matter Search Project.

Peak Magnification from MPEG Simulation
Magnification from Microlensing Candidate

Astronomers are counting the number of so-called microlensing events to try to estimate how much non-illuminating mass is floating around in and just outside the galaxy. The missing mass could help explain the flat rotation curve most galaxies exhibit.

More gravitational lensing information:

• Some background on gravitational lensing.
• Einstein rings, an example of strong gravitational lensing.