The mass of stars that reside in binary systems is relatively easy to assess. For singular stars such as our sun, however, the situation is much trickier. In 1993 the fortuitous alignment of two singular stars of similar brightness, when one transited in front of the other, allowed astronomers to collect key information for determining the mass of the foreground star. The event, known as MACHO-LMC-5, caused the light from the farther star to be magnified by the gravity of the second. By employing the Hubble Space Telescope's powerful Advanced Camera for Surveys, the team observed the two stars separately, which was impossible using ground-based telescopes, and collected enough data to determine the mass of the closer star.
The researchers report in a paper to be published in the Astrophysical Journal that the red star is 1,800 light-years away and one tenth the mass of the sun. Gravitational lensing events that provide sufficient information for measuring the masses of singular stars are rare--only a few occur each decade. Astronomers hope that the planned Space Interferometry Mission (SIM), scheduled for launch in 2009, will allow more singular stars to weigh in.