Starry lens puts a twinkle in quasar's eye.

Starry lens puts a twinkle in quasar's eye

The 1985 discovery of four separate spots of light, or images, representing the same distant quasar provided a dramatic illustration of how the gravitational effect of an intervening galaxy can bend the path of light. Now, for the first time, astronomers have observed what they believe is the focusing of light by a single star in that galaxy. The observation, they say, should allow them to estimate the star's mass and the quasar's size.

"This represents the first detection of a microlensing event," M.J. Irwin of the Institute of Astronomy in Cambridge, England, and his colleagues write in the December ASTRONOMICAL JOURNAL.

The quasar, designated QSO 2237+ 0305, lies almost directly behind the center of a bright nearby galaxy (SN: 1/5/85, p.9). The galaxy's gravity splits and focuses the quasar's light into four images that collectively look like a four-leaf clover.

If the light forming one of these images happened to pass close to a star in that galaxy, the star's gravity would also focus the light. But the magnitude of that effect would change as the star moved within the galaxy, causing the image's brightness to vary over a period of a few months.

In August 1988, Irwin and his colleagues found one of the quasar's images 70 percent brighter than it had been in the previous year. A month later, the image had faded a little. The change was too rapid to result from the motion of the galaxy as a whole and too slow to have been caused by shifts in the quasar's own brightness, they say.

The astronomers calculate that the object responsible for altering the image's brightness has a mass between one-thousandth and one-tenth that of the sun. This suggests the lensing object may even be a brown dwarf -- a difficult-to-detect lump of gas larger than a planet but having too little mass to sustain the fusion reactions that occur at the cores of stars.

Additional observations of brightness variations over time should provide detailed information about the quasar's size and structure. "Continued monitoring of the 2237+0305 system to accumulate data on a number of individual events offers the possibility of constraining both the size of the quasar-continuum-emitting region and the mass distribution function for stars and any other population of compact objects within the intervening galaxy," the astronomers say. In other words, it's a system worth watching.