LONG BEACH, Calif., May 22 -- Stars twinkle because of disturbances in the atmosphere blanketing our planet. Although the effect inspired a well-known nursery rhyme, it hampers astronomers' attempts to study many heavenly bodies. A group at Lawrence Livermore National Laboratory has now developed a system, known as a sodium guide star, that allows astronomers to accurately account for atmospheric disturbances and produce much clearer stellar images. Deanna Pennington, a physicist in the lab's laser program, described their research during a press conference yesterday at CLEO/QELS 2002. The guide star consists of a laser that illuminates sodium atoms in an atmospheric layer 60 miles above the earth, producing a small, bright spot of light high in the sky. By aiming the laser at a portion of the sky near a star of interest and measuring fluctuations in the guide star's light, astronomers can monitor atmospheric distortions wherever they choose to point their telescopes.The Konstanz-Düsseldorf team made use of two devices known as "optical cavities" pointing in different directions. Such cavities basically consist of two mirrors held at a constant distance. The round-trip time of a light beam between the mirrors is a direct measure of the speed of light perpendicular to the mirror surfaces. Any dependence of this speed on the direction of space would show up when the setup is rotated. However, errors creep in through variations of the cavity length, caused by temperature effects and material aging processes. Thus, the researchers employed cavities made from an ultra-pure sapphire crystal, which is virtually impervious to aging effects, and operated it at the temperature of liquid helium, near absolute zero. Length changes are thus reduced to a level significantly lower than what can be achieved at room temperature. Using advanced laser techniques for reading out the cavity round-trip time, the team could obtain a new limit on possible violations of light propagation isotropy.