For the first time, astronomers have watched a variable star swell and shrink. The star, called R Leonis, brightens and dims on a year-long schedule, and an innovative array of telescopes has now shown that it changes diameter by up to 35% over the same period. The observation, to be published in Monthly Notices of the Royal Astronomical Society, promises insight into what controls the dimmer switch in these stars, called Mira variables, and why they eventually shed much of their mass and turn into white dwarfs.
Ordinarily, picking out detail on something as distant as R Leonis, more than 300 light-years away, defeats even large telescopes. The Cambridge instrument, dubbed COAST, or Cambridge Optical Aperture Synthesis Telescope, does the job by capturing light with four small mirrors--each just 16 centimeters across--spaced as much as 6 meters apart (Science, 16 February 1996, p. 907 ). By blending light from the separate telescopes, COAST simulates a telescope with an aperture of six meters and--just as important--is able to see through atmospheric distortion.
The Cambridge astronomers monitored R Leonis for two years. Although it has no more than twice the sun's mass, its diameter varies from 450 times the diameter of the sun to 600 times, a variation that team member Chris Haniff calls "outrageous." An internal instability may drive the cycle: When the star is most compact, its atmosphere dams up radiation, which forces the star to expand so the energy can dissipate.
The observation "is very exciting in itself," says Michael Feast, an astronomer at the University of Cape Town, South Africa. But he adds, "I also think it's exciting for what it shows [the Cambridge interferometer] can do." Next on the agenda for the group is understanding how Mira variables lose material until they are down to a tiny fraction of the sun's mass. The COAST measurements show, for example, that R Leonis expands at the rate of up to 10 kilometers per second, and Haniff suggests that it might throw off material as it reaches its maximum size. Another possibility, says John Baldwin, the team leader, is that the surface of the star is churning, with "large convection cells `boiling,' so that mass comes to the surface in some great sort of blob and then is thrown off."