A common chemical used insunscreen has been found to contribute to wild changes in brightness of certainolder stars that represent the likely end stage of our own Sun's evolution.

Researchers at theHarvard-Smithsonian Center for Astrophysics found titanium oxide in the outeratmosphere of a star, something that had been predicted in the 1930s. Moleculesof the substance absorb much of the star's light, causing it to dim.

When stars like our Sun near theends of their lives, the swell to many times their original size. When the Sunreaches this so-called "red giant" phase of evolution, it willprobably swallow the Earth. But it will also contract and expand many times.From afar, it will appear to pulsate in brightness.

The first star known to varydramatically in this manner was discovered in the 16th Century and was namedMira. Now all stars that behave this way are called Mira variables. But theexact cause of the most dramatic variations has not been known.

"Many variable stars changetheir brightness by small amounts because they pulsate like a beating heart,alternately growing smaller and hotter, then larger and cooler," saidJoshua Goldston, one of the researchers involved in the new study. Suchpulsations can only explain a portion of the brightness changes seen in someMira variables.

Goldston and his colleague, MarkReid, say the titanium oxide can increase the opacity of the star's atmosphere.Light from inner, hotter regions is absorbed. Only light coming from the outer,cooler layers of the star escapes. And as a star swells, the temperature of itsouter reaches becomes lower, so less light is produced there. The result is athousand-fold reduction in visible light, the researchers say.

"That dimming is like exchanginga bank of stadium lights for a single night-light," says Reid.

The finding will be published inthe April 1 issue of the AstrophysicalJournal.

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