Scientists have discovered why some stars dim and seem to temporarily disappear, a new study suggests. According to a report to be published in the April 1 issue of the Astrophysical Journal, light-absorbing chemicals form in the atmosphere of these so-called Mira variable stars, causing them to dim to one thousandth of their maximum brightness--and temporarily vanish from view.

Many stars change their brightness to a small degree because of pulsations--like those of a beating heart--that cause the star to alternately contract (and grow hotter) and expand (and cool). Such pulsations, however, can account for brightness changes up to a factor of only 50, a fraction of those displayed by Mira variable stars. In fact, these stars emit so little optical light that they seem to the human eye to disappear.

In an attempt to explain the vanishing stars, astronomers first postulated in 1933 that light-absorbing metal oxides could form in their surrounding atmospheres. But because of the limitations in computing power available at the time, they could not substantiate their claims. Now Mark Reid and Joshua Goldston of the Harvard-Smithsonian Center for Astrophysics have confirmed that titanium oxide--a chemical commonly used in sunscreen--forms in the atmosphere of these stars, thereby increasing its opacity. As a result, light from the innermost (and hottest) regions of the star is absorbed and only light from the cooler outer layers reaches observers on Earth. (The images at right represent an artist's conception of a Mira variable star at its maximum brightness (top) and its minimum (bottom). Says Reid, "That dimming is like exchanging a bank of stadium lights for a single nightlight."