During the past 40 years, the star fg sagittae has changed color from blue to yellow—direct evidence that its temperature has dropped dramatically. its cooling is probably due to a rapid increase in its size, an inference that is indirectly supported by two independent pieces of observational evidence. like many stars, fg sagittae rhythmically pulsates, beating like a heart, and over the past 30 years the period of this pulsation has noticeably lengthened. given that a star's pulsation period is determined by its radius, fg sagittae must have gotten larger. moreover, fg sagittae's total luminosity—the amount of light it emits over all wavelengths—has been observed to remain constant despite the star's plummeting temperature. it is known that a star's luminosity is the product of its temperature and its radius multiplied together: if one of the two factors has decreased while over the same period luminosity has remained constant, the other factor must have increased.