Star Light, Star Bright
|Eta Carinae: Space Telescope photo courtesy Jeff Hester (Ariz. St. U., NASA)|
Determining the present age of the Universe depends on knowing how fast the distant galaxies are moving way from us and also on figuring out how far away they lie. As everyone knows, time equals distance divided by velocity. Velocities are measured very accurately by the Doppler red shifts of spectral lines in a galaxy's light. To obtain accurate distances astronomers have traditionally relied on a class of bright yellow stars known as Cepheid variables, which pulse rhythmically like a living heart. The slower the beat, the greater the Cepheid's true brightness. The distance of the Cepheid and hence the distance of its host galaxy are obtained by measuring the Cepheid's apparent brightness and inferring its true brightness from its measured pulsation period. In principle this is no different from estimating the distance of a far-off street lamp at night by noting how faint it is.
Applied to astronomy, this method works well only so far as we can detect distant Cepheids, which is not far enough to yield an accurate estimate of the Universe's age. To extend the cosmic yardstick ten times farther, Richard Stothers and Chao-wen Chin have studied the luminosities and variability of a rare class of stars known as luminous blue variables (LBVs), which are less regularly periodic but much brighter than Cepheids. A classic example is Eta Carinae in our own galaxy (see photo). Unlike Cepheids, the variability of the LBVs stems from their occasional violent ejections of matter into space, not from a pulsation. Computer models that can explain this behavior have been formulated by Stothers and Chin. The predicted average interval between major outbursts agrees with observations, and turns out to be shorter the brighter the LBV. Although the characteristic LBV periods range from four to 20 years as compared to only a few months for bright Cepheids, the long wait needed by astronomers to obtain galactic distances with this method is expected to ultimately provide a valuable cosmological dividend, the age of the Universe.
Stothers, R.B., and C.-W. Chin. 1995. A period-luminosity relation for the slow variations of luminous blue variables. Astrophys. J. 451, L61-L64.
Croswell, K. 1995. Blue stars twinkle in a new light. New Scientist 147 (9 September), 17.