Weekly Science Update

Wednesday, July 02, 2008
The North Star
Polaris, the North Star, is not only renowned as a reliable beacon for early navigators. It is a Cepheid variable star, that is, a star whose mass and age are just right to prompt periodic oscillations in its photosphere, with a pulsation time proportional to the star's intrinsic luminosity. This extraordinary property of Cepheid variables, discovered and calibrated at Harvard by Henrietta Leavitt in 1908, allows them to act as reliable beacons for modern cosmic navigators. The period of a distant Cepheid is relatively easily measured with precision. By comparing its apparent brightness with the calibrated intrinsic brightness, a precise distance is determined. Cepheids in distant galaxies that are receding from us provide the basis for the famous distance-velocity relationship of galaxies that underpins the big bang model of an expanding universe.

The critical period-luminosity relationship of Cepheids is a function of stellar mass and age, yet no completely direct measurement of a Cepheid mass has ever been made. For any star, the most direct measurement of the mass requires a companion object so that the dynamical orbital motions of the two around each other can be exactly calculated; other mass determinations are more indirect. Fortunately, it turns out that Polaris has such a companion. Actually, it has two known companions, one so far away (and easily spotted in small telescopes) that its orbital period has not yielded a useful mass measurement. The second companion is a small and faint star only about fifteen AU from the star (one AU is a distance corresponding to the mean earth-sun distance), orbiting every 29 years.

Three CfA astronomers, Nancy Evans, Margarita Karovska, and Dimitar Sasselov, together with five colleagues, have just released a paper in which they combined new Hubble Space Telescope data with other results to determine the orbital parameters of the Polaris close binary system for the first time. They measure a mass of Polaris of 4.5 (+2.2, -1.4) solar masses, and the mass of its companion as 1.26 (+.14, -.07) solar masses. The results are consistent with earlier estimates, but the new work for the first time obtains a direct measure of the star's mass, and thus places a new level of reliability on our physical understanding of this cosmic beacon.

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