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Pulsations in the Pleiades.

THE EXOPLANET-HUNTING Kepler spacecraft long monitored hundreds of thousands of distant stars with an exquisite sensitivity that--ironically --put the brightest stars out of its reach. But not anymore: Publishing in the November issue of The Monthly Notices of the Royal Astronomical Society, Timothy White (Aarhus University, Denmark) and colleagues tried out an innovative new technique known as halo photometry on Kepler observations of the brilliant "Seven Sisters" of the Pleiades.

CCD cameras capture photons in "wells," each of which corresponds to a single pixel in the resulting image. But a bright source can cause a well to overflow, saturating the central pixel(s) and creating a "bleed trail." White and colleagues instead measured brightness using photons recorded in the unsaturated pixels around the central one(s) and showed that this halo is a reliable measure of stellar brightness.

The seven brightest stars of the Pleiades are blue-giant stars brighter and more massive than the Sun, nearing the end of their short lives. As expected for this type of star, the new observations reveal that they all pulsate, slowly varying in brightness by less than 1%. Their light curves are shown at left.

Among them, Maia (20 Tauri) is unusual. It has a slow rotation and calm atmosphere, both of which allow unusual concentrations of heavy elements to circulate near its surface. Kepler observations reveal that Maia varies over an exceptionally long period, brightening and fading with a period of 10 days. The cause, the authors argue, is most likely a large, chemically enriched spot on its surface. If the spot is fixed, then it probably has some relation to the star's magnetic field, but more data are needed to learn how and why these chemical spots exist.

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Title Annotation:STELLAR
Author:Young, Monica
Publication:Sky & Telescope
Article Type:Brief article
Date:Dec 29, 2017
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