An image of the nearby Pleiades star cluster. CfA astronomer Guillermo Torres has completed a detailed eleven-year study of intermediate mass stars in this cluster using SAO’s Fred L. Whipple Observatory Tillinghast facility. Because these stars are rapidly rotating, previous spectroscopic measurements of their motions and properties had been unreliable.
Credit: Smithsonian Magazine

The Pleiades star cluster, a brilliant collection of several hundred stars visible in the winter sky near the constellation of Orion, has been admired by people for thousands of years; it is cited in the Bible and the works of Greek authors. It is a relatively young star cluster whose estimated age is only about 100 million years, meaning that it was born long after the Jurassic dinosaurs had passed into history. The Pleiades cluster is also relatively close by, only about 448 light-years away, which is why it is such a visual splendor and also why it is such a tempting target for astronomers.

The brightest stars in the Pleiades cluster have been studied spectroscopically for more than a century. The stellar spectrum not only reveals a star’s character, the positions and shapes of its atomic lines reveal the star’s motions, both its line-of-sight motion in space (its radial velocity) and its rotation. One of the most extensive spectroscopic programs in the last century lasted for nearly 20 years focused on the moderate mass stars, leading to the discovery and characterization of many new binary stars in the cluster.

But while it is relatively easy to measure the radial velocities of moderate mass stars, more massive ones are more challenging because they have fewer lines and the line profiles are typically very broad because these stars tend to rotate rapidly, some with rotational velocities in excess of 200 km/sec.

As a result, the radial velocities of the more massive stars in the Pleiades are generally of poor quality. Nevertheless, over the years more than a dozen of them have been claimed to be possible binary stars, or variable, and in some cases tentative orbits have even been published, but few of these conclusions have been confirmed.

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