My first Magazine Sky & Telescope - 03.2019

p THE PLEIADES Perhaps the most famous star cluster in the sky, the bright stars of the Pleiades — often called the Seven Sisters — can be seen without binoculars even from a city. The cluster lies roughly 450 light-years away toward the constellation Taurus. parallaxes for a little more than 100,000 stars, all within 300 light-years of Earth. The precision achieved was about 1 milliarcsecond (1 mas), which is 1 / 1,000 of an arcsecond or 1 / 3,600,000 of 1°. That is like seeing an astronaut standing on the Moon from Earth. Along with its expected successes, the Hipparcos mis- sion delivered several surprises. The most notable was the distance to a famous stellar cluster, the Pleiades (Messier 45, in Taurus). This group of bluish stars is easily visible by eye in dark autumn skies — although there are many more members than your naked eye can discern. Hipparcos found a distance of roughly 380 light-years, rather less than the 440 light-years of previous calculations, which were based on the stars’ brightnesses and considerations of stellar physics (see S&T: June 1999, p. 40). This was an embarrassing problem. On the one hand, if astronomers had been using the wrong distance for the Ple- iades, then it could have implications on a much larger scale: The Pleiades are a nearby open cluster and for this reason are frequently used to test our models of stellar evolution and to calculate the distances to farther stars. On the other hand, if Hipparcos were wrong, then the dubious result might challenge the entire Hipparcos catalog. Was there some sk yandtele scope.com • M A RCH 2 019 27

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