My first Magazine Sky & Telescope - 03.2019

magnitude 15. So while Hipparcos could spot an astronaut on the Moon from Earth, Gaia would be able to see a penny. What value would Gaia ﬁ nd for the distance to the famous cluster? Using the ﬁ rst data release from 2016, one team calculated that the 164 cluster stars they included in their analysis gave a distance of 437 light-years. It was a clear con- ﬁ rmation that Hipparcos’s value was wrong. How does this analysis work? Using the same method, we can calculate the distance to the Pleiades based on Gaia’s second data release, announced in April 2018: First we have to pick the Pleiades out from everything else that shines in the same part of the sky. A download of all the (nearly 700,000!) sources lying within 5° of the Pleiades’ position gives a cone of observations, with its tip in the solar system and extending indeﬁ nitely into space. Somewhere inside that cone lie the Pleiades, as well as many ﬁ eld stars in front of and beyond the cluster. The extraction of the Pleiades from such a large stellar population is, fortunately, quite simple. The stars of an open cluster move together and therefore share the same proper motion across the sky. Gaia records a proper motion for the Pleiades of around 6 mas per year, in a particular southeast- ern direction. This leaves 1,876 stars of our original 700,000. Almost all of them belong to the cluster. If we sort these remaining stars based on their parallaxes, we ﬁnd that the vast majority are clumped together, with an average shift in their apparent positions of 7.3 mas. Selecting those stars with parallaxes between 5 and 9.5 leaves 1,594 stars, most of them surely belonging to the cluster. When we factor in various sources of error, we are left with a very precise ﬁnal result of 7.317 ± 0.002 mas, which corresponds to a distance of 445.8 ± 0.1 light-years. It comes almost as a relief: The Pleiades are where they should be, and stellar physics is all right. Pleiades 700 600 500 Taygeta Atlas Pleione Maia Merope Electra Alcyone To Earth p UNUSUAL STREAM When you plot the positions of the cluster’s members in 3D, the brightest stars form a string stretching away from the swarm and pointing toward the solar system (on the right side of this image). The effect might be a byproduct of the data analysis, but other research has seen similar patterns in open clusters. The Pleiades have not relinquished all their mysteries, however. The precise determination of the positions allows us to plot the cluster in 3D. When doing so we can see that the brightest stars (the traditional Seven Sisters, with proper names after the Pleiades of mythology) form a string stretch- ing away from the swarm and pointing toward our solar sys- tem. This is, frankly, bizarre. What are the chances that the brightest, most massive stars are not only on the same side of the cluster but form a tail? It’s highly likely that this feature is an artifact of the measurement. The parallaxes of these brightest stars have particularly large errors, because Gaia is ﬁ ne-tuned for stars between magnitudes 6 and 15, and the Seven Sisters are much brighter. It’s also possible that at least some of these bright stars are not actually part of the cluster: The closest ones, Maia and Merope, are moving faster across the sky than the bulk of the cluster’s members. Then again, it could be that the cluster is simply elongated. And it’s not a wild idea: Other research has seen similar patterns in open clusters across the Milky Way. We can only hope that further analysis and reﬁ nements in the catalog will shed light on these matters. ¢ GUILLERMO ABRAMSON is a physicist at the Statistical and Interdisciplinary Physics Group of the Bariloche Atomic Center and Balseiro Institute, in Bariloche, Argentina. This work made use of Gaia mission data processed by the Gaia Data Process- ing and Analysis Consortium. 400 300 200 FURTHER READING: To learn the fascinating history of stel- 100 Field stars 0 0 1 2 3 4 5 6 7 8 9 10 Parallax (milliarcseconds) p SORTING PARALLAXES There are nearly 2,000 sources that both lie within 5° of the Pleiades and share the cluster’s proper motion across the sky. To tease out the stars that belong to the Pleiades, we plot all the sources’ parallaxes. The cluster’s members clump together. lar parallax, read Parallax: The Race to Measure the Cosmos, by Alan W. Hirshfeld (W. H. Freeman, 2001). “Hipparcos: The Stars in Three Dimensions” (S&T: June 1999, p. 40) recounts the results of the Hipparcos mission, including the mystery of the Pleiades. The paper “Gaia Data Release 2: Using Gaia Parallaxes” by Xavier Luri (University of Barcelona, Spain) and colleagues (Astronomy & Astrophysics 2018) discusses the methods to obtain distance from parallax in the context of Gaia DR2. sk yandtele scope.com • M A RCH 2 019 29

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