Sky & Telescope - March 2019 - 29

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 ﬁnd for the distance to the famous
cluster? Using the ﬁrst 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 conﬁrmation 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 indeﬁnitely into space. Somewhere
inside that cone lie the Pleiades, as well as many ﬁeld 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 southeastern 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

Number of objects

SORTING PA R A LL A X ES: S&T ILLUSTR ATION, SOURCE: GUILLER M O
A BR A MSON; 3D PLOT: GUILLER M O A BR A MSON / CELESTIA

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 stretching away from the swarm and pointing toward our solar system. 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 ﬁne-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 reﬁnements 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 Processing and Analysis Consortium.

400
300
200
100

Field stars

0
0

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.

FURTHER READING: To learn the fascinating history of stellar 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

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Sky & Telescope - March 2019

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