20 AUSTRALIAN SKY & TELESCOPE April 2019
Putting the Pleiades
in their place
Hipparcos’s successor, Gaia, provided the
opportunity astronomers needed. Like
Hipparcos, Gaia is an ESA satellite of
unusual design: It also looks sideways as
it spins, its two telescopes scanning the
samestripofskyoneaftertheotheras
thespacecraftslowlyrotatesitsview.Its
seconddatareleasecontainsthepositions,parallaxesand
proper motions of more than 1.3 billion sources and reaches
as faint as magnitude 21. The uncertainty of its parallaxes is
around 40 microarcseconds (40 μas) for objects brighter than
magnitude 15. So while Hipparcos could spot an astronaut on
theMoonfromEarth,Gaiawouldbeabletoseeacoin.
What value would Gaia find for the distance to the famous
cluster?Usingthefirstdatareleasefrom2016,oneteam
calculated that the 164 cluster stars they included in their
analysisgaveadistanceof437light-years.Itwasaclear
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’
positiongivesaconeofobservations,withitstipintheSolar
Systemandextendingindefinitelyintospace.SomewhereMaiaMeropeTo EarthTaygeta
Atlas Electra
AlcyonePleioneSUNUSUAL STREAM When you plot the positions of the Pleiades’
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.inside that cone lie the Pleiades, as well
as many 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.
The Pleiades have not relinquished all their mysteries,
however. The precise determination of the positions enables
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 star 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.XWHAT IS PARALLAX? Parallax is the shift
in an object’s position against the background
scene when viewed from two different locations.
Nearby stars have measurable parallaxes
due to Earth’s motion around the Sun, which
astronomers can use to calculate the stars’
distances. However, calculating the true distance
using the parallax requires eliminating all sources
of error in the angle measurement.PARALLAX: LEAH TISCIONE /S&T;3D PLOT: GUILLERMO ABRAMSON /CELESTIA1 a.u.Apparent
shift = 2pDistance
to starParallax
angle (p)STAR SLEUTHING