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

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.

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

29


http://www.skyandtelescope.com

Sky & Telescope - March 2019

Table of Contents for the Digital Edition of Sky & Telescope - March 2019

Contents
Sky & Telescope - March 2019 - Cover1
Sky & Telescope - March 2019 - Cover2
Sky & Telescope - March 2019 - 1
Sky & Telescope - March 2019 - Contents
Sky & Telescope - March 2019 - 3
Sky & Telescope - March 2019 - 4
Sky & Telescope - March 2019 - 5
Sky & Telescope - March 2019 - 6
Sky & Telescope - March 2019 - 7
Sky & Telescope - March 2019 - 8
Sky & Telescope - March 2019 - 9
Sky & Telescope - March 2019 - 10
Sky & Telescope - March 2019 - 11
Sky & Telescope - March 2019 - 12
Sky & Telescope - March 2019 - 13
Sky & Telescope - March 2019 - 14
Sky & Telescope - March 2019 - 15
Sky & Telescope - March 2019 - 16
Sky & Telescope - March 2019 - 17
Sky & Telescope - March 2019 - 18
Sky & Telescope - March 2019 - 19
Sky & Telescope - March 2019 - 20
Sky & Telescope - March 2019 - 21
Sky & Telescope - March 2019 - 22
Sky & Telescope - March 2019 - 23
Sky & Telescope - March 2019 - 24
Sky & Telescope - March 2019 - 25
Sky & Telescope - March 2019 - 26
Sky & Telescope - March 2019 - 27
Sky & Telescope - March 2019 - 28
Sky & Telescope - March 2019 - 29
Sky & Telescope - March 2019 - 30
Sky & Telescope - March 2019 - 31
Sky & Telescope - March 2019 - 32
Sky & Telescope - March 2019 - 33
Sky & Telescope - March 2019 - 34
Sky & Telescope - March 2019 - 35
Sky & Telescope - March 2019 - 36
Sky & Telescope - March 2019 - 37
Sky & Telescope - March 2019 - 38
Sky & Telescope - March 2019 - 39
Sky & Telescope - March 2019 - 40
Sky & Telescope - March 2019 - 41
Sky & Telescope - March 2019 - 42
Sky & Telescope - March 2019 - 43
Sky & Telescope - March 2019 - 44
Sky & Telescope - March 2019 - 45
Sky & Telescope - March 2019 - 46
Sky & Telescope - March 2019 - 47
Sky & Telescope - March 2019 - 48
Sky & Telescope - March 2019 - 49
Sky & Telescope - March 2019 - 50
Sky & Telescope - March 2019 - 51
Sky & Telescope - March 2019 - 52
Sky & Telescope - March 2019 - 53
Sky & Telescope - March 2019 - 54
Sky & Telescope - March 2019 - 55
Sky & Telescope - March 2019 - 56
Sky & Telescope - March 2019 - 57
Sky & Telescope - March 2019 - 58
Sky & Telescope - March 2019 - 59
Sky & Telescope - March 2019 - 60
Sky & Telescope - March 2019 - 61
Sky & Telescope - March 2019 - 62
Sky & Telescope - March 2019 - 63
Sky & Telescope - March 2019 - 64
Sky & Telescope - March 2019 - 65
Sky & Telescope - March 2019 - 66
Sky & Telescope - March 2019 - 67
Sky & Telescope - March 2019 - 68
Sky & Telescope - March 2019 - 69
Sky & Telescope - March 2019 - 70
Sky & Telescope - March 2019 - 71
Sky & Telescope - March 2019 - 72
Sky & Telescope - March 2019 - 73
Sky & Telescope - March 2019 - 74
Sky & Telescope - March 2019 - 75
Sky & Telescope - March 2019 - 76
Sky & Telescope - March 2019 - 77
Sky & Telescope - March 2019 - 78
Sky & Telescope - March 2019 - 79
Sky & Telescope - March 2019 - 80
Sky & Telescope - March 2019 - 81
Sky & Telescope - March 2019 - 82
Sky & Telescope - March 2019 - 83
Sky & Telescope - March 2019 - 84
Sky & Telescope - March 2019 - Cover3
Sky & Telescope - March 2019 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202501
https://www.nxtbook.com/nxtbooks/aas/st_202412
https://www.nxtbook.com/nxtbooks/aas/st_202411
https://www.nxtbook.com/nxtbooks/aas/st_202410
https://www.nxtbook.com/nxtbooks/aas/st_202409
https://www.nxtbook.com/nxtbooks/aas/st_202408
https://www.nxtbook.com/nxtbooks/aas/st_202407
https://www.nxtbook.com/nxtbooks/aas/st_202406
https://www.nxtbook.com/nxtbooks/aas/st_202405
https://www.nxtbook.com/nxtbooks/aas/st_202404_qr
https://www.nxtbook.com/nxtbooks/aas/st_202404
https://www.nxtbook.com/nxtbooks/aas/st_202403
https://www.nxtbook.com/nxtbooks/aas/st_202402
https://www.nxtbook.com/nxtbooks/aas/st_202401_aus
https://www.nxtbook.com/nxtbooks/aas/st_202401
https://www.nxtbook.com/nxtbooks/aas/st_202312_aus
https://www.nxtbook.com/nxtbooks/aas/st_202312
https://www.nxtbook.com/nxtbooks/aas/st_202311
https://www.nxtbook.com/nxtbooks/aas/st_202310
https://www.nxtbook.com/nxtbooks/aas/st_202309
https://www.nxtbook.com/nxtbooks/aas/st_202308
https://www.nxtbook.com/nxtbooks/aas/st_202307
https://www.nxtbook.com/nxtbooks/aas/st_202306
https://www.nxtbook.com/nxtbooks/aas/st_202305
https://www.nxtbook.com/nxtbooks/aas/st_202304
https://www.nxtbook.com/nxtbooks/aas/st_202304_qr
https://www.nxtbook.com/nxtbooks/aas/st_202303
https://www.nxtbook.com/nxtbooks/aas/st_202302
https://www.nxtbook.com/nxtbooks/aas/st_202301
https://www.nxtbook.com/nxtbooks/aas/st_202212
https://www.nxtbook.com/nxtbooks/aas/st_202211
https://www.nxtbook.com/nxtbooks/aas/st_202210
https://www.nxtbook.com/nxtbooks/aas/st_202209
https://www.nxtbook.com/nxtbooks/aas/st_202208
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com