Sky & Telescope - March 2019 - 28

Star Sleuthing

and so on and so forth, changing methods
along the way until we reach the conﬁnes
Apparent
of the observable universe. Much of modshift = 2 p
ern astronomical knowledge, from stellar
physics to the structure and evolution of
the universe, depends on a good calibration of this distance ladder - and thus on
knowing the distances to nearby stars to
a T. Controversy over the Pleiades' distance
was therefore disconcerting.
instrumental or systematic error astronoParallax
Suspicion quickly fell on Hipparcos, as
mers had overlooked? Was there a problem
angle ( p)
additional measurements made with other
with just the Pleiades, or also with other
Distance
instruments and methods contradicted
measurements? Or were the Pleiades really
to star
its result. In 2004, observations of three
closer and thus didn't ﬁt into our models
Pleiads done by the Hubble Space Teleof stellar formation and evolution?
scope gave a distance of 435 light-years. In
Astronomers now have an answer.
1 a.u.
2014, an extremely precise measurement
made by combining data from radio teleThe Importance of Being
scopes all over the world gave a result of
Clumped
444 light-years. Measurement after measurement agreed with
Open clusters play a crucial role in astronomy. Because a
the greater distance, making the one from Hipparcos look
cluster's stars formed together from the same interstellar
anomalously small, even though successive reassessments of
cloud, we know that they are the same age. As such, they
the spacecraft's data only narrowed in on the smaller value.
are excellent laboratories to test physical models of stellar
It took many years to solve the mystery. The issue is apparevolution. What's more, by knowing their distance and using
ently a matter of instrument calibration, due to Hipparcos's
it to derive their intrinsic brightness, astronomers can then
intricate observing method. Instead of looking at a ﬁ xed spot
use these models to calculate the distances to farther stars,
in the sky, the telescope rotated about itself and changed the
those that are removed from the reach of direct geometrical
orientation of its rotation axis over time, a common strategy
methods like parallax.
for all-sky surveys (see page 20). Using this method, HipIn this regard, the Pleiades play a keystone role in the calibration of the cosmic distances ladder, which proceeds step by parcos built up a map of the celestial sphere by determining
the relative angular distances between stars and how those
step, from the Sun to the nearest stars, then to farther stars,
distances changed with time. Calculating a particular star's
parallax required distinguishing the star's motion from those
around it. But closely packed stars - like those in a cluster -
Trigonometric Parallax
Gaia DR2
gave tightly correlated measurements. This called for differHipparcos
Gaia DR1
ent calibrations at different spatial scales and resulted in an
VLBI
unexpected source of error for the important and compact
open clusters.
Spectroscopic twins
But even when they knew what the problem likely was,
astronomers had trouble correcting Hipparcos's data to
produce a distance that agreed with the others. It could be
Moving cluster
that there were multiple sources of error. So, what scientists
wanted was conﬁrmation of the larger distance from an
Fit of isochrone
instrument that worked as Hipparcos did.

Putting the Pleiades in Their Place

Binary orbits

350

400

450

500

Distance (light-years)
p HIPPARCOS'S ANOMALY Shown are distances to the Pleiades according to a variety of methods. Note the anomalous distance measured
by Hipparcos (purple), the precise result of radio interferometry (green),
and the early result given by the first Gaia data release (DR1, yellow). The
article uses Gaia's second data release (light blue).

M A RCH 2 019 * SK Y & TELESCOPE

Hipparcos's successor, the hat-shaped 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 same strip of sky one
after the other as the spacecraft slowly rotates its view. Its
second data release contains the positions, parallaxes, and
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

PA R A LL A X: LE AH TISCIONE / S&T; HIPPA RCOS A NO M A LY: S&T ILLUSTR ATION,
SOURCE: C. MELIS E T A L. / SCIENCE 2014

u WHAT 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.

Sky & Telescope - March 2019

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