http://www.skyandtelescope.com.au 19made by combining data from radio telescopes all over the
world gave a result of 444 light-years. Measurement after
measurement agreed with the greater distance, making the
one from Hipparcos look anomalously small, even though
successive reassessments of the spacecraft’s data only
narrowed in on the smaller value.
It took many years to solve the mystery. The issue is
apparently a matter of instrument calibration, due to
Hipparcos’s intricate observing method. Instead of looking
at a fixed spot in the sky, the telescope rotated about itself
and changed the orientation of its rotation axis over time, a
common strategy for all-sky surveys (see ‘Spinning through
space,’ in this issue). Using this method, Hipparcos built
up a map of the celestial sphere by determining the relative
angular distances between stars and how those 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 — gave
tightly correlated measurements. This called for different
calibrations at different spatial scales and resulted in an
unexpectedsourceoferrorfortheimportantandcompact
open clusters.
But even when they knew what the problem likely was,
astronomers had trouble correcting Hipparcos’s data to
produceadistancethatagreedwiththeothers.Itcouldbe
thatthereweremultiplesourcesoferror.So,whatscientists
wanted was confirmation of the larger distance from an
instrument that worked as Hipparcos did.NICOLLE R. FULLER / SAYO-ART LLC; PLEIADES PHOTO: DAVIDE DE MARTIN & THE ESA / ESO / NASA PHOTOSHOP FITS LIBERATOR
STHE PLEIADES Perhaps the most famous star cluster in the sky, the
bright stars of the Pleiades — often called the Seven Sisters — can be
seen without binoculars even from a city. The cluster lies roughly 450
light-years away toward the constellation Taurus.The second data release from the Gaia
mission solves a decades-long controversy
about the distance to the Pleiades cluster.The importance of being clumped
Open clusters play a crucial role in astronomy. Because a
cluster’s stars formed together from the same interstellar
cloud, we know that they are the same age. As such, they
are excellent laboratories to test physical models of stellar
evolution. What’s more, by knowing their distance and using
it to derive their intrinsic brightness, astronomers can then
use these models to calculate the distances to farther stars,
those that are removed from the reach of direct geometrical
methods like parallax.
In this regard, the Pleiades play a keystone role in the
calibration of the cosmic distances ladder, which proceeds
step by step, from the Sun to the nearest stars, then to
farther stars, and so on and so forth, changing methods
along the way until we reach the confines of the observable
universe. Much of modern 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.
Suspicion quickly fell on Hipparcos, as additional
measurements made with other instruments and methods
contradicted its result. In 2004, observations of three Pleiads
done by the Hubble Space Telescope gave a distance of 435
light-years. In 2014, an extremely precise measurement