Gaia Weighs in on the Pleiades Distance Controversy

By Joan Schmelz

Distance is one of the most challenging properties to measure in astronomy – it is bootstrapped from nearby objects like the Sun and planets all the way out to galaxies and quasars. The Pleiades, a nearby star cluster, had served as a cornerstone for astronomical distance derivations and set the scale for other clusters. Results from various ground-based techniques all agreed that the distance was about 133 parsecs, making the Pleiades a solid rung on the lower end of the “Cosmic Distance Ladder.” This important role was called into question by results from the parallax satellite, Hipparcos, the gold standard of distance measurements. The distance measured by Hipparcos is 120.2 ± 1.5 parsecs, significantly and disturbingly different from traditional ground-based values and setting up the so-called “Pleiades distance controversy.” Although this amounts to only a 10% difference in the distance, the result propagates through the system and affects the size, age, and physics of the universe and objects in the universe. This disagreement led to significant shifts in the cosmic distance scale and controversial revisions of physical models required to obtain the Hipparcos result. To resolve this controversy, a multi-year VLBI observing campaign using the High Sensitivity Array was conducted to derive a new independent, distance to the Pleiades. The first four parallax results derived from these measurements determined a distance to the Pleiades of 136.2 ± 1.2 pc (see Figure). This determination is in line with the original results from ground-based measurements, but incompatible with that suggested by Hipparcos (Melis et al. 2014). Now the Gaia mission, Hipparcos’s successor, has made an initial measurement of 134 ± 6 pc, consistent with the Arecibo VLBI result (Gaia Collaboration 2016).



Summary of Pleiades distances from different measurements, showing 1σ errors. The VLBI result, which uses Arecibo (red), is the most accurate determination to date and is consistent with previous ground-based measurements (black). The Hipparcos results (blue) set up the controversy, but the new measurements from Gaia (green) confirm those of Arecibo VLBI (Figure adapted from Melis et al. 2014).

When an observing program requires the detections of weak signals to resolve a fundamental astronomical controversy, there is no substitute for the collecting area of the Arecibo Observatory.

Arecibo is an essential component of the High Sensitivity Array; its unparalleled collecting area is required to detect the weak double and triple radio star systems and decouple their proper motion from their orbital motion. The resolution of the Pleiades distance controversy would not have been possible without Arecibo.

Paper Reference: 2014, Science, 345, 1029

Title: Gaia Data Release 1: Summary of the Astrometric, Photometric, and Survey Properties

Authors: Gaia Collaboration

Paper Reference: Astronomy & Astrophysics no. aa29512-16, Sep 2016

Title: A VLBI Resolution of the Pleiades Distance Controversy

Authors: Melis, Reid, Mioduszewski, Stauffer, Bower