Carl Melis’s Precision Distance Measurement of the Pleiades Makes the Cover of Science

F1.medium-1Carl Melis and collaborators have just published the most accurate distance measurement to the Pleiades Star Cluster – reported on the cover of Science magazine – and one conclusion stands out: Hipparcos was wrong.

Not the second century BCE Greek astronomer, but the European satellite astrometry mission that took place in the early 1990s, which made accurate parallax distance measurements of over 118,000 stars (and less accurate measures for 2.5 million). It is the gold standard for distances in astronomy, but one result has been controversial for some time: the distance to the Pleiades.

The Pleiades is one of a few benchmark systems upon which much of what we know about stars and the cosmic distance scale hinges. As one of the closest, young (~115 Myr) clusters, its members can be accurately characterized, placed on a color-magnitude (Hertzsprung-Russell) diagram, and compared to stellar evolution models. The Pleaides essentially defines the zero-age Main Sequence, the locus from which stars begin their hydrogen-burning lives.  The cluster is also young enough and massive enough to host 11 fairly bright B-type stars, all visible to the naked eye but not necessarily resolvable without binoculars. The properties of these stars can be used to calibrate the distances of clusters containing B stars throughout the Galaxy, allowing us to scale up distances in our Solar Neighborhood to that of the Milky Way, a step up the cosmic distance ladder.


Both of these uses of the Pleiades require accurate distances, and measurements prior to 1990 came in around 130 pc. So it came to some surprise when Hipparcos reported a distance of 116±3 pc (later revised up to 120.2±1.9 pc), more than 10% closer and about 1 milliarcsecond in parallax angle different. The closer distance caused problems with stellar evolutionary models, forcing astronomers to consider (and ultimately reject) the possibility that the Pleiades stars might be somehow peculiar. Several investigations followed, determining distances through ground-based astrometry, isochrone fitting, moving cluster analysis, modeling of binary orbits, and even re-analysis of the Hipparcos data, and consistently found values closer to 130-135 pc. A few studies identified possible systematic effects in the Hipparcos measurement. Nevertheless, the Hipparcos team have stood by their data, which until Carl’s study remained the most precise measurement available.


Carl and his team made a new ground-based astrometric measurement using the exquisite precision and absolute accuracy afforded by Very Long Baseline Interferometry (VLBI).  By combining coordinated observations from radio telescopes around the planet, they had a instrument with an effective dish the size of the Earth, resulting in extremely high precision angle measurements. Importantly, this technique is also accurate, as observations of high redshift quasars puts their measurements on an absolute, rather than relative, scale.  Carl and his team were able to measure the parallaxes of five Pleiades stars to an accuracy of 25 microarcseconds, about 7 billionths of a degree.


Combining their measurements, and accounting for up to a 2.5 pc offset to the center of the cluster in their uncertainties, they measured a distance of 136.2±1.2 pc to the Pleiades, the most accurate and precise measurement made to date.  Importantly, the value is over 7 sigma larger than the Hipparcos measurement, and consistent with the many other measurements made over the past 2 decades.  As stated in the paper:

Our results conclusively show that the Hipparcos-measured distance to the Pleiades cluster is in error.

Other than expanding the distance to the Pleiades – and hence the size of the Universe – by 10%, this result illustrates how an independent check can be made on space-based astrometry measurements. This is critically important with the recent launch of the GAIA satellite, the follow-up to Hipparcos and designed in the same manner.  It may be useful to check both the Hipparcos and GAIA measurements with VLBI astrometry in the near future.


The result was published as Melis et al. 2014, Science, 345, 1029-1032. It can also be accessed through the arXiv.  The authors include Mark Reid, Amy Mioduszewski, John Stauffer and Geoffrey Bowers.

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