Pleiades – near or far?

One of the distance-measuring techniques I described in Measuring the Universe was that of main sequence fitting. It’s a method for deriving the distance to an open star cluster. The idea is that you lay a plot of the HR diagram of the distance cluster on top of the HR diagram for a nearby cluster at a known distance (such as the Hyades). By comparing the brightness you can estimate the distance. Simple.

The problem with this method is that the Hyades is quite an old cluster of stars; we can use it to calibrate the lower part of the main sequence but not the upper part, where bright young stars reside. That’s where the Pleiades come in useful. It’s a much younger cluster, with some luminous stars. (The Pleiades is often known as the Seven Sisters, because of seven bright stars that are visible to the naked eye, but the cluster contains many more stars: Galileo counted 36 stars with his first crude telescope, and we now know that the cluster contains more than a thousand stars.) So if we know the distance to the Pleiades then we can be confident in using main sequence fitting as a tool for measuring distance. The Pleiades thus provides an important rung of the cosmological distance ladder.

Pleiades

A Hubble image of the Pleiades star cluster (Credit: NASA/ESA/Caltech)

The problem is that astronomers are not particularly confident about the distance to the Pleiades.

Before I began my research for Measuring the Universe, the commonly accepted distance to the Pleiades was about 134 parsecs; a variety of independent techniques and measurements arrived at that answer. More recent measurements using the Hubble Space Telescope suggest a distance of 135 to 140 parsecs. While I was writing Measuring the Universe, however, results from the Hipparcos astrometry mission produced a Pleiades distance of just under 120 parsecs – a significantly smaller distance. Since the Hipparcos scientists obtained their distances from parallax observations – which rely just on geometry, and thus should be reliable – one can’t easily dismiss their findings. So how far away is the Pleiades cluster? About 120 parsecs distant? Or 135–140 parsecs distant? It’s an important question.

In a recent paper (“A VLBI resolution of the Pleiades distance controversy“), Carl Melis and colleagues describe their own parallax-based measurements of the Pleiades distance. They used very long baseline radio interferometry to determine an absolute distance to five stars in the cluster. Their result? They obtain a distance of 136 ± 1.2 parsecs, which is consistent with all those other measurements. Their result seems to consolidate this particular rung of the distance ladder – but it does raise a question: why did Hipparcos produce a distance that was about 12% too small?