Distance
The distance to the Pleiades can be used as an important first step to calibrate the cosmic distance ladder. As the cluster is so close to the Earth, its distance is relatively easy to measure and has been estimated by many methods. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster, which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades. Yet some authors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmic distance ladder can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distances as established by Hipparcos and independent means (e.g., the Hyades, Coma Berenices cluster, etc.).
Measurements of the distance have elicited much controversy. Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique that should yield the most direct and accurate results. Later work consistently argued that the Hipparcos distance measurement for the Pleiades was erroneous. In particular, distances derived to the cluster via the Hubble Space Telescope and infrared color-magnitude diagram fitting favor a distance between 135–140 pc. However, the author of the 2007–2009 catalog of revised Hipparcos parallaxes reasserted that the distance to the Pleiades is ~120 pc, and challenged the dissenting evidence. Recently, Francis and Anderson proposed that a systematic effect on Hipparcos parallax errors for stars in clusters biases calculation using the weighted mean, and gave a Hipparcos parallax distance of 126 pc, and photometric distance 132 pc based on stars in the AB Doradus, Tucana-Horologium moving group and Beta Pictoris moving groups, which are similar in age and composition to the Pleiades. Those authors note that the difference between these results can be attributed to random error.
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