Radio telescope network settles dispute over the Pleiades

The Pleiades are a cluster of hot young stars that have been crucial to understanding how stars form.

Astronomers have finally settled a dispute over just how far the Pleiades star cluster – one of the most familiar sights in the night sky – are from Earth.

Until the 1990s the consensus was 430 light years, but then the European satellite Hipparcos, which was launched in 1989 to precisely measure the positions and distances of thousands of stars, came up with a distance of about 390 light-years.

You wouldn't've thought 40 light years was such a big deal given the immense distances involved, but Carl Melis, of the University of California, San Diego, say it was.

That may not seem like a huge difference, but, in order to fit the physical characteristics of the Pleiades stars, it challenged our general understanding of how stars form and evolve. To fit the Hipparcos distance measurement, some astronomers even suggested that some type of new and unknown physics had to be at work in such young stars.

The Pleiades, or the "Seven Sisters", is made up of hundreds of young, hot stars formed about 100 million years ago and have been vital for scientists to understand how similar clusters form. They also used the stars as a yardstick for estimating the distance to more distant, clusters.

To settle the issue, Melis and his colleagues used a global network of radio telescopes including the Very Long Baseline Array (VLBA), a system of 10 radio telescopes ranging from Hawaii to the Virgin Islands; the Robert C. Byrd Green Bank Telescope in West Virginia; the 1,000-foot-diameter William E. Gordon Telescope of the Arecibo Observatory in Puerto Rico; and the Effelsberg Radio Telescope in Germany.

That gave the equivalent of a telescope the size of the Earth to provide precision measurements. Amy Miouduszewski, of the National Radio Astronomy Observatory, describes the task as "the equivalent of measuring the thickness of a quarter in Los Angeles as seen from New York".

The result? The Pleiades is officially 443 light-years from Earth, a figure, the astronomers say, that is accurate to within 1%.

"This is a relief," Melis said, because the newly-measured distance is close enough to the pre-Hipparcos distance that the standard scientific models of star formation accurately represent the stars in the Pleiades.

"The question now is what happened to Hipparcos?" Melis said. No one quite knows yet, but it has called into question the spacecraft's measurements of distances to 118,000 stars. Another spacecraft, Gaia, launched last December, will use similar technology to measure distances of about one billion stars but that may now have to be cross-checked using the radio telescope technique.

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