Radio telescopes have resolved the debate about the distance to the Pleiades - 444 light years

A new measurement claims that the distance to the Pleiades star cluster measured by the European Space Agency's Hipparchus satellite is incorrect and that previous measurements from ground-based telescopes were right all along

Autumn is approaching and with it the cluster of the Pleiades (Kima stars in the Hebrew sources) that will shine early in the night sky. The cluster is named after the seven sisters, daughters of the Titan Atlas and the nymph Pleiades and companions of Artemis. In Hebrew it is called Kima and is mentioned in the Bible in the book of Job (Lach, XNUMX) and in the book of Amos (XNUMX, XNUMX). In the Babylonian Talmud (Blessings Tractate, Noah page, end of the seer chapter), where it is claimed that there are a hundred stars in the cluster. (Source: Wikipedia) In Japanese, the cluster's name is Subaru and the seven-star model appears on the logo of the car company bearing this name.

Indeed, while the group has been known since ancient times, in the last decade a dispute has troubled astronomers as to the distance to it.

Now, a new measurement claims that the distance to the Pleiades star cluster measured by the European Space Agency's Hipparchus satellite is incorrect and that previous measurements from ground-based telescopes were right all along.

The Pleiades star cluster is a perfect laboratory for studying stellar evolution. All the stars - the seven visible to the eye and hundreds more that can only be seen in telescopes, were born from the same cloud of gas, they are of the same age and have the same chemical composition. However, they differ from each other in their mass. Accurate models, however, are highly dependent on distance. Therefore it is critical that astronomers know the exact distance to the star cluster.

The distance measurement has an implication not only for measuring the distance of this particular cluster but it is also used as a gauge to calibrate the distances to the other objects in the universe. In other words, knowing the exact distance to a cluster will help measure accurate distances to the most distant galaxies.

But accurate measurement of the vast distances in space is complicated. Trigonometric parallax - the tiny visible change in the position of the object against background stars as a result of the displacement of the observation point is the most accurate method in astronomy.

The consensus was that the Pleiades lie 435 light years from Earth. However, the European Space Agency's Hipparchus satellite, launched in 1989, accurately studied the position and distances of thousands of stars using parallax, and calculated that the cluster was only 392 light years from Earth with an error of less than 1%.

"It may not seem like a huge difference, but in order to match the physical properties of the Pleiades stars, it challenged our general understanding of how stars form and evolve," said Carl Melis, of the University of California, San Diego, the lead researcher. "To accommodate Hipparchus' distance measurement, some astronomers have even suggested the existence of some kind of new and unknown physics in such young stars."

If the cluster is really 10% closer than we all thought, the stars should be much fainter than the stellar evolution models suggest, so the debate revolves around the question of who was wrong, the spacecraft or the model?

To resolve the discrepancy, Melis and his colleagues used a new technique known as very long-baseline radio interferometry. By linking distant telescopes together, the astronomers created a virtual telescope, with a data collection surface the size of the distance between the telescopes.

The network included the baseline of the Very Long Array (a system of 10 radio telescopes ranging from Hawaii to the Virgin Islands), the Green Bank Telescope in West Virginia, the Gordon Telescope at the Arecibo Observatory in Puerto Rico, and the Apelsberg Radio Telescope in Germany.

"Using these telescopes together gave us the equivalent of an Earth-sized telescope," said Amy Miodzewski, of the National Radio Astronomy Observatory (NRAO). "This gave us the ability to make extremely accurate position measurements, equivalent to measuring the thickness of a coin in Los Angeles as seen from New York."

After a year and a half of observations, the team determined that the distance to the Pleiades is 444.0 light-years within an error range of 1% - that is, closer to the previous ground-based results and not to those of Hipparchus.

"The question now is what happened to Hiparchus?" said Melis.

The spacecraft measured the position of about 120,000 nearby stars and, in principle, could measure the distances more precisely than with ground-based telescopes. If this result holds, astronomers will have to deal with why Hipparchus' observations estimated the distance so inaccurately.

The European Space Agency's observatory - the Gaia spacecraft, which was launched in December 2013 - will use similar technology to measure the distances of about a billion stars. Although it is now ready to begin its science mission, the mission team will need to perform special treatment, utilizing the work of ground-based radio telescopes to ensure accurate measurements.

The findings were published in the August 29 issue of the journal Science and available online.

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