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11 September 2014updated 01 Jul 2021 11:42am

Pleiades row: the fault in our star measurements

Either our understanding of how stars form needs a big overhaul, or one of the current missions of the European Space Agency could turn out to be something of a white elephant.

By Michael Brooks

According to the US National Radio Astronomy Observatory (NRAO), the puzzle has been solved. According to European astronomers, there never was a puzzle – the Americans just need to admit that they are wrong. The rest of us are left with an unanswered question: just how far away are the Pleiades, a group of stars in the Taurus constellation?

The controversy over the answer – or, rather, answers – to this question has serious ramifications. Either our understanding of how stars form needs a big overhaul, or one of the current missions of the European Space Agency (ESA) could turn out to be something of a white elephant.

We can trace the issue back to the time of Abba and the three-day week. The ESA’s Hipparcos satellite was launched in 1989 but was built using 1970s technology. Its four-year mission was to provide a map of outer space. By surveying the relative positions of 118,000 stars, it created what remains our most complete chart of the heavens.

Here is where the problems began. Until Hipparcos, the distance to the Pleiades was thought to be roughly 430 light years. In 1997, the Hipparcos team announced that the stars were in fact closer: 392 light years away. As Hipparcos was the best instrument we had, the new measurement was widely accepted.

Until now, that is. Using an array of radio telescopes scattered around the world, US astronomers have looked at how the Pleiades’ stars move in the sky as the earth turns. By comparing their positions to a galaxy so far away that it seems not to move at all, they have calculated the distance to the Pleiades to be 443 light years.

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The Europeans refuse to accept the new result. So, while the NRAO issued a press release claiming that the Hipparcos result “was wrong”, European astronomers politely told the journal Nature that everyone trusts Hipparcos, “except for the people who are looking at the Pleiades in the States”.

There is more at stake than international relations. The correct distance to the Pleiades matters for two reasons. First, if Hipparcos was wrong because of something in its instruments, the ESA craft Gaia, which has just embarked on a new survey, may be subject to the same kind of error. For a mission that costs €740m and is going to take five years to complete, it would be nice to know that there isn’t a fundamental design flaw.

Second, if Hipparcos is right, something is wrong with our understanding of how young stars form. The US measurements relied on received wisdom about how bright the Pleiades’ stars should be, given their age. Astronomers were confident that we have a good handle on this but if we don’t, the new result could be incorrect – as could many other “facts” about the history of the cosmos.

It is hard to know how things will play out. When it became a nationalistic issue all hope of reasonable dialogue disappeared. Now it’s down to Gaia.

US astronomers are suggesting that Hipparcos’s weakness was its 1970s camera technology. It might, they say, have been overwhelmed by an unexpected intensity of the starlight. Gaia will use its state-of-the-art 900-megapixel camera to map a billion stars. Its data should be accurate enough to provide another measurement of the distance from Planet Earth to many of these stars. Its operators can only hope that it works properly – and that it will somehow bridge astronomy’s transatlantic divide. 

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