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Last Updated: Friday, 28 September 2007, 17:23 GMT 18:23 UK
Scientist reworks star distances
Hipparcos (Esa)
Hipparcos' data underpins the system of cosmic measurement
The most accurate catalogue of the distances to more than 100,000 stars has just been released.

Cambridge astronomer Dr Floor van Leeuwen has spent the past 10 years checking and recalculating data gathered by the Hipparcos satellite.

It collected the information in the 1990s, but questions were raised about apparent errors in the results.

Dr van Leeuwen, who saw a flaw in the way Hipparcos worked, has now corrected the star distances.

"It's been necessary to get more information out of the data than was published before," he told the BBC.

"These missions are very expensive, and getting everything out should be the aim of every scientist working on a mission like Hipparcos."

Systematic skew

The catalogue (Hipparcos - The New Reduction of the Raw Data) will allow astronomers to probe more deeply into the properties of stars and galaxies.

Astronomers can only determine the physical properties of stars by comparing their luminosity with their distance from Earth. Without an accurate measurement of distance, the star's true luminosity or size cannot be known.

Similarly, the simple trigonometric parallax method used by Hipparcos to measure very accurately distances to relatively near objects is a first stepping stone in the process of inferring the distances to ever-more remote objects.

Pleiades (Nasa)
Hipparcos' distance to Pleiades was shorter than it should have been
The first Hipparcos catalogues were published in 1997, but a debate then raged because some of the distances listed appeared shorter than the figures obtained by ground-based observations.

Dr van Leeuwen identified a systematic flaw resulting from temperature fluctuations experienced by Hipparcos as it moved around the Earth. This made the spacecraft twist slightly and skew some of its data.

"The discovery of the problem with the satellite left me with no option but to recalculate the data," Dr van Leeuwen explained. "I knew that it could be done and I knew that the existing data could be significantly improved in all aspects, so I had no choice.

"It was an extremely painful process. You can spend a whole weekend examining one small part of the data, and making the resultant corrections can take two weeks. But the result is that we now have a catalogue more accurate than ever before, and one in which we know that all the calculations work."

Ultimately, Hipparcos' astrometric data has altered our view of the cosmos. Its distance scale found the Universe to be bigger and younger than some thought.

The European Space Agency will launch the Gaia spacecraft early in the next decade. It will compile a catalogue of vastly more stars at even greater distances.

The new Hipparcos data is published by Springer as Volume 350 in the Astrophysics and Space Science Library Series.

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