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Study questions cosmic measurements

Satellite observations that are used as yardsticks for the size of the universe may be off by as much as 10 percent, scientists say.
/ Source: The Associated Press

A European satellite with a telescope designed to precisely measure the position and motion of stars may be a little off in some cases — by as much as 10 percent, actually, a figure that could throw off estimates about the size of the universe, astronomers say.

The Hipparcos satellite, named after the ancient Greek credited with inventing astronomy, was launched by the European Space Agency in 1989 and collected data for four years before it was shut down in 1993.

In 1997, European astronomers released a 17-volume three-dimensional map of the universe based on Hipparcos observations. It indicated the universe is now about 11 billion years old and pushed up the estimated size of the universe by 10 percent to 15 percent.

But its measurement of the distance to the star cluster known as the Pleiades, or the Seven Sisters, has been called into question and caused a major controversy among astronomers.

In a study published Thursday in the journal Nature, Caltech astronomers Xiaopei Pan, Michael Shao and Shri Kulkarni present evidence that appears to confirm Hipparcos missed the mark by a significant amount.

“Hipparcos is, in fact, systematically off from other estimates we have,” said William Van Altena of Yale University.

But Van Altena said it may just be an isolated problem because of the small area of the sky covered by the Pleiades and the way the satellite tried to measure it.

“I would say this does not cast any doubts on the Hipparcos results as a whole,” he said.

Explaining discrepancies
Star clusters such as the Pleiades are valuable tools for calculating distance by measuring the angle between them and the Earth, known as the parallax. But the atmosphere scatters the light received by ground-based telescopes, limiting their resolution — a disadvantage that does not affect the Hipparcos satellite orbiting in the vacuum of space.

Unlike the more expensive Hubble Space Telescope, the $775 million Hipparcos did not produce dramatic photos of the cosmos; it just reported measurements to astronomers.

The parallax measurements taken by Hipparcos gave a different distance in light years — the distance light travels in one year in space — than ground-based telescopes, and with other distance estimates based on the color and brightness of stars.

Pan, Shao and Kulkarni decided to take the additional step of measuring the movement of a pair of binary stars in the Pleiades cluster as another way to estimate distance.

They found their results were consistent with traditional distance estimates but varied from Hipparcos. “It was a little bit of a surprise, but not a great surprise,” Shao said.

“We had very good agreement with the other estimates, so that’s why I have confidence our results are correct and Hipparcos is off,” Pan added.

Who's right?
Princeton astronomer Bohdan Paczynski, like his counterpart at Yale, Van Altena, says he believes the Caltech researchers are right and Hipparcos is wrong.

In a separate commentary published in Nature, Paczynski said the error may be due to an unusual orbit caused by failure of some of the Hipparcos booster rockets shortly after launch. A pending study that will confirm the Caltech measurements is expected to resolve the controversy.

But in the meantime, “if we are to have confidence in our measurements of astronomical distances, this problem must be solved,” Paczynski said.