A modest European space probe has found that the Universe is bigger and older than many believe. By Heather Couper and Nigel Henbest
A quiet revolution has shaken the world of astronomy in recent weeks. A telescope orbiting the Earth has found that most stars are farther away than astronomers had believed, meaning the Universe is bigger, and older, than we had thought.

The orbiting observatory behind the revolution is not Nasa's much-feted Hubble Space Telescope, launched at a cost of more than pounds 1bn to answer those very questions. Instead, it is a European mission, without headline- grabbing photographs or spacewalks, whose price-tag was a tenth that of Hubble.

The Cinderella of this tale is Hipparcos, the High-Precision Parallax- Collecting Satellite. After it was launched in 1989, a rocket failure stranded Hipparcos in the wrong orbit. But astronomers rewrote the software that analyses its observations, and the first results, announced in Venice last month, have lived up to the original hopes.

Hipparcos measures star-distances by seeing how nearby stars seem to move against the background stars as the earth moves round its orbit. It is like observing how a finger held in front of your face seems to move against the background as you view it first with one eye and then the other. Astronomers have long measured this parallax effect with ground- based telescopes, but above the Earth's atmosphere Hipparcos has a much clearer view.

Once these distances are known accurately, astronomers can work out how bright the stars really are. And Hipparcos has overturned a fundamental "rule" of astronomy: that if two "ordinary" stars (not red giants or white dwarfs, for example) are the same temperature, they shine with the same brilliance. When Hipparcos investigated stars with the sun's temperature but lying in the Seven Sisters star cluster (the Pleiades), it found that these stars were only two-thirds as luminous as the Sun. As yet, no one has come up with a convincing explanation.

To account for their brightness in the sky, the dim stars of the Pleiades must lie closer than previously thought. But they are the exception. Generally, Hipparcos has found that stars are farther away. For example, when astronomers picked out a selection of stars thought to lie within 250 light-years of the Sun, Hipparcos found that half of them lay beyond this distance.

Astronomers measure distances to the limits of the Universe by erecting a "ladder" of distances, which starts with the nearest stars and extends by different kinds of measurements, in steps, to the farthest galaxies. With Hipparcos's new results, it appears that everything in the Universe is about 5 per cent farther off. This may not seem a lot, but it solves a puzzle that has been bothering cosmologists for years.

We can date when the Big Bang happened by backtracking the motion of galaxies in the expanding Universe. Recent measurements have come up with a figure of around 10 billion years. Embarrassingly, the oldest stars we know seem to be more than 12 billion years old - which would mean they were born before the Big Bang.

Hipparcos has cut the discrepancy on two fronts. First, a larger Universe must be correspondingly older. Second, the ages of the stars depend on the rate they burn up fuel. As we now know that most stars are farther away, they must be more luminous than previously thought, and so using fuel more quickly than we realised - and hence younger than astronomers had estimated. Putting both results together, the Big Bang happened some 11 billion years ago, with the first stars - the oldest ones we see around us - forming soon afterwards.

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13 0552 moon at first quarter

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25 Mercury at superior conjunction

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