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17. A riddle solved: older Universe, younger stars

Europe gave an extraordinary gift to the world's astronomers in 1997. ESA's Hipparcos Catalogue told them the positions and motions of 120 000 stars, far more accurately than they ever knew before. It was the product of a unique space mission, using a satellite to fix the stars and so to improve mankind's grasp of the Universe.

Michael Feast

For four years, 1989-1993, ESA's Hipparcos satellite scanned the sky, measuring the angles between stars. The largest computations in the history of astronomy then figured out where each star was, and how it moved, with an accuracy 100 times better than previous sky surveys.

"Hipparcos cured a headache for cosmologists," said Michael Feast of Cape Town University, South Africa. "We now judge the Universe to be bigger and therefore older, by about a billion years. The oldest stars are much younger than supposed, by about four billion years. If the Universe is about 12 billion years old, everything fits nicely."

Figure 39: Solar neighbourhood

Scientists now possess, for the first time, a good three-dimensional picture of the bright stars in our neighbourhood. Hipparcos measured the distances of many stars, which were previously a matter of guesswork. For example Polaris, the Pole Star, is 430 light-years away.

Hipparcos hit the headlines in 1997 when it showed that the chief measuring rod for the Universe was wrongly marked. Bright blue stars called Cepheids, of which Polaris is one, vary in luminosity in predictable ways. Astronomers use them to gauge distances of galaxies and the scale of the cosmos. But Hipparcos revealed them to be farther away than previously supposed. This made the Universe about 10 percent older.

Also farther away than expected are the oldest known stars, the so- called halo stars. The change in distances cut their ages by a few billion years. Combined with the change in the cosmic scale, this solved a riddle in astronomy. Before Hipparcos the old stars seemed to predate the Universe. That was as nonsensical as mountains older than the Earth!

Last Update: 13 December 2005

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