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The age of stars has long been debated. Our universe is about 14 billion years old, and the oldest stars were estimated to be 15 or more billion years old. But how can the stars be older than the universe? According to Brian Chaboyer, Assistant Professor of Physics and Astronomy at Dartmouth, and his colloborators, this age crisis is over, based on data from the Hipparcos satellite and new, more accurate calculations. The oldest stars are only 13 billion years old, confirming what was common sense: the universe came first.
"Knowing the ages of the oldest stars provides an important constraint for cosmological models. The models predict the present age of the universe based upon the expansion history of the universe. This prediction can now be more accurately tested using the stellar ages," said Chaboyer.
In the May 2001 issue of Scientific American, Chaboyer explains his method to end the age controversy. To study stellar evolution, researchers examine the oldest stars, and these are found in distant globular clusters, groups of about 100,000 stars found in the outskirts of the Milky Way, our galaxy. The evolution of stars is governed by their mass. Higher mass stars are more luminous as they burn their nuclear fuel more quickly, but they have shorter lifetimes than lower mass stars. Unfortunately, astronomers cannot measure the mass or luminosity of a star directly. Instead, they must measure the apparent brightness of a star in the sky and convert it to luminosity or mass. For the conversion, they need to determine the distance to the star.
Measuring distance in space, however, is one of the most difficult tasks in astronomy. Chaboyer and his colleagues use a geometric method called "parallax" to calculate cosmic distances. Parallax involves measuring the angle to a distant object from two different vantage points; researchers can calculate the distance to the object by looking at the difference of the angles and the separation of the vantage points. To measure parallaxes to stars, astronomers measure the angles from different places in Earth's orbit, then compare those angles to the diameter of the Earth's orbit. Unfortunately, the farther the star from Earth, the smaller the parallax and the larger the error.
The Hipparcos satellite, launched in 1989 by the European Space Agency, was able to determine the distance to globulars with greater accuracy. During its four-year mission, Hipparcos reached stars 10 times farther away than ever before. It didn't reach any globular clusters, however, but it did find some stars that are now acting as a liaison to the clusters helping Chaboyer pinpoint distance with greater accuracy. So armed with more precise parallax measurements, he found that the stars were 10 percent farther away than previously thought. Therefore, because the stars are intrinsically more luminous, they are younger than previous research revealed.
Chaboyer will continue to fine-tune our knowledge of stellar evolution. He has been honored with a National Science Foundation CAREER Award and a Cottrell Scholar Award from the Research Corporation for his work and his teaching in this field. He is a principal investigator on the Space Interferometry Mission telescope, a NASA satellite scheduled for launch in 2008. This mission aims to obtain accurate parallax measurements in our Milky Way and to distant globular clusters to better understand the formation of the galaxy and the age of the universe.
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