PASADENA — A California Institute of Technology astronomer has obtained data that could resolve the "age problem" of the universe, in which certain stars appear to be older than the universe itself.
Dr. Neill Reid, using information collected by the European Space Agency's Hipparcos satellite, has determined that a key distance measure used to compute the age of certain Milky Way stars is off by 10 to 15 percent. The new data leads to the conclusion that the oldest stars are actually 11 to 13 billion years old, rather than 16 to 18 billion years old, as had been thought.
The new results will be of great interest to cosmologists, Reid says, because estimates of the age of the universe, based on tracking back the current rate of expansion, suggest that the Big Bang occurred no more than about 13 billion years ago. Therefore, astronomers will no longer be confronted with the nettling discrepancy between the ages of stars and the age of the universe.
"This gives us an alternate way of estimating the age of the universe," says Reid. "The ideal situation would be to have the same answer, independently given by stellar modeling and cosmology."
Reid's method focuses on a type of star (known as subdwarfs) found in globular clusters, which are spherical accumulations of hundreds of thousands of individual stars. These have long been known to be among the earliest objects to form in the universe, since the stars are composed mainly of the primordial elements hydrogen and helium, and because the clusters themselves are distributed throughout a sphere 100,000 light-years in diameter, rather than confined, like the sun, within the flattened pancake of the galactic disk. Astronomers can determine quantitative ages for the clusters by measuring the luminosity (the intrinsic brightness) of the brightest sunlike stars in each cluster. Those measurements require that the distances to the clusters be known accurately.
Reid looked at some 30 stars within about 200 light-years of Earth. Using the Hipparcos satellite, he was able to obtain very accurate distances to these stars by the parallax method. Parallax is a common method for determining relatively nearby objects. Just as a tree 10 feet away will seem to shift its position against the distant background when an observer closes one eye and then the other, a nearby star will shift its position slightly if the observer waits six months for Earth to reach the opposite side of its orbit. And if the distance between the two observing sites (the baseline) is known very accurately, the observer can then compute the distance to the object by treating the object and the two observing sites as a giant triangle.
Reid chose the 30 stars for special study (out of the 100,000 for which Hipparcos obtained parallax data) because they, like the globular cluster stars, are composed primarily of hydrogen and helium. Thus, these stars also can be assumed to be very old, and may indeed themselves once have been members of globulars that were torn apart as they orbited the galaxy.
Once distances have been measured, these nearby stars act as standard candles whose brightness can be compared to similar stars in the globular clusters. While this is a well-known technique, older investigations were only able to use lower-accuracy, pre-Hipparcos parallaxes for 10 of the 30 stars.
Reid's conclusion is that the clusters are about 10 to 15 percent farther from Earth than previously thought. This, in turn, means that the stars in those clusters are actually about 20 percent brighter than previously thought, because luminosity falls off as distance increases. Brighter stars have shorter lifetimes, so this means that the clusters themselves must be younger than once assumed.
British astronomers Michael Feast and Robin Catchpole recently arrived at very similar conclusions, also based on new data from Hipparcos, but using a different, and less direct, line of argument. They used new measurements of a type of variable known as Cepheids to determine a revised distance to the Large Magellanic Cloud, a galaxy orbiting the Milky Way.
Feast and Catchpole used another type of variable star, the RR Lyrae variables, to bridge between the LMC and globular clusters. The fact that these two independent methods give the same answer makes that answer more believable, says Reid. "Most people previously believed that 14 billion years was the youngest age you could have for these stars," Reid says. "I think it's now accurate to say that the oldest you could make them is 14 billion years.
"No longer are we faced with the paradox of a universe younger than its stellar constituents," says Reid.
The work is set to appear in July in the Astrophysical Journal.