An American astronomer using a new technique reported yesterday that the universe may be a mere 10 billion or 11 billion years old -- younger than many scientists believe.

Determining the age of the universe is one of the most important and yet hotly disputed problems in astronomy, with experts in wide disagreement. Their estimates, using numerous approaches, range from 7 billion to 25 billion years, but cautious estimates put the figure toward the middle range in the teens.

Harvey Butcher, director of the Kapteyn Astronomical Institute observatory in the Netherlands, used a new twist on a method in which the age of stars is determined by radioactive decay, similar to the way scientists determine the age of rocks. He reported his findings in the British journal Nature.

If Butcher's conclusions are correct, "both the accepted physics of stellar evolution and the age of the universe require substantial revision," said Gerry Gilmore of the Institute of Astronomy in Cambridge, England, in a column in the same issue.

The primary approaches to determining the age of the universe depend on the rate at which it is expanding -- following what is known as the Big Bang -- and the rate at which that expansion is slowing due to the gravitational pull of the stars and other matter in it, scientists say. Another way to measure age and thereby test the Big Bang approach is based on theories of how stars evolve.

Using instruments at the European Southern Observatory in Chile, Butcher analyzed starlight to compare the quantity of radioactive thorium, a rare chemical element, with the quantity of neodymium, which is nonradioactive and therefore stable, in stars near the sun. This is a variation on an established technique that uses uranium instead of neodymium.

Scientists expect that half of the thorium will have decayed in 14 billion years, and that the relative abundance of the two elements will indicate how long they have been in the stars. Butcher finds that there is no apparent change in the ratio of the two elements and concludes that therefore the oldest stars cannot be more than 12 billion years old, instead of the 16 billion to 18 billion years estimated recently by using established techniques.

However, his approach depends on many assumptions about when the elements came to be in the star and what other factors, such as nuclear fires burning at the star's core, might affect the relative abundance of the elements.

To resolve some of these questions, "you need bigger telescopes," Butcher told United Press International. New ground telescopes are in the works and scientists say the Hubble Space Telescope, to be launched aboard the space shuttle, should make a big contribution.