By VASANTHA ARORA

ASHINGTON : The Hubble space telescope of the National Aeronautics and Space Administration (NASA) has uncovered the oldest burnt-out stars in the Milky Way galaxy.

These extremely old, dim "clock work stars" provide a completely independent reading on the age of the universe from previous methods, says NASA. The ancient white dwarf stars, as seen by Hubble, are reportedly 12 to 13 billion years old.

Since earlier Hubble observations show that the first stars formed less than one billion years after the universe's birth in the big bang, finding the oldest stars puts astronomers well within an arm's reach of calculating the absolute age of the universe. Although Hubble research sets the age of the universe at 13 to 14 billion years based on the rate of expansion of space, the universe's birthday is such a fundamental and profound value that astronomers have long sought other age-dating techniques to crosscheck their conclusions.

"The new observation short-circuits getting to the age question, and offers a completely independent way of pinning down that value, "says Harvey Richer of the University of British Columbia at Vancouver in Canada.

The new age-dating observations were done by Mr Richer and his colleagues by using Hubble to hunt for the elusive ancient stars hidden inside a globular star cluster 7,000 light years away in the constellation Scorpius.

Conceptually, the new age-dating observation is as elegantly simple as estimating how long ago a campfire burned by measuring the temperature of the smoldering coals.

For Hubble, the "coals" are white dwarf stars, the burned-out remnants of the earliest stars in our galaxy. Hot, dense spheres of carbon "ash" left behind by the long-dead star's nuclear furnace, white dwarfs cool down at a predictable rate. The older the dwarf, the cooler it is, making it a perfect "clock" that has been ticking for almost as long as the universe has existed.

This approach has been recognised as more reliable than age-dating the oldest stars still burning by nuclear fusion, which relies on complex models and calculations about how a star burns its nuclear fuel and ages. White dwarfs are easier to age-date because they are simply cooling, but the trick has been finding the dimmest and hence longest-running "clocks".

As white dwarfs cool they grow fainter, and this required that Hubble train a steady gaze on the ancient globular star cluster M4 for eight days over a 67-day period. This allowed for even fainter dwarfs to become visible, until at last the coolest-and the oldest-dwarfs were seen.

These starts are so feeble, they are less than one-billionth the apparent brightness of the faintest stars that can be seen by the naked eye.

Globular clusters are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk (as further confirmed by Richer's observations). Today, 150 globular clusters survive in the galactic halo.

The globular cluster M4 was selected because it is the nearest to earth, so the intrinsically feeblest white dwarfs are still apparently bright enough to be picked out by Hubble. (IANS)