Don Savage
Headquarters, Washington                                  April 24, 2002
(Phone: 202/358-1547)

Nancy Neal
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-0039)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4514)

RELEASE: 02-73

HUBBLE UNCOVERS OLDEST "CLOCKS" IN SPACE TO READ AGE OF UNIVERSE 

 Pushing the limits of its powerful vision, NASA's Hubble Space Telescope 
has uncovered the oldest burned-out stars in our Milky Way Galaxy. These 
extremely old, dim "clockwork stars" provide a completely independent 
reading on the age of the universe from previous methods.

The ancient white dwarf stars, as seen by Hubble, turn out to be 12 to 
13 billion years old. Because 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 arm's reach of calculating the absolute age of the universe. 

Though previous 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 cross-check their 
conclusions. "This 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, 
Vancouver, Canada.

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

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 recognized 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 oldest -- dwarfs were 
seen. These stars are so feeble (30th magnitude -- considerably fainter 
than originally anticipated for any Hubble telescope imaging with the 
original cameras), 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.

In 1928, Edwin Hubble's measurements of galaxies made him realize that 
the universe was uniformly expanding, which meant the universe had a 
finite age that could be estimated by mathematically "running the 
expansion backward." Edwin Hubble first estimated the universe was only 
two billion years old. Uncertainties over the true expansion rate led to 
a spirited debate in the late 1970s, with estimates ranging from 8 
billion to 18 billion years. Estimates of the ages of the oldest normal 
"main-sequence" stars were at odds with the lower value, since stars 
could not be older than the universe itself.

In 1997 Hubble Space Telescope astronomers broke this impasse by 
announcing a reliable age for the universe, calculated from a very 
precise measurement of the expansion rate. The picture soon got more 
complicated when astronomers using Hubble and ground-based observatories 
discovered the universe was not expanding at a constant rate, but 
accelerating due to an unknown repulsive force termed "dark energy." 
When dark energy is factored into the universe's expansion history, 
astronomers arrive at an age for the universe of 13-14 billion years. 
This age is now independently verified by the ages of the "clockwork" 
white dwarfs measured by Hubble.

-end-

Images, animation, illustrations and additional information are 
available on the Internet at:

http://oposite.stsci.edu/pubinfo/pr/2002/10

http://oposite.stsci.edu/pubinfo/latest.html