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.


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