This is a Digitized Sky Survey image of the oldest star with a
well-determined age in our galaxy. The aging star, cataloged as HD
140283, lies 190.1 light-years away. Hubble Space Telescope was used to
narrow the measurement uncertainty on the star's distance, and this
helped refine the calculation of a more precise age of 14.5 billion
years (plus or minus 800 million years).
The star is rapidly passing through our local stellar neighborhood.
The star's orbit carries it through the plane of our galaxy from the
galactic halo that has a population of ancient stars. The
Anglo-Australian Observatory (AAO) UK Schmidt telescope photographed
the star in blue light. Credit: Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO. More Images
A team of astronomers using NASA's Hubble Space Telescope has taken
an important step closer to finding the birth certificate of a star
that's been around for a very long time.
"We have found that this is the oldest known star with a
well-determined age," said Howard Bond of Pennsylvania State University
in University Park, Pa., and the Space Telescope Science Institute in
Baltimore, Md.
The star could be as old as 14.5 billion years (plus or minus 0.8
billion years), which at first glance would make it older than the
universe's calculated age of about 13.8 billion years, an obvious
dilemma.
But earlier estimates from observations dating back to 2000 placed
the star as old as 16 billion years. And this age range presented a
potential dilemma for cosmologists. "Maybe the cosmology is wrong,
stellar physics is wrong, or the star's distance is wrong," Bond said.
"So we set out to refine the distance."
The new Hubble age estimates reduce the range of measurement
uncertainty, so that the star's age overlaps with the universe's age —
as independently determined by the rate of expansion of space, an
analysis of the microwave background from the big bang, and
measurements of radioactive decay.
This "Methuselah star," cataloged as HD 140283, has been known about
for more than a century because of its fast motion across the sky. The
high rate of motion is evidence that the star is simply a visitor to
our stellar neighborhood. Its orbit carries it down through the plane
of our galaxy from the ancient halo of stars that encircle the Milky
Way, and will eventually slingshot back to the galactic halo.
This conclusion was bolstered by the 1950s astronomers who were able
to measure a deficiency of heavier elements in the star as compared to
other stars in our galactic neighborhood. The halo stars are among the
first inhabitants of our galaxy and collectively represent an older
population from the stars, like our Sun, that formed later in the disk.
This means that the star formed at a very early time before the
universe was largely "polluted" with heavier elements forged inside
stars through nucleosynthesis. (The Methuselah star has an anemic
1/250th as much of the heavy element content of our Sun and other stars
in our solar neighborhood.)
The star, which is at the very first stages of expanding into a red
giant, can be seen with binoculars as a 7th-magnitude object in the
constellation Libra.
Hubble's observational prowess was used to refine the distance to the
star, which comes out to be 190.1 light-years. Bond and his team
performed this measurement by using trigonometric parallax, where an
apparent shift in the position of a star is caused by a change in the
observer's position. The results are published in the February 13 issue
of the Astrophysical Journal Letters.
The parallax of nearby stars can be measured by observing them from
opposite points in Earth's orbit around the Sun. The star's true
distance from Earth can then be precisely calculated through
straightforward triangulation.
Once the true distance is known, an exact value for the star's
intrinsic brightness can be calculated. Knowing a star's intrinsic
brightness is a fundamental prerequisite to estimating its age.
Before the Hubble observation, the European Space Agency's Hipparcos
satellite made a precise measurement of the star's parallax, but with
an age measurement uncertainty of 2 billion years. One of Hubble's
three Fine Guidance Sensors measured the position of the Methuselah
star. It turns out that the star's parallax came out to be virtually
identical to the Hipparcos measurements. But Hubble's precision is five
times better than that of Hipparcos. Bond's team managed to shrink the
uncertainty so that the age estimate was five times more precise.
With a better handle on the star's brightness Bond's team refined the
star's age by applying contemporary theories about the star's burn
rate, chemical abundances, and internal structure. New ideas are that
leftover helium diffuses deeper into the core and so the star has less
hydrogen to burn via nuclear fusion. This means it uses fuel faster and
that correspondingly lowers the age.
Also, the star has a higher than predicted oxygen-to-iron ratio, and
this too lowers the age. Bond thinks that further oxygen measurement
could reduce the star's age even more, because the star would have
formed at a slightly later time when the universe was richer in oxygen
abundance. Lowering the upper age limit would make the star
unequivocally younger than the universe.
"Put all of those ingredients together and you get an age of 14.5
billion years, with a residual uncertainty that makes the star's age
compatible with the age of the universe," said Bond. "This is the best
star in the sky to do precision age calculations by virtue of its
closeness and brightness."
This Methuselah star has seen many changes over its long life. It was
likely born in a primeval dwarf galaxy. The dwarf galaxy eventually
was gravitationally shredded and sucked in by the emerging Milky Way
over 12 billion years ago.
The star retains its elongated orbit from that cannibalism event.
Therefore, it's just passing through the solar neighborhood at a
rocket-like speed of 800,000 miles per hour. It takes just 1,500 years
to traverse a piece of sky with the angular width of the full Moon. The
star's proper motion angular rate is so fast (0.13 milliarcseconds an
hour) that Hubble could actually photograph its movement in a few
hours.
CONTACT
Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Barbara Kennedy
Pennsylvania State University, Space Park, Pa.
814-863-4682
science@psu.edu
Howard Bond
Space Telescope Science Institute, Baltimore, Md., and
Pennsylvania State University, Space Park, Pa.
410-561-0571
bond@stsci.edu
