"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, an age range that
presented a potential dilemma for cosmologists. 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.
"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 H D 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.
Because the aging star is relatively nearby, familiar stars and
constellations as seen from Earth are in the sky, but in different
locations. At upper left is the constellation Orion, which looks
distorted from our new perspective in space. Just to the upper left of
the foreground star is the Pleiades cluster. To the lower left of the cluster, our Sun has dimmed to an apparent magnitude of +7, placing it below naked-eye visibility.
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 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 that than 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."
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 literally a few hours.
Journal reference: Astrophysical Journal Letters
Source: The Daily Galaxy via NASA's Goddard Space Flight Center
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