March 8, 2013
Hubble Helps Astronomers Refine The Age Of The Methuselah Star
April Flowers for redOrbit.com - Your Universe Online
The ink is drying on a brand new birth certificate for a star that's been around for a very long time. New calculations using NASA'S Hubble Space Telescope have enabled a group of astronomers to take an important step closer to reconciling the age of this star, and the age of the Universe.
"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's age could be as old as 14.5 billion years, plus or minus 0.8 billion years. This is a problem because current calculations place the age of the entire Universe at only 13.8 billion years. Earlier estimates from observations made in 2000 place the star's age at 16 billion years. This presented a problem for cosmologists. What do you do to reconcile such a dilemma? You go back to the numbers.
"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 age estimate from Hubble reduces the range of measurement uncertainty — that plus or minus 0.8 billion years — so that the star's age overlaps with the universe's age. This was independently determined by the rate of expansion of space, an analysis of the microwave background from the big bang, and measurements of radioactive decay.
HD 140283, also called the "Methuselah star," has been known for more than a century because of its fast motion across the sky, evidence that the star is simply a visitor to our stellar neighborhood. Methuselah's orbit carries it down through the plane of our galaxy from the ancient halo of stars encircling the Milky Way. It will eventually slingshot back to the galactic halo.
In the 1950s, astronomers were able to measure a deficiency of heavier elements in the star as compared to other stars in our galactic neighborhood. Stars from the halo are some of the first formed in our galaxy and represent a much older population than stars like our Sun that formed later in the disk. Stars formed in this very early universe lack the "pollution" of heavy elements forged inside stars through nucleosynthesis. Compared to Sol and other stars in our solar neighborhood, the Methuselah star has an anemic 1/250th as much of the heavy element content.
Methuselah is in the very first stages of expanding into a red giant, and can be seen with binoculars as a 7th-magnitude object in the constellation Libra.
Hubble observations were used to refine the distance to the star, which worked out to be 190.1 light-years. This measurement was made using trigonometric parallax, where an apparent shift in the position of a star is caused by a change in the observer's position.
By observing them from opposite points in Earth's orbit around the Sun, the parallax of nearby stars can be measured. Then, using straightforward triangulation, the star's true distance from Earth can be calculated. Once that variable is known, the star's exact value for intrinsic brightness can be calculated as well, a fundamental prerequisite to estimating a star's age.
Prior to this study, the European Space Agency's (ESA) Hipparcos satellite made a precise measurement of the star's parallax, but with an age measurement uncertainty of 2 billion years. The astronomers used one of Hubble's three Fine Guidance Sensors to measure the position of the star, arriving at nearly identical measurements as those from the Hipparcos. Hubble's precision, however, is five times better than that of Hipparcos, shrinking the uncertainty so that the age estimate was five times more precise.
With the star's brightness and distance known, Bond and his team applied contemporary theories concerning the star's burn rate, chemical abundances, and internal structure. New theories postulate that leftover helium diffuses deeper into the core and so the star has less hydrogen to burn via nuclear fusion, meaning that it uses fuel faster. This lowers the age estimate.
The age estimate is also lowered because the star has a higher than predicted oxygen-to-iron ratio. Bond hypothesizes that further measurements of oxygen ratios could lower the age even farther because the star would have formed at a time when the universe had a richer oxygen abundance.
"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."
More than likely, the Methuselah star was born in a primeval dwarf galaxy that was eventually gravitationally shredded and sucked in by the emerging Milky Way over 12 billion years ago. The star's elongated orbit is a holdover from that cannibalism event. With rocket-like speeds of 800,000 miles per hour, the star is just passing through the solar neighborhood.