Astronomers appear closer to solving a celestial puzzle with the help of multiple space telescopes and some dogged data gathering.
The enigma involves an aging star, Methuselah, dated at 14.5 billion years old, plus or minus 800 million years.
Methuselah, oldest known star. Image Credit/Digitized Sky Survey
At that age, Methuselah potentially out dates the best estimates from experts for the age of the universe, 13.8 billion years.
The tale is all the more intriguing because this budding red giant is transiting the Milky Way on a fast track. An apparent refugee of a primeval dwarf galaxy that was ripped apart by gravitational forces, Methuselah seems to have been sucked into an emerging Milky Way galaxy over 12 billion years ago.
“We have found that this is the oldest known star with a well-determined age,” said Howard Bond of Pennsylvania State University and the Space Telescope Science Institute in Baltimore, Md. Efforts by Bond’s team, assisted by the Hubble Space Telescope and the European Space Agency’s Hipparcos satellite, have helped to lower earlier estimates of 16 billion years for Methuselah’s age that date back to 2000.
Spacewalking shuttle astronauts upgrade Hubble Space Telescope. Photo Credit/NASA Photooto
“Maybe the cosmology is wrong, stellar physics is wrong, or the star’s distance is wrong,” said Bond, who explained the issues his team faced as it addressed the age dilemma. The work is outlined in the Feb. 13 issue of the Astrophysical Journal Letters.
At 14.5 billion years, with the acknowledged 800 million year uncertainty, Methuselah and a 13.8 billion year old universe now appear compatible -if just barely.
Bonds team used Hubble age estimates to reduce the range of measurement uncertainty, so that the Methuselah’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.
Methuselah’s tenure is bolstered by the star’s lack of heavy elements — the chemical elements synthesized by the fusion processes of the earliest stars, which had only hydrogen, the lightest element, and some slightly heavier helium as fuels.
Hubble’s observational powers were employed to refine distance estimates to the star, now set at a pretty close 190.1 light-years. With observations at different points in the observatory’s orbit around the Earth, astronomers were able to use trigonometric parallax to achieve a five-fold improvement in their distance calculations, an important step in establishing a value for Methuselah’s intrinsic brightness. Intrinsic brightness is a pre-requisite for assigning age.
Previously, astronomers were working with distance estimates made with observations by Hipparcos to achieve an age estimate with 2 billion year uncertainty. Hubble’s more precise optics actually verified the distance observations made with Hipparcos but added the increased precision.
Next, Bond’s group re-assessed the star’s fusion or burn rate, chemical abundances and internal structure in line with emerging theories. Those efforts suggested Methuselah is burning fuel at a rate fast enough to lower the star’s age.
Those re-calculations were found to be in line with a higher than predicted oxygen-to-iron ratio that also justified a younger age for the senior star. The higher oxygen abundance would suggest Methuselah formed when the universe was mature enough for earlier stars to forge an abundance of the heavier element.
More refinements of the oxygen level could lower Methuselah’s age even more.
“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,” explained Bond.