Astronomers have discovered the Methuselah of stars — a denizen of our Solar System's neighborhood that is at least 13.2 billion years old and formed shortly after the Big Bang.
“We believe this star is the oldest known in the universe with a well determined age,” says Howard Bond of Pennsylvania State University in University Park, who announced the finding 10 January at a meeting of the American Astronomical Society in Long Beach, California.
The venerable star, dubbed HD 140283, lies at a comparatively short distance of 186 light years from our Solar System and has been studied by astronomers for more than a century. Researchers have long known that the object consists almost entirely of hydrogen and helium, a hallmark of having formed early in the history of the universe, before successive generations of stars had a chance to forge heavier elements. But no one knew exactly how old it was.
Determining the star’s age required several steps. First, the team made a new and more accurate determination of the star’s distance, using 11 sets of observations recorded between 2003 and 2011 with the Hubble Space Telescope’s fine guidance sensors, which measure the position of target stars relative to reference stars. Having measured the distance and the brightness of the star as it appears on the sky, the astronomers were able to calculate the star’s intrinsic luminosity with unusual precision.
The team then exploited the fact that HD 140283 has advanced to a phase in which it is exhausting the hydrogen at its core. In that phase, the star's slowly dimming luminosity is a highly sensitive indicator of its age, says Bond. His team calculates that the star is 13.9 billion years old, give or take 700 million years. Within the experimental error bars, the age does not conflict with the age of the universe, 13.77 billion years.
The star's age is therefore at least 13.2 billion years — which was the age of another known Methuselah — and possibly somewhat older. Its age is also known with considerably better confidence than in that previous case, Bond says.
The discovery does, however, place constraints on early star formation, says Volker Bromm of the University of Texas in Austin, who was not part of the study. The very first generation of stars coalesced from primordial gas, which did not contain appreciable amounts of elements heavier than helium, he notes. That means that as old as HD 140283 is, its chemical composition — a low but nonzero abundance of heavy elements — shows that the star must have formed after the first stellar generation.
Conditions for making the second generation of stars, then, “must have been in place very early,” says Bromm. In the standard scenario, the very first stars coalesced a few hundred million years after the Big Bang, he notes. Massive and short lived, the first-generation stars died in supernova explosions that heated surrounding gas and seeded it with heavier elements.
But before the second generation of stars could form, that gas had to cool down. The early age of the second-generation star HD 140283 hints that the cooling time, or delay, between the first and second generations might have been extremely short, perhaps only a few tens of million years, Bromm notes.