Methuselah: The oldest star in the universe

Scientists tried to determine the age of what they believed to be the universe's oldest star in 2000. They obtained data from the Hipparcos satellite of the European Space Agency (ESA)(opens in new tab) and determined that HD140283, or Methuselah as it is more generally known, was a startling 16 billion years old.

Such a number seemed somewhat perplexing. How can a star be older than the cosmos when the age of the universe, as measured by studies of the cosmic microwave background, is 13.8 billion years old?

According to astronomer Howard Bond of Pennsylvania State University, "It was a substantial discrepancy." Bond and his colleagues decided to find out the truth and check the figure's correctness in light of this. Their findings were equally astounding.

The oldest star in the universe is HD140283 — or Methuselah as it's commonly known. This Digitized Sky Survey image shows Methuselah star, located 190.1 light-years away. Astronomers refined the star's age to about 14.3 billion years (which is older than the universe), plus or minus 800 million years. Image released March 7, 2013. (Image credit: Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO)

Methuselah, named after a biblical patriarch who is claimed to have lived to the age of 969 and hence was the longest-living person in the Bible, has been the subject of astronomical observation for more than a century. The strange star is about 190 light-years from Earth in the constellation Libra, and it zips through the sky at an incredible 800,000 mph (1.3 million kilometers per hour).

It was clear that the star was old. The metal-poor subgiant is predominantly made of hydrogen and helium and contains very little iron. Such composition meant the star must have come into being when helium and hydrogen dominated the universe and before iron became commonplace (the heavier elements only appeared when massive stars created them in their cores) 

But could Methuselah really be more than two billion years older than its environment? Surely that is just not possible. Either the star was older than the universe or the universe was not as "young" as scientists thought it to be. Or maybe the dating was simply all wrong. What was it to be?

This is a backyard view of the sky surrounding the ancient Methuselah star, cataloged as HD 140283. Image released March 7, 2013. (Image credit: A. Fujii and Z. Levay (STScI))

 A mystery of this magnitude could not be ignored so Bond and his colleagues attempted to unearth the truth by pouring over 11 sets of observations that had been recorded between 2003 and 2011. 

These observations had been made by the Fine Guidance Sensors of the Hubble Space Telescope, which noted the positions, distances and energy output of stars. In acquiring parallax, spectroscopy and photometry measurements, the scientists could determine a better sense of age.

"One of the uncertainties with the age of HD 140283 was the precise distance of the star," Bond said. "It was important to get this right because we can better determine its luminosity and, from that, its age — the brighter the intrinsic luminosity, the younger the star.

"We were looking for the parallax effect, which meant we were viewing the star six months apart to look for the shift in its position due to the orbital motion of the Earth, which tells us the distance."

Bond adds that there were also uncertainties in the theoretical modeling of the stars, such as the exact rates of nuclear reactions in the core and the importance of elements diffusing downwards in the outer layers. So they worked on the idea that leftover helium diffuses deeper into the core, leaving less hydrogen to burn via nuclear fusion. With fuel used faster, the age is lowered.

"Another factor that was important was, of all things, the amount of oxygen in the star," Bond said. HD 140283 had a higher than predicted oxygen-to-iron ratio and, since oxygen was not abundant in the universe for a few million years, it pointed again to a lower age for the star. 

As a result of all of this work, Bond and his collaborators estimated HD 140283's age to be 14.46 billion years. It was a significant reduction on the 16 billion previously claimed but it was still more than the age of the universe itself.

In that sense, it didn't clear up the mystery and, on the face of it, simply ensured Methuselah remained a curiosity. But the scientists posed a residual uncertainty of 800 million years, which Bond said made the star's age compatible with the age of the universe. It was a major breakthrough. 

"Like all measured estimates, it is subject to both random and systematic error," said physicist Robert Matthews of Aston University in Birmingham, UK, who was not involved in the study. "The overlap in the error bars gives some indication of the probability of a clash with cosmological age determinations" 

"In other words, the best-supported age of the star conflicts with that for the derived age of the universe [as determined by the cosmic microwave background], and the conflict can only be resolved by pushing the error bars to their extreme limits."

Further refinements saw the age of HD 140283 fall a bit more. A 2014 follow-up study, for instance, updated the star's age to 14.27 billion years. "Again, if one includes all sources of uncertainty — both in the observational measurements and the theoretical modeling — the error is about 700 or 800 million years, so there is no conflict because 13.8 billion years lies within the star's error bar," Bond said.

Additionally, in May 2021, a different team of astronomers updated the best hypotheses for the age and mass of Methuselah, and after modelling the evolution of stars, they determined that it is 12 billion years old. Even though the sun, at 4.6 billion years old, is still a baby compared to HD 140283, this nevertheless places the star's age well within the universe's age range. Or does it?

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