This Digitized Sky Survey image shows
the oldest star with a well-determined age in our galaxy. Called
the Methuselah star, HD 140283 is 190.1 light-years away. Astronomers refined
the star's age to about 14.5 billion years (which is older than
the universe), plus or minus 800 million years. Image released March 7, 2013
.
For more than 100 years, astronomers have been observing a curious star located some 190 light years away from Earth in
the constellation Libra. It rapidly journeys across
the sky at 800,000 mph (1.3 million kilometers per hour). But more interesting than that, HD 140283 — or Methuselah as it's commonly known — is also one of
the universe's oldest known stars.
In 2000, scientists sought to date
the star using observations via
the European Space Agency's (ESA) Hipparcos satellite, which estimated an age of 16 billion years old. Such a figure was rather mind-blowing and also pretty baffling. As astronomer Howard Bond of Pennsylvania State University pointed out,
the age of
the universe — determined from observations of
the cosmic microwave background — is 13.8 billion years old. "It was a serious discrepancy," he said.
Related: The Methuselah Star: Oldest Known
Star Revealed (Gallery)
Taken at face value,
the star's predicted age raised a major problem.
How could a star be older than
the universe? Or, conversely, how could
the universe be younger? It was certainly clear that Methuselah — named in reference to a biblical patriarch who is said to have died aged 969, making him
the longest lived of all
the figures in
the Bible — was old, since
the m*etal-poor subgiant is predominantly made of hydrogen and helium and contains very little iron. It's composition meant
the star must have come into being before iron became commonplace.
But more than two billion years older than its environment? Surely that is just not possible.
CLOSE
Taking a closer look at
the age of Methuselah
Bond and his colleagues set themselves to
the task of figuring out whether or not that initial figure of 16 billion was accurate. They pored over 11 sets of observations that had been recorded between 2003 and 2011 by
the Fine Guidance Sensors of
the Hubble Space Telescope, which make a note of
the positions, distances and energy output of stars. In acquiring parallax, spectroscopy and photometry measurements, a better sense of age could be determined.
"One of
the uncertainties with
the age of HD 140283 was
the precise distance of
the star," Bond told All About Space. "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."
There were also uncertainties in
the theoretical modelling of
the stars, such as
the exact rates of nuclear rea*ctions in
the core and
the importance of elements diffusing downwards in
the outer layers, he said. 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.
This is a backyard view of
the sky surrounding
the ancient star, cataloged as HD 140283, which lies 190.1 light-years from Earth. The star is
the oldest known to astronomers to date. Image released March 7, 2013. (Image credit: A. Fujii and Z. Levay (STScI))
"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.
Bond and his collaborators estimated HD 140283's age to be 14.46 billion years — a significant reduction on
the 16 billion previously claimed. That was, however, still more than
the age of
the universe itself, 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, even though it wasn't entirely perfect.
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"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," Matthews said. "In other words,
the best supported age of
the star is in conflict 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 updated
the star's age to 14.27 billion years. "The conclusion reached was that
the age is about 14 billion years and, again, if one includes all sources of uncertainty — both in
the observational measurements and
the theoretical modelling —
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.
Scientists have been keen to discover when
the universe began — that is, when
the Big Bang occurred and left its imprint on
the fabric of
the cosmos. (Image credit: NASA)
Taking a closer look at
the age of
the universe
For Bond,
the similarities between
the age of
the universe and that of this old nearby star — both of which have been determined by different m*ethods of analysis — is "an amazing scientific achievement which provides very strong evidence for
the Big Bang picture of
the universe". He said
the problem with
the age of
the oldest stars is far less severe than it was in
the 1990s when
the stellar ages were approaching 18 billion years or, in one case, 20 billion years. "With
the uncertainties of
the determinations,
the ages are now agreeing," Bond said.
Yet Matthews believes
the problem has not yet been resolved. Astronomers at an international conference of top cosmologists at
the Kavli Institute for Theoretical Physics in Santa Barbara, California, in July 2019 were puzzling over studies that suggested different ages for
the universe. They were looking at measurements of galaxies that are relatively nearby which suggest
the universe is younger by hundreds of millions of years compared to
the age determined by
the cosmic microwave background.
Related: Big Bang to Civilization: 10 Amazing Origin Events
In fact, far from being 13.8 billion years old, as estimated by
the European Planck space telescope's detailed measurements of cosmic radiation in 2013,
the universe may be as young as 11.4 billion years. One of those behind
the studies is Nobel laureate Adam Riess of
the Space Telescope Science Institute in Baltimore, Maryland.
The conclusions are b*ased on
the idea of an expanding universe, as shown in 1929 by Edwin Hubble. This is fundamental to
the Big Bang —
the understanding that there was once a state of hot denseness that exploded out, stretching space. It indicates a starting point that should be measurable, but fresh findings are suggesting that
the expansion rate is actually around 10% higher than
the one suggested by Planck.
Indeed,
the Planck team determined that
the expansion rate was 67.4 km per second per megaparsec, but more recent measurements taken of
the expansion rate of
the universe point to values of 73 or 74. That means there is a difference between
the measurement of how fast
the universe is expanding today and
the predictions of how fast it should be expanding b*ased on
the physics of
the early universe, Riess said. It's leading to a reassessment of accepted theories while also showing there is still much to learn about dark matter and dark energy, which are thought to be behind this conundrum.
Related: The 11 Biggest Unanswered Questions About Dark Matter
A higher value for
the Hubble Constant indicates a shorter age for
the universe. A constant of 67.74 km per second per megaparsec would lead to an age of 13.8 billion years, whereas one of 73, or even as high as 77 as some studies have shown, would indicate a universe age no greater than 12.7 billion years. It's a mismatch that suggests, once again, that HD 140283 is older than
the universe. It has also since been superseded by a 2019 study published in
the journal Science that proposed a Hubble Constant of 82.4 — suggesting that
the universe's age is only 11.4 billion years.
Matthews believes
the answers lie in greater cosmological refinement. "I suspect that
the observational cosmologists have missed something that creates this paradox, rather than
the stellar astrophysicists," he said, pointing to
the measurements of
the stars being perhaps more accurate. "That's not because
the cosmologists are in any way sloppier, but because age determination of
the universe is subject to more and arguably trickier observational and theoretical uncertainties than that of stars."
Nebula and stars in deep space. (Image credit: Vadim Sadovski/Shutterstock)
So, how will scientists figure this out?
What could be making
the universe potentially appear younger than this particular star?
"There are two options, and
the history of science suggests that in such cases
the reality is a mix of both," Matthews said. "In this case that would be sources of observational error that haven't been fully understood, plus some gaps in
the theory of
the dynamics of
the universe, such as
the strength of dark energy, which has been
the prime driver of
the cosmic expansion for many billions of years now."
Related: Dark Matter and Dark Energy: The Mystery Explained (Infographic)
He suggests
the possibility that
the current "age paradox" reflects time variation in dark energy, and thus a change in
the rate of acceleration — a possibility theorists have found might be compatible with ideas about
the fundamental nature of gravity, such as so-called causal set theory. New research into gravitational waves could help to resolve
the paradox, Matthews said.
To do this, scientists would look at
the ripples in
the fabric of space and time created by pairs of dead stars, rather than relying on
the cosmic microwave background or
the monitoring of nearby objects such as Cepheid variables and supernovae to measure
the Hubble Constant —
the former resulting in
the speed of 67 km per second per megaparsec and
the latter in 73.
Trouble is, measuring gravitational waves is no easy task, given they were only directly detected for
the first time in 2015. But according to Stephen Feeney, an astrophysicist at
the Flatiron Institute in New York, a breakthrough could be made over
the course of
the next decade. The idea is to collect data from collisions between pairs of neutron stars using
the visible light these events emit to figure out
the speed they are moving relative to Earth. It also entails analyzing
the resulting gravitational waves for an idea of distance — both of which can combine to give a measurement of
the Hubble Constant that should be
the most accurate yet.
The mystery of
the age of HD 140283 is leading to something bigger and more scientifically complex, altering
the understanding of how
the universe works.
"The most likely explanations for
the paradox are some overlooked observational effect and/or something big missing from our understanding of
the dynamics of
the cosmic expansion," Matthews said. Precisely what that "something" is, is sure to keep astronomers challenged for some time.