One of the oldest known objects in the universe wanders through the Milky Way.
The star cluster M92, a tightly packed ball of stars about 27,000 light-years from Earth, is about 13.8 billion years old, researchers report in a paper submitted June 3 to arXiv.org. A newly refined age estimate makes this cluster of stars nearly the same age as the Universe.
Clarifying the age of clusters such as M92 can help place constraints on the age of the Universe itself. It may also help solve cosmic puzzles about how the universe evolved.
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The age is “on the edge of the age of the universe as estimated by other groups,” says astronomer Martin Yin of Dartmouth College. “This helps us put a lower limit on the age of the universe. We don’t expect M92 to be born before the universe, do we?’
Globular clusters like M92 are tight knots of stars that are thought to have formed at the same time. This makes it easier for astronomers to measure the ages of stars. Stars that are born with different masses have different fates: the big ones quickly use up their fuel and die young, while the small ones are delayed. Figuring out how many stars in a cluster have aged over the years of burning fuel gives an idea of when the entire cluster was born.
But these estimates are based on assumptions about how stellar evolution works. Ying and his colleagues wanted to find a measure of age that circumvented these assumptions.
Using a computer, the team created 20,000 synthetic stellar populations for M92, each for a different possible age of the cluster. They then compared the colors and luminosities for each of these populations to Hubble Space Telescope observations of M92 and calculated the age that best matches the collection.
This is not the first time astronomers have measured the age of M92, but previous estimates relied on just one synthetic collection of stars. Comparing thousands of them reduced the uncertainty introduced by the assumptions built into each one. The new technique reduced uncertainty in the cluster’s age by about 50 percent, Ying says. The team found that the cluster is 13.8 billion years old, plus or minus 750 million years. This is strikingly close to the best estimate of the age of the universe: a little over 13.8 billion years, plus or minus 24 million years, according to measurements of the first light emitted after the Big Bang by the Planck satellite.
The age of clusters like M92 is important in part because of the growing tension about how fast the universe is growing. Since the 1990s, astronomers have known that the universe is expanding at an increasing rate thanks to a mysterious substance called dark energy. But recent measurements of the speed of this expansion, a number called the Hubble constant, don’t match
One way around this tension is to accept a different age of the universe, says cosmologist and study co-author Mike Boylan-Kolchin of the University of Texas at Austin.
“We often think of it as if Moses came down from Mount Sinai with ‘13.8 billion years’ written on the tablets, but that’s not quite the case,” he says. “If one takes the Hubble tension seriously, one must also say that we don’t know the age of the universe very well.”
This is where the M92 comes in. Before spacecraft measured the earliest luminosities of the cosmos, the age of globular clusters was the best way to place a limit on the age of the universe. That practice fell out of fashion for a while, says cosmologist Wendy Friedman of the University of Chicago, who was not involved in the new work.
But improvements in calculations, theory, and measurements of distances to clusters like M92 are giving it another try.
“The Hubble voltage itself is a really tough nut to crack,” Friedman says. By itself, this measurement is not precise enough to resolve the dispute. But “the more restrictions we have, the better,” she says. “It shows the way to the future.”
