A new technique has been used to estimate the age of globular star cluster M92 at 13.8 billion years, plus or minus 0.75 billion years. This is an almost exact match for the estimated age of the Universe, albeit with wider error bars. Either this nearby collection of stars formed as soon as possible after the Big Bang, or we’ve been underestimating how old the Universe really is.
Globular clusters are, as the name suggests, near-spherical collections of tens of thousands of tightly packed stars sitting outside galaxies. In general, they are very old – but some are considerably older than others, and these attract the most interest from astronomers as they provide us with insight into the universe’s earliest days.
M92 is a favorite object for northern hemisphere amateur astronomers, being easily visible in binoculars, but to professionals it’s particularly significant as a candidate for the oldest globular cluster in our near vicinity. Indeed, as a forthcoming paper notes, M92 is often used as a benchmark against which we measure newly forming galaxies seen soon after the Big Bang.
Nevertheless, M92’s true age has been a matter of considerable debate. Estimates have ranged from 11 billion to 14.8 billion years old. The latter figure is ± 2.5 billion, which is just as well because our best estimate for the universe's age is 13.78 billion years. A star cluster a billion years older would be embarrassing, to say the least.
Even the more plausible estimates have quite wide error bars, so a team tried a new approach. Finding M92’s age “Helps us set the lower bound of the age of the universe,” study author, Dartmouth College PhD student Martin Ying, told Science News. “We don’t expect M92 to be born before the universe, right?”
Previous estimates of the age of M92, like with other old clusters, worked by counting stars at different stages of their life cycle. Globular clusters are believed to turn all their original gas into stars quite quickly, so the stars are mostly born in a relatively tight window, although secondary populations do exist. Stars with short to medium-length life cycles have finished fusing their core hydrogen by now, so by studying which stars have turned into red giants we can estimate their ages.
However, the authors of the new paper point out; “Most previous studies […] are limited in that they do not take into consideration the wide range of uncertainty in constructing stellar models.” For all we have learned about stars’ life cycles, there’s still debate about how long an object of specific size and composition will live.
To get around this, the authors generated model star clusters using a variety of stellar models, varying nuclear reaction rates, helium abundance, convection and a variety of other factors thought to influence how fast stars evolve. These were compared with observations of 18,000 of M92’s stars to find the closest matches.
This produced an age of 13.80 ± 0.75 billion years, around half the error of previous methods. If the authors are right, either the true value is at the lower end of this range, or there is a flaw in our estimates of time since the Big Bang.
Most of the remaining error the paper acknowledges is provided by uncertainty about M92’s distance from Earth, despite calibration against two Milky Way stars that have an almost identical composition of heavier elements as M92. Both stars’ distances have been precisely measured by the Hubble Space Telescope. The best estimate for M92’s distance is 27,700 light years, but small variations from this would affect our estimates for the brightness and reddening of the stars within.
M92 is approaching us at more than 100 kilometers per second (62 miles per second), so if we wait a few million years, we’ll get a better view. Building bigger telescopes is faster, however.