A Nearby Star Cluster May Be Almost as Old as the Universe

A new method was used to estimate the age of the 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 soon after the Big Bang, or we’re underestimating how old the Universe is.

Globular clusters are, as the name suggests, near-spherical collections of tens of thousands of massive stars sitting outside galaxies. In general, they are very old – but some are older than others, and they are attracting more interest from astronomers as they give us an insight into the early days of the universe.

M92 is a favorite object for amateur astronomers in the northern hemisphere, easily visible in binoculars, but to professionals it is particularly significant as a candidate for the oldest globular cluster in our immediate area. In fact, as a forthcoming paper notes, M92 is often used as a benchmark against which we measure the newly forming galaxies seen immediately after the Big Bang.

However, M92’s true age is a matter of great debate. Estimates range from 11 billion to 14.8 billion years old. The final figure is ± 2.5 billion, which is also correct since our best estimate for the age of the universe is 13.78 billion years. A star cluster older than a billion years would be a shame, after all.

Even the more plausible estimates have wide error bars, so one team tried a new approach. Finding the age of M92 “helps us set a lower limit on the age of the universe,” study author Dartmouth College PhD student Martin Ying told Science News. “We didn’t expect M92 to be born before the universe, right?”

Previous estimates of M92’s age, like other old clusters, worked by counting stars at different stages of their life cycle. Globular clusters are believed to rapidly convert all of their original gas into stars, so stars are often born in a relatively tight window, although a secondary population exists. Stars with short to medium-length life cycles have finished fusing their primary hydrogen by now, so by studying which stars became 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 account the wide range of uncertainty in developing stellar models.” For all that we have learned about the life cycles of stars, there is still debate about how long something of a certain size and composition will survive.

To cope with this, the authors developed star cluster models using different stellar models, different nuclear reaction rates, helium abundances, convection and various factors thought to influence whether how fast are the stars. These were compared to observations of 18,000 M92 stars to find the closest match.

This yielded an age of 13.80 ± 0.75 billion years, about half the error of earlier methods. If the authors are correct, either the true value is at the lower end of this range, or there is a flaw in our estimates of the time since the Big Bang.

Much of the remaining error identified by the paper is provided by the uncertainty about M92’s distance from Earth, despite being calibrated against two Milky Way stars with nearly the same composition of heavier elements as M92. The distances of both stars have been accurately measured by the Hubble Space Telescope. The best estimate for M92’s distance is 27,700 light years, but small variations from this will affect our estimates for the luminosity and reddening of the inner stars.

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. However, building larger telescopes is faster.

The paper was published in the Astrophysical Journal, and a preprint is available at ArXiv.org.