Globular clusters are spherical associations of old stars, thought to have formed during the earliest days of our universe, nearly 14 billion years ago. They contain some of the oldest stars in our galaxy, and the same appears true for other galaxies as well. But new research led by astronomers at the University of Warwick is now challenging this belief, showing that globular clusters may be a full 4 billion years younger than previously thought. Published May 24 in Monthly Notices of the Royal Astronomical Society , the paper states that globular clusters may be closer to 9 billion years in age than the previously measured value of 13 billion years.How did this age discrepancy arise? The age of globular clusters has long been determined by studying the light from their stars. Astronomers compare the properties of the integrated (total) light received from globular clusters with templates of starlight produced by stars of different ages and types. (It’s worth noting that at one point, these models showed that globular clusters were older than the universe, so this wouldn’t be the first time astronomers have updated their age estimates for these objects.)This new study takes a different approach, using new models called Binary Population and Spectral Synthesis (BPASS) models, which take into account the effects of binary stars and how the evolution of binary systems — which are known to be common — affects the light received from globular clusters. The idea is this: interactions between binary stars can change the properties of the starlight emitted by one star as elements from its atmosphere are stripped away by the gravity of a companion. When taken into account these binary interactions could make the starlight coming from globular clusters look more like the older templates used by astronomers to determine stellar age, prematurely aging the cluster.