Globular Clusters Are Less Evolved than Astronomers Thought

Some of the oldest structures in the Milky Way are the globular clusters. Ancient collections of millions of stars, that have held together by mutual gravity over billions of years. But new data collected by NASA’s Chandra X-Ray Observatory casts doubt on their “ancient nature”. They might be surprisingly less mature than astronomers previously believed.

According to conventional wisdom, globular clusters pass through three phases of evolution in the development of their structure: adolescence, middle age, and old age. Keep in mind, we’re talking about the age of the cluster here, not the age of the individual stars in the cluster.

One way to calculate the age of a cluster is to look for the presence of binary X-ray sources. These happen when two stars get so close to one another that they begin to transfer mass. The transfered material piles up into an accretion disk around one star, which can blaze brightly in the X-ray spectrum. Globular clusters should form these X-ray binaries in their middle age, and then lose them again as they reach old age.

Recent images from NASA’s Chandra X-Ray Observatory revealed the number of bright X-ray sources in two globular clusters: NGC 6397 and NGC 6121. While they were expecting to see less double stars in NGC 6397, it was just the opposite.

Instead of most globular clusters being in their middle ages, astronomers are starting to think that many are in an adolescent stage of evolution. When astronomers surveyed 13 globular clusters, 10 were in adolescence and only 3 were middle aged.

With so many clusters in the earlier stags of their evolution, the later stages must take much longer to reach than astronomers previously believed. Even though the clusters are already billions of years old, they’ve barely reached their prime.

Original Source: Chandra News Release