Credit: NASA/CXC/Northwestern Univ/J.Fregau

Globular clusters are spectacular spherical agglomerations of stars. A large globular cluster is over 100 light years across, and contains hundreds of thousands of stars. The globular clusters are believed to be among the oldest known objects in the Universe. However, new data from the NASA’s Chandra X-ray Observatory (CXO) show that the globular clusters are probably less evolved than previously thought.  

Theories demonstrate that globular clusters pass three evolutionary phases: adolescence, middle age, and old age. These "ages" correspond to the evolutionary state of the cluster, not the physical ages of the individual stars.

CXO, which explores the Universe in X-rays from Earth orbit, recently imaged two globular clusters, NGC 6397 and NGC 6121, located in our Galaxy, the Milky Way. These images show the centers of the two clusters. They are part of a new study that argues that globular clusters might be surprisingly less mature in their development than previously thought.

The X-ray sources in these two globular clusters are a product of their environment. The central regions of globular clusters are so densely populated by stars that single and double stars can interact, resulting in the formation of double stars that transfer material and radiate X-rays. Since such binary stars are predicted to mostly be formed in the middle of the lifetime of a globular cluster and then disappear during more advanced evolutionary stages, the relative number of X-ray sources hints the stage of evolution of the cluster.

The number of double stars, and bright X-ray sources, in a globular cluster also depends on the rate of stellar encounters near the center of the cluster. NGC 6397 has a lower encounter rate than NGC 6121, so it should have fewer X-ray sources, assuming that it is in the same stage of evolution. Instead, NGC 6397 has a clear surplus of X-ray sources, suggesting that it is in a more advanced stage of evolution (middle age) than NGC 6121, which appears to still be in adolescence.

The conclusion of this new study is that most globular clusters may be in the adolescent stage of their evolution, rather than middle age as previously thought. Also, the few globular clusters, like NGC 6397, previously believed to be in old age may actually only be in middle age. Thirteen globular clusters were included in the study; ten of these clusters appear to be in adolescence and three appear to be middle-aged.

CXO is ideal for studying double stars in globular clusters because its superb spatial resolution and sensitivity allow it to resolve large numbers of X-ray sources. Optical observations are not as informative for accurate censuses of double stars in globular clusters because tens or hundreds of thousands of bright single stars are also detected in optical images, making the identification of double stars in close orbits less precise.

About 150 globular clusters belong to our Galaxy. They are located in the Galaxy’s vast stellar halo, and orbit the Milky Way’s center in immense orbits. NGC 6121 and NGC 6397 are estimated to be over 7,000 light years distant.

Further Reading

Oldest Known Objects May Be Surprisingly Immature
http://chandra.harvard.edu/press/08_releases/press_042808.html

Aymen Mohamed Ibrahem  
Senior Astronomy Specialist