Chandra Release - April 28, 2008

Visual Description: NGC 6121 and NGC 6397

The X-ray images of the globular clusters NGC 6397 and NGC 6121 are shown. The image consists of two panels, with NGC 6397 at left and NGC 6121 at right. In both panels, the background is dark, and a few sources are scattered within the cluster, appearing as small, bright points of light in yellow, white and light blue. In the first panel, the stars are concentrated in the center of the image, with some spreading out towards the edges.  In the second panel, the stars appear much less concentrated, creating a sparse, slightly asymmetrical distribution. Globular clusters are dense bunches of up to millions of stars that are among the oldest known objects in the Universe.  The X-ray sources in these two globular clusters are a product of their environment. The centers of globular clusters are so crowded that single and double stars can interact, resulting in the formation of double stars that transfer mass and give off X-rays. Since such double stars are expected to mostly be formed in the middle of a globular cluster’s evolution and then lost when the cluster gets older, the relative number of X-ray sources gives clues about the stage of evolution the cluster is in. The number of double stars - and bright X-ray sources - in a globular cluster also depends on the encounter rate 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.