Research Highlight

Secrets of X-ray stars in a distant galaxy unveiled

doi:10.1038/nindia.2014.96 Published online 17 July 2014

Researchers have gained new insights into globular clusters — dense spherical collections of stars — by studying low-mass X-ray-emitting binary stars in NGC 1399, an elliptical galaxy about 65 million light years from Earth1. These insights will help scientists better understand how such globular clusters contribute to the structure and evolution of distant galaxies like NGC 1399.

Globular clusters harbour X-ray binary stars, a special class of binary stars that consist of a normal star and a collapsed star, such as a white dwarf, a neutron star or a black hole. Such pairs of stars emit X-rays when the gravity of the collapsed star pulls material from the normal star. X-ray emissions from such stars allow astrophysicists to investigate the nature and evolution of stellar remnants and their host stellar systems.

However, observational studies of such X-ray star systems are rare, as they need lengthy observations. To probe such clusters of X-ray stars, the researchers analysed X-ray images of such star clusters in NGC 1399. These X-ray images were captured by two space telescopes: the Hubble Space Telescope and the Chandra X-ray Observatory.

The researchers found that NGC 1399 possesses a population of low-mass X-ray binaries with luminosities below the detection limit of individual sources. They identified that the faint globular cluster with a low-mass X-ray binary star population has similar properties to its bright counterparts.

They also found that the average X-ray luminosity of red globular clusters is about three times greater than that of blue globular clusters. As all low-mass X-ray binary stars have similar intrinsic average luminosity distributions, the researchers claim that red globular clusters are three times more likely to host faint low-mass X-ray binary stars than blue ones.


  1. D’Ago, G. et al. The luminosity function of low mass X-ray binaries in the globular cluster system of NGC 1399. AstronAstrophysics 567, A2 (2014) doi:10.1051/0004-6361/201322722