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This ancient stellar jewelry box, a globular cluster called NGC 6397, glitters with the light from hundreds of thousands of stars. Astronomers used the Hubble Space Telescope to gauge the cluster4s distance at 7800 light-years away. NGC 6397 is one of the closest globular clusters to Earth. The cluster's blue stars are near the end of their lives. These stars have used up their hydrogen fuel that makes them shine. Now they are converting helium to energy in their cores, which fuses at a higher temperature and appears blue.
The reddish glow is from red giant stars that have consumed their hydrogen fuel and have expanded in size. The myriad small white objects include stars like our Sun.
When Eduardo Vitral and Gary A. Mamon of the Institut d'Astrophysique de Paris set out to study the core of NGC 6397, they expected to find evidence for an “intermediate-mass” black hole (IMBH). These are smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of massive stars. IMBH are the long-sought “missing link” in black hole evolution and their mere existence is hotly debated, although a few candidates have been found. Instead, the scientists found evidence of a concentration of smaller black holes lurking there. New data from the Hubble Space Telescope have led to the first measurement of the extent of a collection of black holes in a core-collapsed globular cluster.
This image is composed of a series of observations taken with Hubble's Advanced Camera for Surveys. The research team used Hubble's Wide Field Camera 3 to measure the distance to the cluster.
Image Credit: NASA, ESA, and T. Brown and S. Casertano (STScI)
Acknowledgement: NASA, ESA, and J. Anderson (STScI)
Image enhancement: Jean-Baptiste Faure
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