The densely packed globular cluster
NGC 6325 glistens in this image from the NASA/ESA Hubble Space
Telescope. This concentrated group of stars lies around 26 000 light
years from Earth in the constellation Ophiuchus.
Globular clusters like NGC 6325 are tightly bound collections of stars with anywhere from tens of thousands to millions of members. They can be found in all types of galaxies,
and act as natural laboratories for astronomers studying star
formation. This is because the constituent stars of globular clusters
tend to form at roughly the same time and with similar initial
composition, meaning that astronomers can use them to fine-tune their
theories of how stars evolve.
Astronomers inspected this particular cluster not to understand star
formation, but to search for a hidden monster. Though it might look
peaceful, astronomers suspect this cluster could contain an
intermediate-mass black hole
that is subtly affecting the motion of surrounding stars. Previous
research found that the distribution of stars in some highly
concentrated globular clusters — those with stars packed relatively
tightly together — was slightly different from what astronomers
expected.
This discrepancy suggested that at least some of these densely packed
globular clusters — including perhaps NGC 6325 — could have a black
hole lurking at the centre. To explore this hypothesis further,
astronomers turned to Hubble’s Wide Field Camera 3 to observe a larger
sample of densely populated globular clusters, which included this
star-studded image of NGC 6325. Additional data from Hubble’s Advanced
Camera for Surveys were also incorporated into this image.
