NGC 6397
NGC 6397 (also known as Caldwell 86) is a globular cluster in the constellation Ara. It is located about 7,800 light-years from Earth, making it one of the two nearest globular clusters to Earth (the other one being Messier 4). The cluster contains around 400,000 stars, and can be seen with the naked eye under good observing conditions. NGC 6397 is one of at least 20 globular clusters of the Milky Way Galaxy that have undergone a core collapse, meaning that the core has contracted to a very dense stellar agglomeration. Astronomical research. Estimating the age of  the Milky Way. In 2004, a team of astronomers focused on the cluster to estimate the age of the Milky Way Galaxy. The team consisted of Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. They used the UV-Visual Echelle Spectrograph of the Very Large Telescope at Cerro Paranal to measure the beryllium content of two stars in the cluster. This allowed them to deduce the time elapsed between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster. This, added to the estimated age  of the stars in the cluster, gives an estimate for the age for the Galaxy: about 13.6 billion years, which is nearly as old as the universe itself. This estimate assumes that the NGC 6397 is not older than the Milky Way. Lower mass limit for stars. In 2006, a study of NGC 6397 using the Hubble Space Telescope was published that showed a clear lower limit in the brightness of the cluster's population of faint stars. The authors deduce that this indicates a lower limit for the mass necessary for stars to develop a core capable of fusion : roughly 0.083 times the mass of the Sun. Black holes. In February 2021, the core of NGC 6397 was reported to contain a relatively dense concentration of compact objects (white dwarfs, neutron stars and black holes), based on the movement of stars near the core derived from data gathered by the Hubble Space Telescope and the European Space Agency's Gaia spacecraft. The total mass of the black holes in this central region has an upper limit of the order of 1,000 solar masses, from many smaller black holes with an estimated average size of 20 solar masses. However, it  is also argued that many of these black holes would have been ejected from the cluster's core due to dynamical interactions, which leads to this invisible concentration being formed of mainly white dwarfs and neutron stars, with only very few black holes.