NGC 6397

NGC 6397 is a globular cluster in Ara which can be seen as a hazy disc in a small telescope. It has a visual magnitude of 5.9 and an apparent diameter of 26 arc minutes.

At 7,200 light years away, this conspicuous globular is one of the two nearest to us (the other is Messier 4, in Scorpius). Currently it seems that M 4 is a bit closer, at about 6,800 light years, but the uncertainty is large enough that the sequence may change.

NGC 6397 is one of at least 20 globulars of our Milky Way Galaxy which have undergone a core collapse, meaning that its core has contracted to a very dense stellar agglomeration; this is the nearest such globular. The cluster contains around 400,000 stars.

In 2004, a team of astronomers focused on NGC 6397 to estimate the age of the Milky Way. They deduced an age for the Galaxy of about 13.6 billion years, which is nearly as old as the universe itself. In 2006, a study of NGC 6397 using the Hubble Space Telescope showed a clear lower limit in the cluster's population of faint stars. The authors deduced that this indicated the lower mass limit for stars capable of fusion, at a value of approximately 0.083 solar masses.