Messier 72 - the NGC 6981 Globular Cluster

Description:

"Galactic clusters have long been the stellar systems most suitable for testing the predictions of stellar evolution theories over the main evolutionary phases of both H- and He-burning structures. In more recent times, the color-magnitude diagrams (CMDs) of Galactic globular clusters (GCs) have become of critical importance in elucidating the early phases of galactic evolution, as well as providing severe constraints to the age of the universe. This has stimulated much effort on both the theoretical and observational fronts. During each night many standard stars were observed, in order to determine color equations, extinction, and zero points. Reductions for the primary and local standards followed exactly the same procedures, to mitigate the introduction of systematic errors. As part of a program studying the RR Lyrae variables in selected Galactic GC's, the clusters NGC 6981 (M72) and NGC 1851 were also observed as part of the same program, and similar observing and reduction techniques were followed for each."

"We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time."

"Globular clusters are effectively approximated by using a King-Michie distribution. Sherbakov proposed that identifying the possible amount of dark matter present in globular clusters would improve understanding of their behavior. This thesis examines the properties of one-component and two-component models in order to maximize the percentage of dark matter. We find that we can obtain excellent density fits for some globular clusters (e.g., NGC 288 and NGC 6981) with up to 95% of the matter being dark."

History of Observation:

"Nebula seen by M. Mechain in the night of August 29-30, 1780, above the neck of Capricorn. M. Messier looked for it on the 4th and 5th October following: the light is as faint as for the preceding; near it is a small telescopic star: the position was determined from the star Nu Aquarii, of fifth magnitude.' (diam 2')".

Locating Messier 72: