Credit: Jim Fordice
Astronomical and Imaging Data
| RA: | 18h 31m 54.10s |
| DEC: | -23° 28′ 30.7″ |
| MAG: | 9.13 |
| Diameter: | 5.8′ |
| Const: | Sgr |
| OTA | Planewave CDK 24 |
| Focal Length | 3962 |
| Camera | QHY 600M |
| Site | Rio Hurtado, Chile |
| Sky Quality | Bortle 1 |
Useful Informations
NGC 6642 is a highly evolved, compact globular cluster located deep within the Galactic bulge of the Milky Way, approximately 26,700 light-years from Earth in the constellation Sagittarius .
Dynamical and Evolutionary Status
NGC 6642 is a prime example of a globular cluster that has undergone significant dynamical evolution due to the harsh tidal forces in the inner region of the Galaxy.
- Core-Collapsed: The cluster is identified as a core-collapsed candidate, meaning its central region is extremely dense and appears to lack a distinct core structure when observed with high resolution. Its current location is very close to its perigalactic distance (closest approach to the Galactic center), which contributes to this high degree of dynamical processing.
- Depleted Mass Function: Studies of the cluster’s stellar population show a pronounced depletion of low-mass stars. The mass function is inverted in the cluster’s core, meaning there is an increase in the relative number of stars as mass increases. This is a tell-tale sign of mass segregation and significant stellar evaporation (stars being stripped away) caused by strong tidal forces over the cluster’s lifetime.
- Blue Stragglers: The core is home to numerous blue straggler stars, which are stars that appear “younger” or lag behind in their aging rate compared to other cluster members. These are often thought to be the result of stellar collisions or mass transfer between binary stars, which is more likely in the dense environment of a core-collapsed cluster.
Stellar Population and Age
NGC 6642 is a very old globular cluster, with an age estimated to be on the order of ∼13 billion years (based on a logarithmic age of log(Age/yr) ≈ 10.14).
- Metallicity: It is a metal-poor cluster, with a metallicity of [Fe/H] ≈ −1.26 to −1.80 dex. This value is typical of the older stellar populations that formed early in the galaxy’s history.
- Horizontal Branch: The cluster displays a well-developed Horizontal Branch (HB) on its color-magnitude diagram, but its morphology is notably redder than is typical for other old halo globular clusters with similar low metallicity. This atypical HB morphology is another puzzle that astronomers use to study the complex evolution of stars in this cluster.
Due to its proximity to the Galactic plane, the cluster light suffers from significant interstellar reddening (dust obscuration), which complicates observational studies of its stellar population.