Credit: Armen Akopian
Astronomical and Imaging Data
| RA: | 18h 10m 17.60s |
| DEC: | -31° 45′ 50.0″ |
| MAG: | 9.26 |
| Diameter: | 4.1′ |
| Const: | Sgr |
| OTA | CCF RC 250mm f/8 |
| Focal Length | 2000mm |
| Camera | QHYCCD QHY268 Pro M |
| Site | Rockwood, TX, United States |
| Sky Quality | Bortle 1 |
Useful Informations
NGC 6558 is a globular cluster located in the constellation of Sagittarius. It is an ancient, dense spherical grouping of stars that is considered a “bulge” cluster because of its proximity and orbit within the central region of the Milky Way galaxy.
Key Scientific Properties
- Age and Metallicity: NGC 6558 is an old cluster, with an estimated age of about 13 billion years. Its stars are considered metal-poor, with a low abundance of elements heavier than hydrogen and helium. The metallicity of the cluster is roughly [Fe/H] = −1.0. This is a characteristic of many of the oldest objects in our galaxy, as they formed from the primordial gas of the early universe.
- Stellar Composition: The cluster has a prominent blue horizontal branch (BHB), a feature in a Hertzsprung-Russell (H-R) diagram that indicates the presence of hot, blue stars. The existence of these stars, along with its specific chemical abundances (e.g., enhanced alpha-elements like oxygen and magnesium), suggests that the cluster’s stars were enriched by yields from core-collapse supernovae in the early universe.
- Structure: NGC 6558 is a very concentrated cluster, indicating a high stellar density. It is thought to have a post-core-collapse structure, meaning that its central region has undergone a gravitational collapse, leading to an even higher density of stars in its core.