Credit: Jim Fordice
Astronomical and Imaging Data
| RA: | 19h 41m 44.41s |
| DEC: | -33° 59′ 58.1″ |
| MAG: | 12.4 |
| Diameter: | 4.4′ |
| Const: | Sgr |
| OTA | Planewave CDK24 |
| Focal Length | 3962mm |
| Camera | QHY 600M |
| Site | Río Hurtado, Chile |
| Sky Quality | Bortle 1 |
Useful Informations
Terzan 8 is a scientifically important globular cluster primarily recognized for its definitive membership in the globular cluster system of the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), a satellite galaxy currently being absorbed by the Milky Way.
Extragalactic Origin and Kinematics
Unlike the majority of the Milky Way’s globular clusters, Terzan 8 did not form within our Galaxy.
- Parent Galaxy: It is considered a bona fide cluster of the Sgr dSph, along with a few others like Terzan 7 and Arp 2. Its stars share a common kinematic signature with the Sgr dwarf galaxy’s core population.
- Distance and Location: The cluster is located in the constellation Sagittarius and is approximately 26 kpc ( 65,000 light-years) from the Sun. Its existence in the Galactic halo is direct evidence of the Milky Way’s growth through the hierarchical merging of smaller stellar systems.
- Tidal Disruption: As the Sgr dSph is stretched and tidally disrupted by the Milky Way’s gravity, Terzan 8 is now found within the debris stream of the dwarf galaxy.
Stellar Population and Composition
Terzan 8’s stellar properties contrast notably with the other Sgr dSph cluster, Terzan 7, but are consistent with older halo clusters.
- Age: Photometric studies have established that Terzan 8 is an old globular cluster, with an age estimated to be coeval with the typical ancient globulars of the Milky Way’s halo (i.e., 12 Gyr), making it significantly older than Terzan 7.
- Metallicity: It is classified as a metal-poor cluster, with an average iron abundance of [Fe/H] -2.27 dex, which is on the low end of metallicities found in the Milky Way’s halo clusters.
- Chemical Signatures: Spectroscopic analysis of its stars confirms that its chemical composition—specifically the abundance ratios of elements like Oxygen, Sodium, and α-elements—links it chemically to the Sgr dSph system, exhibiting a so-called “Sagittarius-flavoured” signature.
- Multiple Populations: As a low-mass globular cluster, it is generally considered to be chemically homogeneous, a key finding in the study of how multiple stellar populations form. Only a small minority of stars, if any, show the enhanced Sodium abundance characteristic of second-generation stars found in more massive clusters.