Credit: Aldo Zanetti
Astronomical and Imaging Data
| RA: | 20h 06m 04.69s |
| DEC: | -21° 55′ 16.2″ |
| MAG: | 8.52 |
| Diameter: | 6.8′ |
| Const: | Sgr |
| OTA | Celestron 9.25 Edge HD |
| Focal Length | 2350 |
| Camera | ASI1600MM |
| Site | Central Italy |
| Sky Quality | Bortle 3 |
Useful Informations
NGC 6864, also known as Messier 75 (M75), is a highly compact and distant globular cluster located in the constellation Sagittarius. It’s a scientifically significant object due to its high stellar density and its location in the outer regions of the Milky Way’s halo.
Key Physical Properties
- Distance and Location: M75 is a distant cluster, situated approximately 67,500 light-years from Earth. Its location in the outer galactic halo makes it an important tracer of the Milky Way’s extensive gravitational potential.
- Structure: The cluster is exceptionally dense and centrally concentrated, classified as a Shapley-Sawyer Concentration Class I. This indicates a high degree of stellar crowding at its core, a characteristic of many of the most massive globular clusters. This dense core has undergone gravitational collapse.
- Age and Metallicity: M75 is an ancient stellar system, with an estimated age of over 13 billion years. Its stars are very metal-poor ([Fe/H]≈−1.3 dex), consistent with their formation from the pristine gas of the early universe.
Scientific Significance
M75’s extreme properties make it a key object for understanding the dynamics of dense stellar systems.
- Dynamical Evolution: The cluster’s high concentration and core-collapsed state provide a prime opportunity to study stellar interactions, including collisions and the formation of exotic objects like blue straggler stars.
- Variable Stars: M75 hosts a significant population of variable stars, particularly RR Lyrae stars, which are crucial for determining the cluster’s distance and for understanding the physics of stellar pulsation in ancient, metal-poor environments.
- Galactic Orbit: The cluster’s remote location in the halo and its highly elliptical orbit help astronomers model the mass distribution of the outer Milky Way, particularly the contribution of the elusive dark matter halo.
Messier 75 is part of the Gaia Sausage, the hypothesized remains of a dwarf galaxy that merged with the Milky Way. It is a halo object with an orbital period of 0.4 billion years to travel around the galaxy on a very pronounced ellipse, specifically eccentricity of 0.87.