Credit: Aldo Zanetti
Astronomical and Imaging Data
| RA: | 18h 36m 23.94s |
| DEC: | -23° 54′ 17.1″ |
| MAG: | 5.10 |
| Diameter: | 32′ |
| Const: | Sgr |
| OTA | Celestron 9.25 Edge HD |
| Focal Length | 2350 |
| Camera | ASI1600MM |
| Site | Po Plain, Italy |
| Sky Quality | Bortle 7 |
Useful Informations
NGC 6656, also known as Messier 22 (M22), is a large, bright, and scientifically important globular cluster located in the constellation Sagittarius. It’s one of the most prominent globular clusters in the night sky and is a valuable target for astronomers studying stellar populations and galactic evolution.
Key Physical Properties
- Proximity and Apparent Size: M22 is one of the nearest globular clusters to Earth, located approximately 10,600 light-years away. Its proximity and large angular size—it spans about 32 arcminutes, wider than the full Moon—make it an exceptionally well-studied object. It’s one of only four globular clusters visible to the naked eye from the Northern Hemisphere.
- Age and Metallicity: The cluster is ancient, with an estimated age of about 12 billion years. Its stars are moderately metal-poor ([Fe/H]≈−1.7 dex), a characteristic typical of halo globular clusters.
- Structure: M22 is a moderately concentrated cluster, classified as Shapley-Sawyer Concentration Class VII. This classification indicates that its stars are not as tightly packed toward the center as in some other globular clusters, making it easier to resolve individual stars.
Notable Features and Scientific Significance
M22 is a unique and intriguing object that challenges some conventional ideas about globular clusters.
- Distinct Stellar Populations: One of M22’s most remarkable features is that it hosts multiple stellar populations, with stars that show distinct variations in their chemical composition. Unlike most globular clusters, which are thought to have formed in a single starburst event, M22 shows evidence of at least two different generations of stars with differing abundances of carbon and nitrogen.
- Presence of a Planetary Nebula: M22 contains a planetary nebula, designated GJJC-1, making it one of only a few globular clusters known to host one. Planetary nebulae are formed when a low-to-intermediate-mass star ejects its outer layers at the end of its life, and their presence in such an old stellar population is a rarity.
- Evidence of a Black Hole: The cluster is suspected of harboring an intermediate-mass black hole at its core, a hypothesis supported by the detection of multiple faint X-ray sources that could be accreting black holes.
- Pulsars: M22 is home to a significant population of millisecond pulsars, which are rapidly rotating neutron stars. The presence of these objects provides a valuable opportunity to study the late stages of stellar evolution in a dense environment.