NGC 6426
Globular cluster in the constellation Ophiuchus
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NGC 6426 is a globular cluster of stars located in the equatorial constellation of Ophiuchus. It was discovered by the German-English astronomer William Herschel on 3 June 1786.[7] This cluster is at a distance of 67,000 light years from the Sun.[3] It has an apparent visual magnitude of 10.9 and an angular diameter of 4.2′, making it difficult to observe with a small telescope.[4]
| NGC 6426 | |
|---|---|
HST image of NGC 6426 | |
| Observation data (J2000 epoch) | |
| Class | IX[1] |
| Constellation | Ophiuchus |
| Right ascension | 17h 44m 54.71s[2] |
| Declination | +03° 10′ 12.5″[2] |
| Distance | 67 kly (20.6 kpc)[3] |
| Apparent magnitude (V) | 10.9[4] |
| Apparent dimensions (V) | 4.2′[4] |
| Physical characteristics | |
| Metallicity | [Fe/H] = –2.34[3] dex |
| Estimated age | 13.0±1.5 Gyr[5] |
| Other designations | NGC 6426, Cr 346[6] |
This cluster is orbiting in the outer galactic halo at a distance of 47 kly (14.4 kpc) from the Galactic Core. It is one of the oldest and most metal-poor clusters in the Milky Way system. NGC 6426 has an angular half-light radius of 0.92′ and a tidal radius of 13.0′,[3] with a Shapley–Sawyer Concentration Class of IX.[1] It is an estimated 13.0±1.5 billion years old.[5] Chemical abundances of four members at the tip of the red giant branch suggests there may have been at least two generations of stars, with the older stars enriching a younger generation with elements including Mg, Si, and Zn. There is also some indication of hypernova enrichment of the pre-cluster medium by lighter alpha process elements.[8]
Based on the spectra of a dozen identified RR Lyrae variables, this is classified as an Oosterhoff type II cluster.[9] In 2012, a carbon star was discovered near the center of the cluster.[10]
