Astronomical and Imaging Data

Useful Informations

Lynga 7 (or BH 184 from the van den Bergh-Hagen catalog) is a significant star cluster that occupies a transitional and debated position in the classification of star clusters, generally accepted now as a metal-rich globular cluster belonging to the Galactic disk system.

---

Classification History and Location

• Classification Shift: Lynga 7 was initially cataloged in 1964 as a sparse open cluster due to its location close to the plane of the Milky Way. However, later photometric studies in 1993 revealed its color magnitude diagram (CMD) contained an old, well-defined red horizontal branch and a characteristic vertical subgiant branch, features strongly indicative of a globular cluster.
• Location and Obscuration: The cluster lies in the constellation Norma, positioned very close to the galactic plane at a low galactic latitude. This location places it within a very crowded stellar field and subjects it to heavy interstellar reddening and dust extinction, which complicate observations and classification. The distance from the Sun is estimated to be approximately 8 kiloparsecs (26,000 light-years), placing it relatively close to the edge of the Galactic disk. Its Galactocentric distance is about 4.3 kiloparsecs.

---

Physical and Chemical Characteristics

• Age: Lynga 7 is an exceptionally old stellar system. Although some initial estimates suggested it was the oldest known open cluster, its overall CMD morphology confirms an age comparable to the ancient globular clusters, indicating it is a part of the oldest stellar populations in the Galaxy.
• Metallicity: It is classified as a metal-rich globular cluster, with an iron abundance ratio of approximately -1.01 dex. This metallicity is high for a typical halo globular cluster but is characteristic of the so called disk globular cluster sub-population, which tends to be more metal-rich than the metal-poor halo clusters.
• Morphology: The CMD of Lynga 7 is similar in morphology to other metal-rich globular clusters like 47 Tucanae, particularly showing a curved Red Giant Branch (RGB) and a dominant Red Horizontal Branch (HB), as opposed to the blue HB seen in metal-poor clusters.
• Significance: As a member of the Galactic disk globular cluster system, Lynga 7 is a crucial tracer for studying the formation and chemical evolution of the inner, older parts of the Milky Way disk, often presenting a possible intermediate population between the old spheroidal halo and the younger disk.

---