Astronomical and Imaging Data

Useful Informations

Terzan 11 is a globular cluster located in the direction of the Galactic Bulge. It is classified among the highly obscured globular clusters, making its observation and characterization a significant challenge that often requires infrared astronomy.

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Discovery and Location

• Discovery: Terzan 11 was discovered by the Armenian French astronomer Agop Terzan in 1971 during a survey of the Milky Way’s central region, one of a set of four clusters labeled Terzan 9 through 12.
• Constellation: The cluster is situated in the constellation Sagittarius, an area of the sky that is intensely crowded with stars and heavily obscured by interstellar dust due to its proximity to the Galactic Center.
• Distance and Position: Its location places it in the inner region of the Milky Way. The distance from the Sun is typically estimated to be around approximately 4.8 kpc. Its coordinates place it close to the Galactic Plane.

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Physical and Stellar Properties

• Apparent Faintness and Size: The cluster is quite faint in the visible band, with an apparent magnitude of about 15.6, and a relatively small angular size, typically cited around one arcminute in diameter. This faintness is primarily due to the severe dust obscuration along the line of sight.
• Extinction: Like other Terzan clusters near the Galactic Plane, Terzan 11 suffers from significant and likely differential extinction from foreground dust and gas clouds. This heavy obscuration means that much of the starlight is blocked or reddened, complicating the determination of its true luminosity and structure from optical data alone.
• Metallicity and Age: As an old globular cluster, its stellar population is ancient. While specific high resolution metallicity data is sparse, it is generally considered to be a metal-poor cluster, consistent with the rest of the Galactic globular cluster population which formed early in the universe’s history. The lack of heavy elements is a key characteristic of these first generation systems.

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Note on Identification

It is important to note that the cluster Terzan 5 was initially misidentified as Terzan 11 by its discoverer in the second cataloging. However, Terzan 5 is now firmly established as a separate and distinct object, recognized for its complex multiple stellar populations, which suggests it may be the remnant core of a disrupted dwarf galaxy rather than a simple globular cluster. The Terzan 11 discussed here is the distinctly cataloged cluster at the coordinates RA ≈ 18h 12m 15.8s and Dec ≈ −22∘44′31′′.

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