Astronomical and Imaging Data

Useful Informations

Palomar 11 (often abbreviated as Pal 11) is a scientifically interesting example of a low-concentration globular cluster in the Milky Way, located in the constellation Aquila. It belongs to the sparsely populated family of Palomar clusters, discovered late in the 1950s due to its faintness and heavy obscuration by intervening interstellar dust.

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Structural and Dynamic Classification

Palomar 11 is classified with a low central concentration using the Trumpler-King system, typically assigned as Class XI.

• Low Concentration: This designation indicates that the cluster is very sparse and diffuse, lacking the tightly packed, bright core typical of many other globular clusters (like M13 or 47 Tucanae). Its stars are spread out over a wide area, making it visually challenging and sometimes giving it the appearance of a very dense open cluster rather than a classical globular.
• Likely a Thick Disk Cluster: Photometric analysis of its color-magnitude diagram (CMD) suggests that Palomar 11 is structurally and chemically similar to the Milky Way’s thick disk globular clusters, such as NGC 5927. Its orbit and properties link it more closely to the older, more massive components of the galactic disk than to the sparsely populated outer stellar halo.

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Age and Chemical Composition (Metallicity)

Palomar 11 is not among the oldest or most metal-poor globular clusters, which makes its study crucial for understanding the Milky Way’s formation.

• Age: It is estimated to be approximately 10.4±0.5 billion years old. This is slightly younger than the oldest clusters (which are often over 13 billion years old), placing it on the younger end of the globular cluster age spectrum.
• Metallicity: Its low but not extremely low metallicity is consistent with its thick disk characteristics. The abundance of iron relative to hydrogen is estimated to be around [Fe/H]≈−0.7 to −1.0 dex (solar abundance is 0.0 dex), meaning its metal content is significantly higher than the extremely metal-poor halo clusters.

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Galactic Position

The cluster’s location and distance place it within the inner to mid-halo region, but its line of sight is heavily affected by galactic dust.
A key factor defining Palomar 11 is the heavy interstellar reddening it experiences. It lies relatively close to the galactic plane, and the dust and gas in the plane significantly absorb its light. This obscuration makes it appear much fainter and redder than it would otherwise be, which is the primary reason it was only discovered relatively recently.

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Observed Dimensions and Brightness

The observed characteristics reflect its sparse nature and the effect of dust.

• Apparent Visual Magnitude (V): The cluster has an integrated brightness of approximately 9.80 magnitudes. This places it well below the limit of naked-eye visibility and makes it a difficult target for amateur telescopes, as its light is spread out over a large area.
• Absolute Visual Magnitude (MV​): Its intrinsic luminosity is calculated as approximately −6.92 magnitudes.
• Apparent Diameter: The cluster spans about 9.8 to 10.0 arcminutes across the sky. Due to its low concentration, this light is very diffuse, giving the cluster an unusually low surface brightness.

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