Astronomical and Imaging Data

Useful Informations

Palomar 9 is an intriguing globular cluster, notable not for being an extremely faint Palomar-survey discovery like some of its namesakes, but rather for being an object in the galactic bulge that is heavily obscured by interstellar dust. In fact, it has a dual designation: Palomar 9 (Pal 9) is simply the modern name for the already-known cluster NGC 6717.

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Identity and Discovery History

Palomar 9 was initially discovered by William Herschel on August 7, 1784, who cataloged it as a faint, round nebula (H III.143). It was not officially recognized as a globular cluster until 1931 by Per Collinder. In 1955, George Abell independently rediscovered it during the Palomar Sky Survey and assigned it the designation Palomar 9, unaware of its prior NGC designation, thereby giving it its well-known double identity. The small, bright clump of stars on its edge was even cataloged separately around 1900 as IC 4802.

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Location and Visibility

• Constellation: Palomar 9 is located in the southern constellation of Sagittarius.
• Distance and Location: It is one of the closest Palomar clusters to the Galactic Center, residing in or near the Galactic Bulge. Its distance from the Sun is relatively small for a globular cluster, approximately 23-24 kilolight-years (kly), and its distance from the Galactic Center is even smaller, only about 7-8 kly.
• Obscuration: Palomar 9 is situated close to the plane of the Milky Way galaxy, which means our line of sight passes through significant amounts of interstellar gas and dust. This heavy obscuration is the primary reason for its inclusion in the “faint” Palomar catalog; the dust dims the light, or extinguishes it, making the cluster appear much fainter than it actually is.
• Apparent Properties: Despite the obscuration, it is one of the easiest Palomar clusters for amateur astronomers to observe, shining with an apparent visual magnitude of ≈ 9.3 and spanning an apparent diameter of ≈ 5 to 10 arc minutes.

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Physical and Structural Characteristics

• Concentration: Palomar 9 is classified as a relatively concentrated cluster, typically given a Shapley-Sawyer concentration class of VIII.
• Structure: Astronomers suspect it has a post-core-collapse morphology—meaning its central region has undergone a gravitational runaway process, resulting in a very dense, steep central profile. This high central density, common in old globular clusters, is what makes it appear highly concentrated.
• Age and Metallicity: It is an old, relatively metal-poor globular cluster, consistent with the typical population of globular clusters found orbiting the Milky Way.
• Luminosity: Its absolute visual magnitude is estimated to be around MV​ ≈ −5.7, indicating an intrinsically average to high luminosity that is severely masked by the intervening dust.

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