Astronomical and Imaging Data

Useful Informations

Palomar 3 (Pal 3) is a distant and faint globular cluster located in the outer halo of the Milky Way galaxy. It is a scientifically significant object, particularly as an example of the less-dense, extended star clusters.

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Key Identifiers and Location

• Designations: Palomar 3, also known as the Sextans Globular Cluster or Sextans C, is one of the 15 faint globular clusters discovered on plates from the Palomar Observatory Sky Survey in the 1950s.
• Constellation: It is located in the constellation Sextans.
• Distance and Velocity: Palomar 3 is notable for its extreme distance, making it one of the most remote globular clusters in the Milky Way. Its distance from the Sun is approximately 302,000 to 320,000 light-years ( ∼ 92 to 98 kpc). It is receding from us with a radial velocity of approximately + 83.4 ± 8.4 km/s.
• Appearance and Concentration: Pal 3 is considered a very sparse and low-surface-brightness globular cluster. It has a very weakly concentrated core, classifying it as a Trumpler class XII (a very loose classification, indicating a low concentration of stars). Its apparent visual magnitude is around 14.26 mag. Its faint nature often makes it difficult to distinguish from background and foreground stars.

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Stellar Population and Age

• Age: Palomar 3 is an ancient structure, estimated to be around 10 billion years old, aligning it with the oldest stars in the galaxy.
• Metallicity: It possesses a low-to-intermediate metallicity (the abundance of elements heavier than hydrogen and helium) in common with other outer halo clusters.
• Evolutionary Stage: Pal 3 is considered a classic example of the “second-parameter” anomaly in globular clusters due to its red horizontal branch (RHB) morphology in combination with its low metallicity. The color of the horizontal branch is primarily determined by metallicity, but when two clusters have the same metallicity but different horizontal branch colors, a “second parameter” (such as age or helium abundance) is required to explain the difference.
• Age Anomaly: Comparative studies suggest Palomar 3 may be ∼1.5 to 2 billion years younger than the most ancient inner halo globular clusters (like M3 or M15), if its chemical abundance ratios are correctly estimated.

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Galactic Context and Origin

• Galactic Location: Along with other Palomar clusters (like Palomar 4 and 14), Pal 3 is a giant cluster located in the far outer halo of the Milky Way, orbiting far from the galactic center.
• Dwarf Galaxy Confusion: Due to its very sparse and low-density appearance, Palomar 3 was initially mistaken for a nearby dwarf galaxy of the spheroidal type, a classification temporarily labeled as “Sextans C” before it was confirmed to be a globular cluster belonging to the Milky Way.
• Neighboring System: It is situated close on the sky (about 4 degrees separation) to the Sextans Dwarf Spheroidal Galaxy, which is an actual satellite galaxy of the Milky Way. However, the significantly different radial velocities of the two objects strongly suggest they are physically unrelated and independent systems.
• Accretion Hypothesis: The group of younger, outer halo globular clusters, which includes Palomar 3, has been hypothesized to have formed in dwarf satellite galaxies that were later accreted and tidally disrupted by the Milky Way.

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