Astronomical and Imaging Data

Useful Informations

The globular cluster Palomar 12 (Pal 12) is a fascinating star system that distinguishes itself from the majority of globular clusters in the Milky Way, particularly due to its probable extragalactic origin.

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Observational Properties and Location

Palomar 12 is a member of the Palomar Globular Clusters group, faint collections of stellar systems discovered on the Palomar Observatory Sky Survey plates.

• Location: It is situated in the constellation Capricornus.
• Distance: The cluster is located approximately 62,000 to 64,000 light-years (around 19.5 kpc) away from the Sun.
• Visual Appearance: It is a faint object with an apparent magnitude of about 11.99 and an apparent angular size of around 2.9 arcminutes. Its low concentration means it is classified as Shapley-Sawyer Class XII, making it one of the most diffuse globular clusters known.
• Physical Size and Mass: Palomar 12 has an estimated radius of about 162 light-years and a total mass of approximately 1.59×104 solar masses (M_⊙​).

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Stellar Composition and Age

Palomar 12 is characterized by properties that are unusual for a typical, old Milky Way globular cluster, suggesting it is a comparatively younger object.

• Age: It has an estimated age of about 6.5 to 9.5 billion years (Gyr), making it roughly 30% younger than the average Galactic globular cluster (which are typically around 12−13 Gyr).
• Metallicity: It is relatively metal-rich for a globular cluster, with a metallicity value of [Fe/H] ≈ −0.85 dex. In astronomical notation, this means its iron content is about 14% that of the Sun.

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Origin and Orbital Dynamics

The most significant scientific aspect of Palomar 12 is its hypothesized extragalactic origin, which is consistent with its non-typical age and metallicity.

• Extragalactic Origin: It is now widely accepted that Palomar 12 was captured by the Milky Way from the Sagittarius Dwarf Elliptical Galaxy (SagDEG).
• Tidal Capture: Kinematic studies of its proper motion suggest that this tidal capture event occurred relatively recently, approximately 1.7 billion years ago, as SagDEG orbits and is slowly being tidally disrupted and consumed by the Milky Way.
• Association with Sagittarius Stream: Palomar 12’s current position and motion place it within the vast Sagittarius Stream—a long, stellar stream formed by the tidal stripping of the SagDEG as it orbits our galaxy.

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