Astronomical and Imaging Data

Useful Informations

NGC 288 is a classic, low-concentration globular cluster located in the southern constellation Sculptor. It is a crucial object for stellar evolution studies due to its characteristics, particularly its distinct color-magnitude diagram.

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Key Physical Properties

• Type and Location: NGC 288 is a globular cluster (GCL) that is part of the Milky Way’s halo population. It is found at an approximate distance of 8.96 kpc (about 29,220 light-years) from Earth.
• Age and Metallicity: It is an exceptionally old stellar system, with an estimated age of approximately 13.5±1.1 Gyr, making it one of the most ancient structures in the Galaxy. Its metallicity is considered metal-poor, with a value of [Fe/H] ≈ −1.14 dex (or ≈ −1.32 dex in other sources), meaning it has significantly fewer heavy elements than the Sun.
• Mass: The cluster has an estimated total mass of around 4.8×104 solar masses (M_⊙​).
• Kinematics: The cluster exhibits a retrograde orbit with respect to the general rotation of the Milky Way, with a mean radial velocity of approximately −45.4 km/s (or −56.3 ± 20.1 km/s in other studies). Its dynamical properties, suggesting an accreted origin from a dwarf galaxy like Gaia-Sausage-Enceladus (GSE), present a contradiction with its very old age and chemical consistency with other “in situ” Milky Way clusters, a paradox that is often addressed by proposing a “Splash” scenario where it formed in the proto-disc and was later dynamically heated by the GSE merger.

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Structural and Stellar Characteristics

• Concentration: NGC 288 is classified as a low-concentration globular cluster (Shapley–Sawyer concentration class X), meaning its stars are more loosely bound together compared to richer, denser globulars.
• Appearance: The cluster appears large, with an apparent visual diameter of about 13.8 arcminutes. It has a visually well-resolved core of about 3′ in diameter, which is surrounded by a much more diffuse, irregular ring of stars. Peripheral members extend notably outward, particularly towards the southwest.
• Stellar Population: As an old globular cluster, its stellar population consists mainly of Population II stars that are coeval (formed at the same time).
  • Color-Magnitude Diagram (CMD): The CMD is a key feature of NGC 288. When compared to the similar-metallicity globular cluster NGC 362, NGC 288 shows a bluer horizontal branch and a redder/fainter main sequence turn-off point, which is interpreted as NGC 288 being older.
  • Variable Stars: The cluster contains various types of variable stars, including RR Lyrae stars (useful for distance determination) and SX Phoenicis stars.
• Tidal Structure: Observations have revealed an extra-tidal clumpy structure extending up to 3.5 times the cluster’s tidal radius, suggesting some stellar material is being stripped, though evidence for long, coherent tidal tails is debated. This extra-tidal material is a result of the cluster’s interaction with the Milky Way’s gravitational field.

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