Astronomical and Imaging Data

Useful Informations

NGC 5897 is a globular cluster belonging to the Milky Way’s halo, notable for its extremely diffuse and sparse structure. It is sometimes informally referred to as the “Ghost Globular” due to its low surface brightness and lack of a densely concentrated core.

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Astrometric and Physical Properties

NGC 5897 is located in the southern constellation of Libra. Its approximate distance from Earth is about 24.1 kilolight-years (7.4 kpc) or roughly 40,000 light-years. It is a relatively faint object with an apparent visual magnitude (V) of approximately 8.5, making it visible through a small telescope. Its large physical size, with a diameter estimated to be over 170 light-years, combined with its low concentration, contributes to its dim, diffuse appearance. The cluster’s heliocentric radial velocity, which is the speed at which it is moving directly away from or toward the Sun, is measured at approximately +101.5 km/s.

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Stellar Population and Chemistry

Metallicity and Age

As a halo globular cluster, NGC 5897 is classified as metal-poor, meaning its stars contain relatively few elements heavier than hydrogen and helium. Its metallicity, expressed as [Fe/H], is typically measured around −1.9 to −2.04 dex, indicating that its abundance of iron is less than one percent of the Sun’s.

This low metallicity suggests the cluster is one of the oldest objects in the Galaxy, having formed very early in the universe, likely before the Milky Way’s disk and spiral arms were established. However, studies of its chemical composition also show that the abundance of alpha-process elements (like Oxygen and Magnesium) is enhanced, a signature consistent with the earliest generations of stars. Some studies have also indicated the presence of multiple star-forming epochs within the cluster, and its age is estimated to be about 2 Gyr older than the cluster M3.

Variable and Exotic Stars

The cluster is known to host a significant population of variable stars, predominantly RR Lyrae variables, which are key distance indicators in astronomy. Photometric studies have also revealed a distinct population of blue straggler stars (BSS) on its color-magnitude diagram (CMD). Blue stragglers are stars that appear younger and more massive than is consistent with the cluster’s main-sequence turnoff point, often thought to be the result of stellar mergers or mass transfer in binary systems.

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Structural Characteristics

NGC 5897 is structurally one of the loosest known globular clusters. Its star density is extremely low, even in the center.

• Concentration Class: It is assigned a Shapley–Sawyer Concentration Class of XI (11 on a scale of I to XII), placing it almost at the bottom of the concentration scale (Class XII is the least concentrated). This lack of a dense central core is its most distinctive structural feature.
• Tidal Features: Modern deep photometric surveys have revealed the presence of an extra-tidal envelope around the cluster. These diffuse, low surface brightness features are stars that are being gravitationally stripped away from the cluster, suggesting that NGC 5897 is undergoing tidal disruption due to the gravitational forces of the Milky Way galaxy.

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