Astronomical and Imaging Data

Useful Informations

NGC 1851 is a massive globular cluster located in the southern constellation of Columba. Also known as Caldwell 73, this dense cluster is notable for several unique characteristics that challenge traditional views of globular clusters as having a single, uniform stellar population.

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Physical Properties & Dynamics

• Distance and Magnitude: NGC 1851 is approximately 39,500 light-years (12.1 kiloparsecs) from the Sun and has an apparent magnitude of 7.3, making it visible with binoculars. Its absolute magnitude is about -7.8, making it one of the more luminous globular clusters.
• Mass and Age: The cluster has a mass of about 5.51×105 solar masses and an estimated age of 9.2 billion years.
• Concentration: NGC 1851 has a Shapley-Sawyer Concentration Class of II, indicating a very dense central concentration of stars. This is one of the highest concentrations among known globular clusters in the Milky Way.
• Tidal Tails: Despite some earlier suggestions, deep imaging has revealed the presence of tidal tails extending from the cluster. These streams of stars, stripped away by the Milky Way’s gravity, suggest that NGC 1851 is tidally disrupting and on a highly eccentric orbit through the galactic halo.

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Stellar Populations

NGC 1851 is a prime example of a globular cluster with multiple stellar populations, a feature that has become a key area of research.

• Compositional Differences: Observations have shown that the cluster’s stars are not uniform in their chemical composition. There are distinct populations with different abundances of heavy elements, especially those produced by the slow neutron-capture process (s-process). This is unusual, as most globular clusters are thought to have formed from a single gas cloud.
• Subgiant Branch Split: The color-magnitude diagram of NGC 1851 shows a split in the subgiant branch. This split suggests that there are two main stellar populations that have different ages or compositions. The presence of these multiple populations has led to speculation that NGC 1851 may be the remnant of a tidally stripped dwarf galaxy, similar to Omega Centauri, which was accreted by the Milky Way.
• Variable Stars and Pulsars: The cluster contains a significant number of variable stars, including RR Lyrae variables. It is an Oosterhoff Type I cluster, based on the properties of its RR Lyrae stars. A number of millisecond pulsars have also been discovered within the cluster, including some in binary systems with other massive objects.

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