NGC 1851, Caldwell 73

Image Title: Globular Cluster NGC 1851

Copyright: Massimo Di Fusco

Date image was taken: December 11, 2025

Location: Rio Hurtado (Chile)

Data Acquisition Method: Remote Observatory (Commercial)

Description and Details: NGC 1851 (also known as Caldwell 73) is a relatively massive globular cluster located in the southern constellation of Columba. It is located 39500 light-years from the Sun, and 54100 light-years from the Galactic Center. The cluster is following a highly eccentric orbit through the galaxy, with an eccentricity of about 0.7.
This object has a Shapley–Sawyer Concentration Class of II, indicating a dense central concentration. It has one of the highest concentrations known for Galactic globular clusters. The stellar components show two separate populations of subgiant stars, with the brighter branch being more concentrated in the outer regions of the cluster. NGC 1851 is an estimated 9.2 billion years old with 551000 times the mass of the Sun.
The cluster is surrounded by a diffuse halo of stars that stretches outward to a radius of 240 pc or more. This feature, if combined with the lack of tidal tail or associated stream of stars, suggests the cluster may be a stripped dwarf galaxy nucleus, similar to Omega Centauri, that has been accreted by the Milky Way. It is also possible the cluster is the result of the merger of two separate clusters, but the fact that they would need to have the same metallicity – what astronomers term the abundance of elements other than hydrogen and helium – makes this scenario less likely.

Data acquired at Chilescope
ASA Newton 500 @1900mm, f/3.8
ASA DDM85 equatorial Mount
FLI PL16803 camera
Astrodon R/G/B filters 3x(2x300")
PixInsight, PS

Name: Massimo Di Fusco

Website or Facebook Profile: https://www.astrobin.com/users/massimo.difusco/