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Using the Hubble Space Telescope (HST), astronomers have observed a globular cluster known as NGC 1754 as part of a systematic study of the most massive and compact clusters in the Large Magellanic Cloud (LMC). Results of the observational campaign, May 15 on the arXiv pre-print server, shed more light on the properties of this cluster.

Globular clusters (GCs) are tightly gravitationally bound collections of tens of thousands to millions of stars. Given that the constituent stars of GCs all formed at approximately the same time and location, and had similar initial composition, astronomers perceive these clusters as natural laboratories enabling studies on the evolution of stars and galaxies.

Discovered in 1836, NGC 1754 is an old globular cluster in the LMC, at a distance of some 170,000 light years away. The cluster has a core radius of approximately 2.87 light years, metallicity at a level of -1.45, and its mass is estimated to be around 100,000 solar masses.

NGC 1754 has recently been observed with HST's Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS). The observations were conducted as part of the study of LMC's most massive and compact GCs by a team of astronomers led by Camilla Giusti of the University of Bologna in Italy.

"For the photometric analysis of the cluster, we used a set of 16 high-resolution images taken by the HST's WFC3 with the UVIS1 channel centered on the cluster. (...) The information from this dataset was complemented with a second one consisting of 13 images taken by the HST's ACS in optical filters (F606W and F814W) and sampling a field region located at ∼ 5′ from the center of the cluster," the researchers explain.

HST observations found that NGC 1754 has a core radius of approximately 2.74 light years, a half-mass radius of 10.2 light years, and its chronological age is 12.8 billion years. Therefore, the findings confirm that NGC 1754 is a compact and old globular cluster.

The metallicity of NGC 1754 was confirmed to be -1.45, and the reddening of the cluster was measured to be at a level of 0.1, which is consistent with values suggested by previous studies.

According to the paper, the results suggest that the process of globular cluster formation started at the same cosmic time, both in the LMC and in the Milky Way. Hence, the astronomers note that this process is likely independent of the characteristics of the host environment.

Furthermore, the findings point to an advanced dynamical age of NGC 1754, possibly on the verge of core collapse. This, according to the authors of the paper, seems to confirm that the natural dynamical evolution of GCs plays a role in shaping the age-core radius distributions observed in the Large Magellanic Cloud.

"The difference in the values of the core radius observed for the old LMC clusters is due to their different dynamical ages, with less compact clusters being less dynamically evolved than the more compact ones," the researchers conclude.

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