NGC 5897 in Libra

May 2026 - Nebula and Cluster of the Month

This month’s object is a globular cluster in Libra, famous for its elusiveness. Unsurprisingly, it was another discovery of William Herschel’s. Herschel obviously lived at a time when light pollution still lay in the future, and we can only dream about the skies he enjoyed, even in urban settings. He first described this object on the night of 25 April 1784 as A very compressed cluster of stars, 8 or 9’, extremely rich, irregularly round or little elongated.

He then observed it again on the night of 10 March 1785, writing A beautiful large cluster of the most minute and most compressed stars of different sizes, 6 or 7’ in diameter. Irregularly round, faint. Red colour.

Initially, he did not recognise that these observations were of the same object and so it entered his catalogue (VI = Very compressed and rich clusters of stars) as both 8H.VI and 19H.VI. It eventually passed into the New General Catalogue with the number 5897, with the description Globular cluster. Pretty faint, large, very irregularly round, very gradually brighter towards the middle, very easily resolved.

NGC 5897 lies about 40,000 light-years from us. The angular diameter, as measured with modern methods, is 10—12 arcminutes, corresponding to an actual diameter of 150—170 light-years. It is very loose in concentration, being classified as a type XI cluster on the Shapley-Sawyer scale of globular cluster concentrations. The very loosest globulars are of type XII. This has consequences both for the physics of the cluster and for its visibility.

A defining characteristic of NGC 5897 is its very low metallicity. The stars within the globular have iron abundances of less than 1% that of the sun. This places it amongst the metal-poor halo globulars and indicates a very great age. In fact, NGC 5897 probably formed before the Milky Way had developed its core, bar and spiral arms.¹

Despite this low metallicity, the stars in the cluster clearly represent several different populations, confirming that even such diffuse clusters undergo multiple star-forming periods and self-enrichment.

A notable feature is the presence of blue straggler stars – stars that appear younger and more massive than the cluster’s main-sequence turn-off population. Their existence is generally interpreted as the result of stellar mergers or mass transfer in binary systems, processes facilitated by dynamical interactions even in relatively low-density clusters.

Age estimates place NGC 5897 at roughly 10—12 Gyr, possibly older than some benchmark clusters such as M3 by about 2 Gyr, reinforcing its status as an early Galactic fossil.

The low density and poor concentration of the globular cluster contribute significantly to its reputation as a difficult object for visual observers. In addition, it is low-lying from our shores, never reaching much over 15° above the horizon from mid-Britain. Although the magnitude is usually quoted as 8.4, this does not compensate for the poor concentration. I must admit now that I have never had a satisfactory view of it from my suburban location. I have suspected it a couple of times, but never strongly enough to report a positive observation.

The key in searching for globular clusters like NGC 5897 is not to look for an obvious diffuse ‘ball’, but instead to look for a small scatter of very faint stars. For a discussion of this, see my article on NGC 5466 (April 2025). The brightest stars in NGC 5897 are of the order of magnitude 13, so not too much of a stretch for a moderate telescope.

Much of what I say there is also true for NGC 5897, in many ways a visual twin to NGC 5466. The problem is exacerbated for NGC 5897 by its low altitude. You should not expect to see a globular cluster like the lovely image at the top of this article. You’d be forgiven for thinking that it should present little difficulty from that. The underexposed image below gives a much better idea of what to expect visually. This shows stars to about magnitude 16 or 17.

Patrick Maloney

Object	RA	Dec	Type	Magnitude
NGC 5897	15h 17m 25s	-21° 08’	Globular cluster	8.4

Reference:

1. Chiti, A. et al. (2025), DELVE-ing into the Milky Way's Globular Clusters, arXiv preprint.

If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Cluster of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.