Nebula and Cluster of the Month Archive 2025

In this series of articles we draw your attention to Nebulae, Clusters and other Galactic objects that are particularly worthly of an observer's time.

• NGC 5466 in Boötes

  April 2025 - Nebula and Cluster of the Month

  I hope you are all enjoying the spring galaxy season. Following on from the galaxies by midnight in the middle of April, we have a small grouping of four globular clusters coming into fine view. Two were observed by Charles Messier and included in his famous catalogue; M3 – see this column for April 2021 – and M53, and two fainter clusters which were discovered by William Herschel; NGC 5053 and NGC 5466.

  This month we’ll be looking at NGC 5466. Discovered on the night of 17 May 1783, Herschel placed it in his category VI (very compressed and rich clusters of stars) as entry no. 9. In his catalogue, published in the Philosophical Transactions of the Royal Society in 1786, he describes it as A cluster of extremely small and compressed stars, 6 or 7’ diameter. Many of the stars visible, the rest so small as to appear nebulous. This is still a very good description, and gives a good idea of what this object looks like through a modest telescope on a good night.

  NGC 5466 lies in the west of Boötes, close to the border with Canes Venatici and 5° west of M3. The nearest Flamsteed-designated star is the 6.2-magnitude 11 Boo. The cluster lies at a distance of about 52,000 light-years. It shows signs of gravitational disruption, particularly in that it has a very long ‘tail’ of stars streaming behind it. The tail, or ‘tidal stream’ is very difficult to detect, but may be as much as 60° long. This is seen as evidence of tidal interactions between the globular cluster and the Milky Way. Some researchers have suggested that there have also been one or more interactions with the Large Magellanic Cloud¹.

  

  Although the catalogue magnitude of this object is given as 9.2, be aware that this is very misleading. NGC 5466 has very poor concentration, with no visually noticeable brightening towards the centre. Its concentration class is XII, the lowest, indicating a very loosely compressed object.

  I first searched for NGC 5466 on 24 March 2014, whilst the object was the Webb Deep-Sky Society’s Object of the Season². I classed my first attempt from my light-polluted home site as a ‘not seen’, though my notes from that night say There is a very small scattering of very faint stars where this object lies. This may be all I can see of it.

  On further reflection, I realised that I should not expect to see a ‘globular cluster’ as such – a ball of stars with a distinctly brighter centre and an unresolved haze of background stars. The low concentration suggested that what I would most likely see (from under light-polluted skies at any rate) would be just what I did see – a scatter of the brightest individual member stars of the cluster and nothing else.

  I returned to the object on the somewhat better night of 27 April 2014. I had more success this time. I detected a large but very dim patch – a very low-level glow. I only recognised it because of the faint scatter of stars across it. All the stars were very faint and difficult to place with any precision as they twinkled in and out of visibility. I made a sketch, reproduced here.

  

  Nearly a year later, on 25 March 2015, I was at my regular dark-sky site in the Lake District. I described the night as cold and fairly crisp. The naked eye limiting magnitude was 5.8 and the object glass of the finder scope iced over. I couldn’t resist the opportunity to look at NGC 5466 under vastly superior conditions to those I could ever get at home.

  It was quite a revelation. I had an amazing view. I could see a huge, pale, circular patch with dozens of stars splashed across it. Only 28 of those stars were certain enough to plot, with many others blinking in and out of sight, tantalisingly on the edge of visibility.

  

  The final chapter (so far) of my personal history with NGC 5466 occurred on 14 March 2025. As the sky conditions at my home are now so poor, I cannot observe visually any more. For a recent birthday, I was bought a SeeStar S50, a marvellous little machine which means although I am no imager (I have no idea at all what a ‘luminance frame’ is, for example) I can at least now do something on a clear night. On this particular night, I returned to NGC 5466. The result is reproduced below, a 30-minute exposure that gives a pretty good impression of what this object looks like visually.

  

  Patrick Maloney

  With many thanks to my family for the SeeStar.

  Object	RA	Dec	Type	Magnitude
  NGC 5466	14h 05m 28s	+28° 32’	Globular cluster	9.2

  References:

  1. See, for example, Yong Yang et al., ‘Revisit NGC 5466 tidal stream with Gaia, SDSS/SEGUE, and LAMOST’, Monthly Notices of the Royal Astronomical Society, Volume 513, Issue 1, June 2022, Pages 853–863.
  2. The Deep-Sky Observer, Issues 163, 165 and 166.

  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.

• NGC 2419 in Lynx

  January 2025 - Nebula and Cluster of the Month

  January brings with it the hope of a better year, of new beginnings. Let’s all hope for clearer skies and for an awakening amongst those who can make a difference, of the damage done by light pollution. I wish you all a happy and peaceful new year.

  On the night of 31 December 1788, William Herschel was examining the sky in the region where the constellations of Gemini, Auriga and Lynx meet. He was engaged in his ‘sweeping’, whereby he swept the telescope across an area of sky to find new ‘nebulae’ and clusters of stars. On this night, he came across a bright nebulous object in the southern regions of Lynx and fixing its position by its difference in Right Ascension and Declination from the 4.9 magnitude star 63 Aurigae, some 5.2° away, he described it as Considerably bright, round, very gradually much brighter in the middle. About 3’ diameter.

  He considered the object bright enough to fit his first class of objects, ‘bright nebulae’ and assigned it the number 218 within that class. It was, in fact, one of the 39 globular clusters discovered by Herschel during his sweeps. At the time, he can hardly have been aware of the significance of this object.

  

  Herschel did not resolve the object into stars and made no indication that he believed it to be anything other than a nebula. In the nineteenth century, the third Earl of Rosse, using his gigantic 72” reflector at Parsonstown in County Offaly, first resolved the object into stars and declared that he believed it to be a globular cluster.

  In 1888, a century after its discovery, 218 H.I entered the New General Catalogue as NGC 2419. There was an oddity about NGC 2419. As a globular cluster, it looked perfectly normal, but its positioning was strange. To an observer on Earth, it is the most remotely placed globular cluster, being well away from the main clustering of these objects in Sagittarius and Scorpius – i.e. towards the galactic centre. NGC 2419 lies about as far as it’s possible to get from the direction of the galactic centre.

  In 1922, the first photographs of NGC 2419 confirmed Rosse’s discovery; the object was indeed a globular cluster.

  In the 1920s, Harlow Shapley, using the period-luminosity relationship of its variable stars, calculated the globular’s distance to be 99,000 light-years. The period-luminosity relationship has since been refined, with the recognition of the difference between delta-Cephei variables and RR Lyrae variables, leading to a revised distance of around 272,000 light-years from Earth, or 300,000 light-years from the galactic centre.

  This colossal distance (three times the galactic diameter) led to the belief that NGC 2419 was not gravitationally bound to the Milky Way and must therefore be an ‘intergalactic wanderer’, and so its popular name was coined. Even today, some sources still insist that this is so, or that the velocity of the globular cluster is greater than the escape velocity of the Milky Way.

  Romantic as it sounds, it’s not true. The popular name is a misnomer. NGC 2419 is actually part of the Milky Way system, taking a mind-boggling three billion years to make a single orbit of the Milky Way.

  It has been noted that if an observer in M31, the Andromeda Galaxy, were to observe our Galaxy, NGC 2419 would be the brightest Milky Way globular visible, being far removed from the dust and gas which would obscure many of the other globular clusters.

  Despite its great distance, NGC 2419 is not the furthest-flung of the Galaxy’s globulars. There are three even more distant; Palomar 3, Palomar 4 and Arp-Madore 1 and these, too, are still gravitationally bound to the Milky Way. However, at magnitudes 13.9, 14.2 and 15.8 respectively, NGC 2419 is still the most far-flung outpost of the Galaxy that we’re likely to see visually.

  There is another oddity about NGC 2419. Most globular clusters consist of stars all of which are approximately the same age. They form a single population, in the terminology of astronomers. It’s believed that globular clusters form in a single event, and the absence of spare gas and dust prevents the formation of further stars once the initial formation is complete.

  NGC 2419, however, has been discovered to have two very distinct populations. One population is considerably richer in helium than the other. There is currently no satisfactory explanation for this anomaly.

  NGC 2419 is physically a large, bright globular cluster. If it were the same distance as M13 in Hercules, it would be half as big again as that globular and a full magnitude brighter. As it is, at its remote distance, it shines at magnitude 10.3, still a relatively easy target for modest telescopes. Fortunately for us, it is compressed, a type II in the 12-type classification of globular cluster concentration. This means that the brightness of the constituent stars is contained within a small diameter, rather than loosely spread out.

  

  It lies about 7.5° north of Castor, α Geminorum, in a fairly blank region of sky. It is about 4.1’ in diameter, and on low finding powers can look like a star, one of a row of three, the other two of which are around seventh magnitude. There are several fainter stars scattered around, but none of them are members of the cluster. The faintest stars on my drawing are around magnitude 13.5. The brightest member star of the cluster is magnitude 17.3, so not much hope of visual resolution unless you have an unfeasibly huge telescope (say a 72”).

  Nevertheless, it is comfortably bright enough to be easily visible, and the brightness difference between the outer periphery and the inner core is quite marked. Despite not being able to resolve the globular, I did note that it appeared somewhat mottled and the core appeared bulbous.

  I’ll be taking a break for the next couple of months. The criteria that I use to select objects for each month mean that there is nothing suitable left for February and March. I’ll be back in April. Until then, I hope your skies are clear and dark.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 2419	07h 38m 08s	+38° 52’ 54”	Globular cluster	10.3