Nebula and Cluster of the Month Archive 2026

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 4361 in Corvus

  April 2026 - Nebula and Cluster of the Month

  The subject of this month’s article first came to my personal attention in 1977, whilst I was eagerly looking through my newly-obtained copy of the Skalnate Pleso Atlas Coeli, first compiled by the Czech astronomer Antonín Bečvář in 1951 and republished in 1962. Within its wonderfully oversized pages, I spotted a green circle, indicating a planetary nebula, framed by the four stars of the kite asterism of Corvus. It was labelled ‘4361’, the ‘NGC’ being implied.

  

  NGC 4361 was first seen by William Herschel on the night of 7 February 1785. He described it then as Very bright, pretty large, irregularly round, brighter in the middle, like 2 nuclei. Interestingly, Herschel did not describe it as a ‘planetary nebula’ and place it in his class IV, but simply as a ‘bright nebula’. It entered his catalogue with the designation 65H.I.

  NGC 4361 has proved significant in the study of planetary nebulae and their central stars because of its unusually high ionisation, low metallicity and an optically thin structure. These factors make it a valuable case-study in the understanding of stellar evolution and nebular physics.

  The central stars of planetary nebulae are low- to intermediate- mass stars which have passed through the asymptotic giant branch of the Hertzsprung-Russell diagram. Late in this stage, the stars expel their outer envelopes to leave the extremely hot central remnant that then illuminates and ionises the material that has been expelled. The temperature of the central star in NGC 4361 is in excess of 100,000K. The high-energy photons emitted from the star are capable of ionising helium twice, producing HII emission lines in the object’s spectrum. The relative strength of the HII lines in the spectrum indicates how highly ionised the nebular material around the star is. NGC 4361 is one of the most highly ionised known planetary nebulae.¹

  The spectrum of the central star also shows low metallicity (basically meaning a dearth of heavier elements). This indicates that the progenitor would have been an old, population II star. It therefore provides a window into the late life of an older generation of Milky Way stars.

  During the investigation into the nebula by Walsh et al (see reference 1), a small region of the planetary nebula was discovered to be emitting at a wavelength of 670.3nm (6703Å). Closer investigation revealed further lines which do not match planetary nebula lines. It was concluded that there is an emission galaxy behind the planetary nebula and shining through it. It has been given the snappy designation NGC 4361-BgGal1224290-184707. (If you really want to look, it’s 24.3” west of the central star. If you can see it, do let me know!)

  

  NGC 4361 shines with a visual magnitude of 10.9. It is just shy of 2’ wide along its longest axis. This should be an easy object, and under good conditions it is, but as many of us live with constant light pollution, especially at low altitude, it can be a challenging object to observe. From mid-Britain, the highest it gets is 17.5°.

  Even though my attention was first drawn to it in 1977, I didn’t get to see it until 2001, when I was lucky enough to be attending the Winter Star Party in the Florida Keys. There I used a 16” (400mm) Newtonian reflector for my first view of this object. I noted a very bright ring, clearly detached from the 13.2-magnitude central star. I could see an elongation in PA about 230°. This ring effect is caused by slightly darker (presumably less abundant) nebulosity in the immediate vicinity of the central star. It’s subtle, and can be quite hard to detect in suboptimal conditions.

  My next view of NGC 4361 was from light-polluted, further-north home in April 2016. The observation was in stark contrast to the 2001 sighting. Very faint, blurred and distorted. The atmosphere was very turbulent at this altitude (17.5°). Quite large but almost no detail seen - perhaps a slight elongation. The middle is brighter and there was the occasional glimpse of a central star. A very disappointing view.

  Undeterred, I tried again from a dark sky site in May of 2016. I was rewarded with a better view (pictured here) and recorded in my log Pretty bright for its altitude (16.5° - I was 1° further north than my home site). Circular with a bright centre. A central star pops in and out of view. There was possibly the occasional glimpse of a ring structure.

  

  This can be a challenging object, but as so often with difficult targets, it pays back the effort put into seeing it.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 4361	12h 24m 31s	-18° 47’	Planetary nebula	10.9

  Reference:

  1. Dissecting the Planetary Nebula NGC 4361 with MUSE, Walsh et al, Astronomy & Astrophysics, Volume 690, A264 (2024).

  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.

• Collinder 95 and NGC 2245 in Monoceros

  January 2026 - Nebula and Cluster of the Month

  I’d like to start by wishing you all a very happy and prosperous new year.

  This month we’re going to look at an object which at first looks straightforward but which presents a conundrum if we dig deeper.

  The object is the open cluster Collinder 95 in Monoceros. It’s not a bright, splashy cluster but nevertheless deserves some attention. It was first listed by the Swedish astronomer Per Arne Collinder (1890—1975) in his catalogue of open clusters, published in 1931. Collinder’s catalogue appears as an appendix to his paper On Structural Properties of Open Galactic Clusters and their Spatial Distribution. The purpose of the study was to provide a systematic collection of objects with measured (or at least estimated) quantities – position, apparent diameter, number of stars, total magnitude, concentration, etc. – as the basis for a discussion on how clusters differ in structure, richness and spatial placement within the Galaxy.

  Collinder listed 471 objects – not all of which were open clusters. There are a few globular clusters and asterisms, but the vast majority are open clusters. Many of the objects were already known and had previous catalogue entries. About 60 were new identifications. Cr 95 is one of these.

  With the proliferation of catalogues these days, Cr 95 now has other designations: Lund 225, OCl 491 and the soulless C0627+099. It also appears to be no. 14 in the obscure Alessi catalogue of ‘unknown open clusters in the Tycho-2 catalogue.’

  

  Cr 95 has in the past been rather poorly studied, often being overlooked because of its relative sparseness. The European Gaia satellite, launched in 2013 and operational until 2025, provided unprecedentedly accurate astrometry for billions of Milky Way stars. This dataset has allowed for far more detailed investigation into hundreds of open clusters. Importantly, Gaia data allows for a much more certain membership selection for stars in sparse or widely scattered clusters. In the case of Cr 95, its existence as a true cluster has been confirmed, and that it lies relatively nearby (a few hundred light-years). It is a young cluster, certainly, but here we hit a snag. Two standard ways for determining the age of an open cluster are isochronal aging and kinematic aging. The isochronal age of a cluster is determined, basically, by the spectral analysis of the member stars. Different mass stars will evolve at different rates, and numerical analysis is performed to determine the cluster’s age from the spectra of its member stars. Kinematic ageing is based on the motion of the stars, which are assumed to have had a single place of birth.

  The problem with Cr 95 is that these derived values do not match. Specifically, the kinematic studies return an older age than the isochronal studies. The conclusion drawn from this inconsistency is that the member stars of Cr 95 did not have a single birthplace.

  Cr 95, then, may be a representative of clusters whose origin is distributed or partly unbound, as opposed to the more common ‘monolithic’ open clusters. The member stars, then, are cousins rather than siblings.

  To find Cr 95, you’ll need to move 8.5° east-north-east of Betelgeuse (α Orionis). Alternatively, start at the much better-known open cluster NGC 2264 and move 2.5° due west.

  The cluster fills an area about the same size as the full moon. The brightest stars are of eighth- and ninth-magnitude, but do stand out against the background. The cluster is better seen on low, finding powers.

  There is much nebulosity mingled with the stars of the cluster. This carries various designations, individual sections being labelled vdB 76, vdB 77, vdB 78 and vdB 79. Overall, it is classed as IC 447.

  The brightest of this nebulosity is in the southern half of the cluster, gathered around a triangle of eighth and ninth magnitude stars. Curling north from this, through the rest of the cluster and beyond, is a great semicircular dark nebula, LDN 1599.

  The image that I took with my SeeStar 50 shows this clearly. Not visible in the image but just off the top left of it, the dark nebula curls round to a bright patch of nebulosity around a 10.4-magnitude star. The star is 3UC201-059373 and the nebula is NGC 2245.

  NGC 2245 is clearly visible in a modest telescope. My observation of it through my 12” Newtonian reflector is shown. On that night, I described it as A small, bright nebula around a tenth-magnitude star, stretching away from it like a short cometary tail.

  

  Whilst the nebulosity around Cr 95 is clear in long-exposure images, it is far from clear visually. I suspect that the clearest sky and some hefty aperture would be needed to see it. I certainly never have.

  If you use the Guide 9 planetarium program, note that Cr 95 is immediately to the east of where the cluster is marked. The position is correct in MegaStar.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  Cr 95	06h 31m 09s	+09° 51’	Open cluster	8-ish
  NGC 2245	06h 32m 42s	+10° 09’	Bright nebula	-