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This narrowband image of planetary nebula NGC 3699 in Centaurus was provided courtesy of Adam Block where he has an even larger version for you to explore.
Around 2015 this planetary nebula was imaged by ESO from its La Silla Observatory in Chile using an instrument on their 3.6-metre telescope. Adam's image was also taken from Chile but, 11 years on, he's using a 0.61m telescope and 32 hours of exposure time to produce this detailed, confusing and fascinating view of NGC 3699 in the southern constellation of Centaurus.
Adam has a great article on this object and his imaging process on Astrobin and, rather than repeat what he's said here, I suggest you read it. As Adam notes, this is probably a polypolar planetary nebula with a binary system at its heart, but there appears to be very little written about it.
The article for NGC 3699 on French Wikipedia claims that the GAIA DR3 parallax corresponds to a distance of about 1228–1534 pc (4,000–5,000 light-years). There's little data available for NGC 3699 anywhere else.
I checked this with Gaia DR3 using Vizier and the location of this particular source (Gaia DR3 5336133687170599040) does appear to be that of Adam's suspected central star. However the estimated effective temperature of around 10,000K seems a little low, although it is easily the highest to be found in the area. Additionally, the Renormalized Unit Weight Error (RUWE) for this star is very close to 1, suggesting that this isn't an unresolved binary system... perhaps Gaia DR4 will help when it arrives.
NGC 3699 is a relatively large and faint object which I suspect puts it beyond the scope of visual observers, and it's location will foil many imagers in the northern hemisphere. Fortunately the availability of remote telescopes under southern skies opens up the possibility to examine many fabulous objects of this kind.
James Whinfrey - Website Administrator.
April 2026 - Picture of the Month
Copeland's Septet (Hickson Compact Group 57) in Leo
This month I bring you a wonderful image of a group of galaxies just above the hind-quarters of Leo known as Copeland's Septet, Hickson 57, Arp 320 or a number of other designations, as is often the way.
This group of galaxies was discovered in 1874 by Ralph Copeland with the Parsons 72-inch telescope whilst an assistant to Lord Rosse at Birr Castle in Ireland. For a while this group was lost due to a positioning error by Copeland which carried through to the compilation of the New General Catalogue of Nebulae and Clusters of Stars (NGC) by Dreyer in 1888. Being a septet, Copeland obviously identified seven members in this group, albeit over different nights of observing.
Paul Hickson made it a group of eight members, with the addition his H component, when it was catalogued as Hickson Compact Group 57 (HCG 57) in 1982 1. In this paper, Hickson examined 100 compact galaxy groups from the Palomar Observatory Sky Survey –now available as the Digitised Sky Survey (DSS)– with respect to their overall morphology and that of their component galaxies. From this study he attempted to draw conclusions about the dynamical evolution of such groups and their extrapolation to larger galaxy clusters, in which it was believed the density of galaxies was broadly the same.
HCG 57 comprises NGC 3753 (HCG 57A), NGC 3746 (HCG 57B), NGC 3750 (HCG 57C), NGC 3754 (HCG 57D), NGC 3748 (HCG 57E), NGC 3751 (HCG 57F), NGC 3745 (HCG 57G) and 2MASX J11375047+2200450 (LEDA 36010, PGC 36010 and also HCG 57H).
Hickson estimated there to be three spiral galaxies amongst them, but this image would suggest that this was an underestimate, which is a shame because a low proportion of spiral galaxies was felt to set these groups apart from the general background.
The NASA/IPAC Extragalactic Database (NED) gives the Hubble distance to HCG 57 as 136.04±9.53 Mpc (about 443.4 Mly) which matches closely with radio frequency based estimates of 135±3 Mpc, and places the group within the Coma Supercluster. The galaxies themselves range between 434.5 Mly (HCG 57A) and 470.7 Mly (HCG 57G) distant.
A study of the distribution of neutral elemental hydrogen (HI) in 16 of the Hickson Compact Groups (HCG) using data from the Very Large Array (VLA) proposed an evolutionary sequence for these groups 2: in phase 1 HI is confined to the individual galaxies themselves; in phase 2 tidal forces are clearly having their effect and the hydrogen is becoming distributed throughout the group, such tidal features are well in evidence in this image of HCG 57; and by the end of phase 3 the galaxies lack any HI, in fact it may be missing from the group entirely.
In a later paper this mechanism was revisited with an analysis of 38 HCGs and found little correlation between the evolutionary stage of the galaxy group based on its morphology and that of its HI deficiency 3 . The author notes that the phase 3 groups are all deficient in HI, however the difference between phases 1 and 2 is negligible and likely attributable to the amount of HI present in the galaxies at the time the group formed. Additionally, there's always the possibility of replenishment from captured galaxies at a later stage that requires consideration.
Galex shows emission in the far-ultraviolet, which is characteristic of star formation in galaxies, is confined to HCG 57B and HCG 57D with perhaps a hint in HCG 57C. It's probably a coincidence that these three galaxies are also at nearly the same distance.
HCG 57B was host to two supernovae since the turn of this century: SN 2002ar and SN 2005ba, type Ia and type II respectively. It has been noted that HCG 57D seems to be the least reddened of the galaxies in the group with clear signs of active star formation, it's also a LINER-type Active Galactic Nucleus (AGN).
A recent paper examined this group from a high-energy perspective using data from Chandra and XMM-Newton X-ray 4. It identified point-like X-ray emissions from HCG 57A, HCG 57B and HCG 57D confirming the presence of AGNs. The authors also note that HCG 57A and HCG 57D form an interacting pair, right in the middle of this image, with HCG 57D having passed through the disk of HCG 57A about 50 million years ago, leaving a substantial gas bridge and areas of shock heating. The turbulent expansion of the resulting shock through the two galaxies are also suggested as a possible mechanism to explain the significant suppression of star formation in the larger HCG 57A. Being much smaller, the shock passed through HCG 57D long ago.
Visual observation of the group doesn't appear to be easy by any means based on Steve Gottlieb's comments, and a large telescope will be necessary, probably 350mm (14-in) or bigger. He notes that the easiest of the galaxies is NGC 3753 (HCG 57A), which is coincidentally also the closest, and that the whole group is possible with persistence and a good sky. As you can see, these compact galaxy groups do make spectacular imaging targets.
Verdes-Montenegro, L., Yun, M. S., Williams, B. A., Huchtmeier, W. K., Del Olmo, A., and Perea, J., “Where is the neutral atomic gas in Hickson groups?”, Astronomy and Astrophysics, vol. 377, EDP, pp. 812–826, 2001. doi:10.1051/0004-6361:20011127. https://ui.adsabs.harvard.edu/abs/2001A%26A...377..812V/abstract (Viewed on 25 March 2026).
Jones, M. G., “Disturbed, diffuse, or just missing? A global study of the H I content of Hickson compact groups”, Astronomy and Astrophysics, vol. 670, Art. no. A21, EDP, 2023. doi:10.1051/0004-6361/202244622. https://ui.adsabs.harvard.edu/abs/2023A%26A...670A..21J/abstract (Viewed on 25 March 2026).
O'Sullivan, E., “HCG 57: Evidence for Shock-heated Intergalactic Gas from X-Rays and Optical Emission Line Spectroscopy”, The Astrophysical Journal, vol. 979, no. 2, Art. no. 240, IOP, 2025. doi:10.3847/1538-4357/ada14b. https://ui.adsabs.harvard.edu/abs/2025ApJ...979..240O/abstract (Viewed on 25 March 2026).
March 2026 - Picture of the Month
Dwarf galaxy Sextans A
A glittering image of irregular dwarf galaxy Sextans A captured by the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory courtesy of KPNO/NOIRLab/NSF/AURA.
This month's colourful image is of the irregular dwarf galaxy Sextans A. It's box shape, which has been sculpted by a succession of supernovae, is filled with bright young and massive stars and H-II regions around its periphery for that splash of colour.
As the name suggests, it's a small galaxy with a diameter of about 2.45 kpc (8,000 light-years). It lies only around 1.4 Mpc (4.6 million light-years) away at the edge of our Local Group. Sextans A is also chemically primitive: it displays very low metallicity being comprised mostly of hydrogen and helium. This is largely due to its low mass as it lacks the gravitational strength to retain the the heavier elements that are energetically ejected in supernovae and which would then contribute to the generation of complex chemical species during the late evolution of massive stars.
However, its low metallicity makes Sextans A a good analogue for early galaxies, and a convenient target for detailed observation due to its proximity.
A recent survey of asymptotic giant branch (AGB) stars in Sextans A with the James Webb Space Telescope (JWST) revealed the presence of Silicon Carbide (SiC) dust. These dwarf galaxies were expected to lack the heavier elements that are required to create SiC dust, in fact they didn't expect to find any dust at all, yet it turned out to be abundant and composed almost entirely of iron. The authors found that one of the stars surveyed was producing dust via an unusual mechanism, demonstrating that solid matter can be created even when many of the usual components are absent.1
The discovery of SiC dust in such a metal poor galaxy is evidence that solid particles can be created even at this apparently early stage of galactic formation.
Polycyclic aromatic hydrocarbons (PAHs) are interpreted as signifying star formation, due to their catalytic effects on the formation of molecular hydrogen and moderation of ionising radiation. They were also thought to be absent in low mass galaxies, since the lack of dust to shield these compounds from the intense UV radiation from young stars would permit rapid decomposition.
Another study used JWST data of Sextans A's interstellar medium to reveal dense pockets of PAHs. In large galaxies PAHs are distributed throughout the galaxy inside large dust and gas clouds, many of which have featured in this column. This result suggests PAHs might not be so rare in such low metal environments, rather it's the sparse distribution of these clumps within small distant galaxies that makes them hard to measure.2
M. L. Boyer et al, “Discovery of SiC and Iron Dust around AGB Stars in the Very Metal-poor Sextans a Dwarf Galaxy with JWST: Implications for Dust Production at High Redshift”, The Astrophysical Journal, Volume 991, Number 1. DOI 10.3847/1538-4357/adf06a
Tarantino, E. J., “JWST Captures Growth of Aromatic Hydrocarbon Dust Particles in the Extremely Metal-poor Galaxy Sextans A”, arXiv e-prints, Art. no. arXiv:2512.04060, 2025. doi:10.48550/arXiv.2512.04060
My feature object this month is the ultra-diffuse galaxy UDG 32 in Hydra, which can just be seen as a smudge to the lower right, although I have to admit that the more spectacular NGC 3314 in the upper left certainly lifts the image.
Ultra-diffuse galaxies (UDGs) are very faint, as this image clearly shows, much like dwarf galaxies. In fact, they might be consider as the same class of object given their similar masses, except that UDGs are the same size as galaxies like the Milky Way, and far from being dwarf in their physical extent. Their faintness arises from their sparse old stellar populations that contain 100—1000 times fewer stars than similar sized galaxies.
The cause of such large yet sparse galaxies remains an open question. They're a diverse group of objects with some comprised of almost nothing but dark matter and others having very little. The main contenders appear to be that UDGs are either failed galaxies or dwarf galaxies that have grown in size, possibly inflated by the action of supernovae. They may represent separate populations of both types of object. 1
There's suggestion that this month's feature, UDG 32, might have been formed from the stellar filaments from NGC 3314A, one of the pair of galaxies lending spice to this image, as it lies about 40 kpc away in its trail. 2
NGC 3314 is a pair of overlapping and non-interacting galaxies imaginatively designated as NGC 3314A, the foreground face-on spiral galaxy of about 16.8 kpc (55,000 light-years) diameter, and NGC 3314B, which is a much larger spiral galaxy whose diameter is about 83 kpc (270,000 light-years). The Hubble Space Telescope has an excellent close-up of the NGC 3314 pair and for their apparent sizes to be so close there must be a large distance between them, about 26 Mpc (85 million light-years). 3
The very nature of UDG 32 makes it unlikely to be a target for imagers, let alone visual observers, without the benefit of a very large telescope. However the NGC 3314 pairing is a possible target for both imaging and visual observation, although the latter will need a large scope. It will be challenging since it's faint and rises less than 10° above the horizon in the UK; those further south may have more luck.
James Whinfrey - Website Administrator.
References
Maria Luisa Buzzo, Duncan A Forbes, Thomas H Jarrett, Francine R Marleau, Pierre-Alain Duc, Jean P Brodie, Aaron J Romanowsky, Anna Ferré-Mateu, Michael Hilker, Jonah S Gannon, Joel Pfeffer, Lydia Haacke, “The multiple classes of ultra-diffuse galaxies: can we tell them apart?”, Monthly Notices of the Royal Astronomical Society, Volume 536, Issue 3, January 2025, Pages 2536–2557, https://doi.org/10.1093/mnras/stae2700
“Formation of an ultra-diffuse galaxy in the stellar filaments of NGC 3314A: Caught in the act?”, Enrichetta Iodice, Antonio La Marca, Michael Hilker, Michele Cantiello, Giuseppe D’Ago, Marco Gullieuszik, Marina Rejkuba, Magda Arnaboldi, Marilena Spavone, Chiara Spiniello, Duncan A. Forbes, Laura Greggio, Roberto Rampazzo, Steffen Mieske, Maurizio Paolillo, Pietro Schipani, A&A 652 L11 (2021), DOI: 10.1051/0004-6361/202141086
Data for the NGC 3314 pair of galaxies taken from NASA/IPAC Extragalactic Database (NED) on 27 January 2026. The dodgy maths of the conversions to light-years is my own.
This HII region in the northern part of Orion was discovered in 1939 by Harold and Charles Lower using an 8-inch f/1 Schmidt camera to make a 20 minute exposure on a photographic film. Brendan's SHO narrowband image above contains a total of 24 hours of exposure and a lot more detail.
As a consequence of the discovery this region is sometimes known as Lower's nebula. It was later catalogued as Cederblad 64 (Ced 64), then Siméis 34 (Sim 34) before reaching Stewart Sharpless' list as number 261 (Sh2-261). The same region later produced three bright nebulae designations in the Lynds' Catalogue of Bright Nebulae: LBN 862, LBN 863 and LBN 864.
Based on photographic plates, Stewart Sharpless estimated Sh2-261 to be 45 arc-minutes in diameter with an 'irregular' form in his 1959 paper 1. In this paper Sharpless considered this HII region to be associated with star HD 41997 which he estimated to be B8.5 class.
Radio measurements of CO emission appear to show that Sh2-261 lies beyond the western edge of the Gemini OB1 Molecular Cloud Complex 2 at a distance of about 1.89 kpc (6200 light-years) 3, although other estimates of its distance vary widely. As a result it's uncertain whether it lies in the Perseus arm of the galaxy or our own Orion spur.
HD 41997 (Gaia DR3 3345950879898371712) is now classified as an O7.5III star with a visual magnitude of 8.45 at a distance of around 1.75 kpc (5705 light-years) and it's a runaway 4 with significant peculiar velocity with respect to [its] environment having been ejected for wherever it formed.
Brendan's wonderful image captures the runaway right where you might expect it, below the high energy blue structure in the central hollow, just to the right of centre, from where it illuminates this whole region of dust and gas. The brightest region surrounding the star corresponds to LBN 864.
Unsurprisingly perhaps, according to The Night Sky Observer's Guide Volume 4, Sh2-261 is barely visible as a half-degree patch of very faint haze with a 16–18 inch telescope and, I assume, a good night sky. So, probably one for the imagers or an ambitious visual observer.
James Whinfrey - Website Administrator.
References
Sharpless, S., “A Catalogue of H II Regions.”, The Astrophysical Journal Supplement Series, vol. 4, IOP, p. 257, 1959. doi:10.1086/190049.
Wang, C., Yang, J., Xu, Y., Li, F., Su, Y., and Zhang, S., “Molecular Gas toward the Gemini OB1 Molecular Cloud Complex. I. Observation Data”, The Astrophysical Journal Supplement Series, vol. 230, no. 1, Art. no. 5, IOP, 2017. doi:10.3847/1538-4365/aa6c6b.
Foster, T. and Brunt, C. M., “A CGPS Look at the Spiral Structure of the Outer Milky Way. I. Distances and Velocities to Star-forming Regions”, The Astronomical Journal, vol. 150, no. 5, Art. no. 147, IOP, 2015. doi:10.1088/0004-6256/150/5/147.
Carretero-Castrillo, M., Ribó, M., and Paredes, J. M., “Galactic runaway O and Be stars found using Gaia DR3”, Astronomy and Astrophysics, vol. 679, Art. no. A109, EDP, 2023. doi:10.1051/0004-6361/202346613.
