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.

A photograph of the fateful page from the Skalnate Pleso Atlas Coeli, a celestial atlas showing the position of NGC 4361 as a green disk in the constellation of Corvus.
A photograph of the fateful page from the Skalnate Pleso Atlas Coeli provided by the author.

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!)

An image of planetary nebula NGC 4361 in Corvus courtesy of Adam Block/Mount Lemmon SkyCenter/University of Arizona.
An image of An image of planetary nebula NGC 4361 in Corvus courtesy of Adam Block/Mount Lemmon SkyCenter/University of Arizona.

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.

A sketch of planetary nebula NGC 4361 in Corvus by Patrick Maloney through his 12-inch newtonian telescope at x150 magnification with an O-III filter.
A sketch of planetary nebula NGC 4361 in Corvus by Patrick Maloney through his 12-inch newtonian telescope at x150 magnification with an O-III filter.

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.