NGC 6369 in Ophiuchus
June 2025 - Nebula and Cluster of the Month
June, in addition to providing us with the brightest nights of the year, is also the month when clusters and nebulae become more easily available. The rich star fields of the Milky Way begin to swing back into prominence, and with them a profusion of Galactic treats.
Amongst the constellations most blessed with these objects is Ophiuchus, rather low in British skies, which means it tends to be overlooked somewhat by visual observers, especially those in light-polluted areas (which is just about everybody these days).
Nevertheless, Ophiuchus is well worth a long study, and this month we’re going to look at one of its delights, the planetary nebula NGC 6369.
As with most of the objects presented here, this one was discovered by William Herschel, who first saw it on 21 May 1784. On that night, he described it as Pretty bright, round, pretty well-defined planetary disc, 30 or 40” diameter.
Recent research into this planetary nebula seems to be largely absent. Distance estimates range from 2000 to 4000 light-years (or even 5000 in some cases). This indicates a true diameter of the object as anywhere between 0.3 and 0.6 light-years (or even 0.7 light-years!). These are fairly typical values for planetary nebulae, if not very precise.
The central star, which shines at magnitude 15.6, is a challenging object for visual observers. Its spectrum is classed as [WO3]. The straight brackets indicate that the spectrum resembles that of a WO3 star, but is not the spectrum of a WO3 star. A WO3 star is an oxygen-rich Wolf-Rayet star (supermassive and superluminous). The central stars of planetary nebulae are white dwarfs, typically with lower masses than the Sun, but considerably hotter and brighter.
The central star appears to be a binary and was once suspected of being a pulsating variable. It has the variable-star designation V2310 Ophiuchi. Examination by the Kepler Space Telescope revealed no variations1.
Visually speaking, Herschel’s description still holds true. My observation was made with a 12” (300mm) Newtonian telescope from light-polluted skies. I found that I could not see the planetary at all without the use of an OIII filter. These filters are invaluable and are an essential piece of kit for any planetary nebula hunter. If you live further south and/or have less light pollution than me to your south, you may well be able to see it without a filter. It is fairly bright at magnitude 11.4.
Once the OIII filter was in place, the object was very clear to see. The nebula presents itself as small, quite bright and circular. The surface brightness is uneven, being noticeably brighter along the north to northwest edge. Other observers have noted this, and white light images bear this out. Averted vision gives a suggestion of annularity, and there was even an occasional hint of the central star (which I admit came as something of a surprise).
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| NGC 6369 | 17h 29m 20s | -23° 45’ | Planetary nebula | 11.4 |
References:
- Jacoby, George H.; Hillwig, Todd C.; Jones, David; Martin, Kayla; De Marco, Orsola; Kronberger, Matthias; Hurowitz, Jonathan L.; Crocker, Alison F.; Dey, Josh (2021). ‘Binary central stars of planetary nebulae identified with Kepler/K2’. Monthly Notices of the Royal Astronomical Society. 506
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.
