NGC 6778 in Aquila

July 2025 - Nebula and Cluster of the Month

It can be difficult to believe, but by the end of July, mid-Britain has regained a small measure of Astronomical darkness. The nights are still short and bright, though, so every opportunity to observe should be seized.

Moving to centre-stage this month is the great northern sweep of the Milky Way through Cassiopeia, Cygnus and Aquila, bringing with it many open clusters and planetary nebulae.

Our object this month has the distinction of having two discoverers and consequently two entries in the NGC. John Herschel made the first known observation of this planetary nebula in 1825, his description being extremely small, stellar. This object entered the New General Catalogue in 1888 as NGC 6785.

In 1863, the object was observed by Albert Marth, discoverer of around 600 NGC objects. His position was slightly different from that of John Herschel, and became NGC 6778. Marth’s description was small, elongated, ill-defined disc. The positions listed in the original NGC are slightly different, but it is now recognised that these two objects are identical.

I shall refer to the object as NGC 6778, as this is how it is usually referenced. The reason why the later discovery should be preferred is simply because it has the lower NGC number, in turn because the original measurements gave this entry the westernmost Right Ascension.

Although small and not very bright (listings vary, but all are between magnitude 12.1 and 12.4), this planetary nebula has been the subject of much study because of its complex morphology and unusual chemical makeup.

The defining feature of NGC 6778 is its binary central star, which has an orbital period of just 3.68 hours. Such a short period indicates that the system underwent a common-envelope phase, during which material from the secondary star spiralled into the envelope of the primary, ejecting the outer layers and creating the nebula's present form¹. This interaction is believed to have generated the nebula’s bipolar lobes, bright knots, and high-speed jets.

Spectroscopic studies show collimated outflows moving at speeds of up to 100 km/s ². These jets disrupt the nebula’s equatorial ring, leading to a chaotic appearance filled with bright knots, filaments, and complex internal structures. Such features are often associated with close binary interactions, highlighting the role of binarity in shaping planetary nebulae³.

One of the most intriguing aspects of NGC 6778 is its abundance discrepancy factor (ADF). This factor compares abundances calculated from spectral recombination lines to those from collisionally excited lines. Typically, planetary nebulae show modest ADFs, but NGC 6778 exhibits an ADF averaging about ten times the normal values and peaking at nearly twenty times near its centre. Such extreme values imply chemically distinct regions within the nebula, potentially representing material ejected during a late thermal pulse or binary interaction event.

From the perspective of the visual observer, NGC 6778 is not particularly spectacular, but not difficult to see. An 8” or 10” (200–250mm) scope will show a very small, rounded disc. Under most conditions, an OIII filter (or failing that, a nebula filter) will be needed to distinguish it from a star.

A 12” or 14” (300–350mm) scope will reveal something of its elongated shape, and possibly even a glimpse of the central star. The magnitude of the central star is given various values in the sources (14.8, 16.9). Some observers report having glimpsed the star with 14” scopes, suggesting the true value is towards the brighter of the given values.

I observed the planetary with my 12” (300mm) Newtonian reflector in July 2014 under different magnifications. At x83, it was seen as a small disc. It was pretty faint but visible to direct vision with no OIII filter. x375 reveals a suggestion of elongation. It was seen to be brighter in the middle, but there was no real sign of a central star. There is some outer, fainter nebulosity. The OIII filter increases the object’s brightness but adds no details.

Pushing my scope, I increased the magnification to x450, the inner core of the nebula now being distinctly brighter than the outer envelope. At this magnification, the OIII filter made little difference and added nothing to the view.

Patrick Maloney

Object	RA	Dec	Type	Magnitude
NGC 6778 aka NGC 6785	19h 18m 25s	-01° 35’	Planetary nebula	12.4

References:

1. Miszalski, B., Jones, D., Rodríguez-Gil, P., et al. (2011) 'Binary central stars of planetary nebulae discovered through photometric variability', MNRAS, 413(2), pp. 1264–1278.
2. Corradi, R.L.M., Sabin, L., Miszalski, B., et al. (2011) 'Fast collimated outflows in planetary nebulae with close binary central stars', MNRAS, 410(2), pp. 1349–1358.
3. Jones, D. and Boffin, H.M.J. (2017) 'Binary stars as the key to understanding planetary nebulae', Nature Astronomy, 1, p. 117.

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.