NGC 40 in Cepheus
September 2025 - Nebula and Cluster of the Month
Our object for September is the planetary nebula NGC 40, in Cepheus. It presents something of a contrast between the descriptions given by early observers and those of the present.
It was first observed (quelle surprise) by William Herschel on the night of 25 November 1788. His description runs A star of ninth magnitude with a very faint, milky nebulosity. The star is either double or not round. Less than 1’ diameter.
He included it in his fourth class (‘planetary nebulae’) as 58 H.IV. It passed through various catalogues; in John Herschel’s Slough Observations of 1833 it is number 8, in John Herschel’s later General Catalogue it was number 20 and then in the New General Catalogue it became number 40. By the time it reached the NGC, the description had become Faint, very small, round, very suddenly much brighter in the middle. Star, mag 12 south preceding.
William Herschel’s description and that of the NGC are separated by 100 years during which time the object remained basically small and very faint. More recent observations tend to differ, as I will show later.
Physically, NGC 40 lies at a distance of about 1618 parsecs (5277 light-years)1, which gives it a true diameter of about one light-year. All pretty normal.
The nebula presents a barrel-shaped appearance, with the long axis aligned to the north-northeast. It has two bipolar lobes, probably caused by two separate mass ejections from the central star. The age of the nebula has been calculated as about 5,800 ± 600 years.2
The central star shines at magnitude 11.6, though this is slightly variable. The star bears the variable star designation of V400 Cep. It is a Wolf-Rayet star, rich in carbon and deficient in hydrogen.
Deep observations of the nebula with large telescopes reveal concentric haloes surrounding the central nebula while a much fainter outer spread of nebulosity extends up to four minutes in diameter, corresponding with a true diameter of about six light-years3.
NGC 40 has been intensively studied and presents a good model for the future of Sun-like stars. One day, our star will produce a planetary nebula like NGC 40, will evolve into a Wolf-Rayet star, then eventually the nebula will dissipate and the Sun will shrink to become a white dwarf and then very slowly cool down.
On now to modern observations. Whilst the early observers called NGC 40 small and faint, that does not appear to be the prevailing opinion of more recent observers. Maybe it’s a matter of expectation. If, like me, you’re used to hunting down virtually stellar, 14th magnitude planetaries, this object comes as a real contrast.
The magnitude quoted for this object varies considerably. Some sources insist on 12.4 whilst others prefer 10.6 or 10.7. It tends to be the older sources that give the lower value. I’m not suggesting that this object has brightened, just that some planetary nebula magnitudes have been reappraised. It is my experience that NGC 40 is much brighter than 12th magnitude.
The late Steve Coe, in his published observations, describes NGC 40 as bright and large
.
In The Night Sky Observer’s Guide Volume 1, NGC 40 is described as viewed through telescopes of the 12”–14” class and telescopes of the 16”–18” class.
The 12”–14” observation describes the nebula as bright and ‘a spectacular object’.
I would concur. My own observation (shown below) is accompanied by a very enthusiastic description: Very, very bright. Very large and immediately obvious even at x83. Pretty circular, but not evenly bright. Brighter arcs were seen on the southeast and northwest edges. Some darker patches were also noted. A small extension away from the disc was noted to the northeast, associated with a small star. The OIII filter adds nothing, merely removing the brilliant central star from view. A lovely object giving a different view with every power.
NGC 40 is well worth a look this September. It presents a wealth of detail under sufficient power. Don’t be afraid of using high power on planetary nebulae. The old adage that wide-field is best for deep-sky is an old wives’ tale and does not apply to most deep-sky objects. Greater magnification gives greater contrast and is a must for planetary nebula observing.
Whilst you are gazing at it, it is interesting to bear in mind that you are looking at the probable future of our own Sun.
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| NGC 40 | 0h 13m 01s | +72° 31’ | Planetary nebula | 10.7 |
References:
- Frew, D. J., Parker, Q. A., & Bojičić, I. S. (2016) ‘The Hα surface brightness–radius relation’, MNRAS, 455, pp. 1459–1488.
- Meaburn, J., López, J. A., Bryce, M., & Redman, M. P. (1996) ‘Kinematics and evolution of planetary nebulae with Wolf–Rayet nuclei’, MNRAS, 282, pp. 1313–1324.
- Chu, Y.-H., Jacoby, G. H., & Arendt, R. (1987) ‘Planetary nebula halos and progenitor mass loss’, ApJ, 322, pp. 899–904.
