Cluster M 46 with Planetary Nebula NGC 2438

Messier 46 + NGC 2438
Messier 46 + NGC 2438: Open cluster Messier 46 with planetary nebula NGC 2438; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 35+20+20+20 min LRGB; Bernese Highlands; © 2011 Radek Chromik

Open Cluster M 46

Messier 46 is a rich open star cluster in a beautiful location in the Milky Way ribbon on the stern of the ship Argo Navis (Puppis). Charles Messer discovered this star cluster in 1771. However, he was only able to partially dissolve the cluster and believed that the group of stars contained a little nebula. An indication that the optical performance of its telescopes was not exactly outstanding from today's point of view.

At least 150 stars (Burnham) between 10th and 13th magnitude have been recognized as group members, but there are probably around 500. The brightest stars in the cluster are blue giants near the spectral class A0, each with about 100 times the solar luminosity. The distances of the cluster vary from 3200 to 6000 light years. The total integrated brightness of the cluster is about 6.6 mag and the apparent extent 25 to 30 arc minutes.

Revised+Historic NGC/IC, Version 22/9, © Dr. Wolfgang Steinicke [277]
DesignationNGC 2437
TypeOCL (III2m)
Right Ascension07h 41m 46.8s
Declination-14° 48' 36"
Diameter20 arcmin
Visual magnitude6.1 mag
Metric Distance1.375 kpc
Dreyer Description!, Cl, vB, vRi, vL, inv planetary
Identification, RemarksM 46, OCL 601

Planetary Nebula NGC 2438

NGC 2438
NGC 2438: Planetary nebula NGC 2438; 500/2500mm Newton + SBIG ST-6; Observatory Bülach; © 8. 2. 1997 Stefan Meister

A special jewel in the open star cluster M 46 is the planetary nebula NGC 2438, which appears to the observer within the cluster, about 7 arc minutes from the center. The PN was first recorded by Sir William Herschel.

A membership of the PN in the star cluster is excluded, since the two have different radial velocities. The distance of the PN is given as 3300 light years, which gives a true diameter of 70'000 AU or a little more than one light year. It appears that the PN is in front of the actual cluster, but the distance between the two objects is not known accurately enough to be definite. The central star appears visually about 17.5 mag bright, but is relatively easy to photograph because of its strong radiation in the blue and UV range. The calculated surface temperature is around 75'000 Kelvin.

«Strasbourg-ESO Catalogue of Galactic Planetary Nebulae» Acker et al., 1992 [141]
DesignationsPN G231.8+04.1: NGC 2438, PK 231+04.2, ARO 46, Sa 2- 13, VV 43, VV' 69
Right Ascension (J2000.0)07h 41m 50s
Declination (J2000.0)-14° 44' 08"
Dimensions 64." (optical), 80." (radio)
Distance 2.0 kpc
Radial Velocity+74.0 km/s ± 4.0 km/s
Expansion Velocity 22.4 km/s (O-III), 20.0 km/s (N-II)
C-Star DesignationsAG82 87
C-Star Magnitude17.7 mag (B filter)
DiscovererHERSCHEL 1827

Planetary Nebula Minkovski 1-18

Minkovski 1-188
Minkovski 1-188: Section of the STScI Digitized Sky Survey [147]

The faint planetary nebula Minkovski 1-18 (M 1-18, PK 231+4.1) lies even further north of M 46. It was discovered in 1946 by the German-American astronomer Rudolph Minkowski at the Mount Wilson Observatory. [397] With an integrated brightness of 15.0 mag and an apparent diameter of 32 arc seconds, the PN is much more inconspicuous than NGC 2438. The distance is 4433 pc (14'460 light years) and it moves away with 18 km/s. [145]

«Strasbourg-ESO Catalogue of Galactic Planetary Nebulae» Acker et al., 1992 [141]
DesignationsPN G231.4+04.3: M 1-18, PK 231+04.1, Sa 2- 15, VV 44, VV' 70
Right Ascension (J2000.0)07h 42m 03s
Declination (J2000.0)-14° 21' 12"
Dimensions 30." (optical)
Radial Velocity+18.0 km/s ± 25.0 km/s
Expansion Velocity 13. km/s (O-III)
C-Star DesignationsAG82 89
C-Star Magnitude20.9 mag (B filter)
DiscovererMINKOWSKI 1946

Finder Chart

To the east of the head of the Canis Maior about the same height as γ CMa and about five degrees south of α Monocerotis lies the bright, open star cluster M 47, which is easily visible to the naked eye on clear nights. About one degree to the east of this is M 46, which differs from M 47 in that it has considerably more, but finer stars. At the northern edge of M 46 lies the planetary nebula NGC 2438. For the precise identification of M1-18 one uses the excerpt from the STScI Digitized Sky Survey.

Chart Cluster M 46 with Planetary Nebula NGC 2438
Cluster M 46 with Planetary Nebula NGC 2438 in constellation Puppis. Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 2438
NGC 2438: Pencil drawing; 200mm SCT; © 20. 2. 1995 Bruno Bleiker

200 mm aperture: Although the planetary nebula NGC 2438 is quite faint, it is immediately noticeable in the eyepiece. However, the property remains more or less structureless. A dilution of the gas is very faintly visible in the center. The central star with 17.5 mag is not visually recognizable. The use of the OIII filter did not bring any significant improvement. The nebula stands out more strongly from the surroundings, but structures hardly emerge. The contrasts in the upper drawing are exaggerated compared to the view in the telescope. Surrounding stars are not shown above for reasons of time and the sheer number. — 1995, Bruno Bleiker

NGC 2438
NGC 2438: Pencil drawing; 200mm SCT; © 4. 1. 1997 Philipp Reza Heck

200 mm aperture: The first thing you notice is the round, slightly oval shape of this small ring nebula. The wide ring is evenly bright and easily recognizable without a nebula filter. Inside the ring, the nebula is a little weaker. The visual impression is accurately recorded in the Vorontsov-Velyaminov system as a PN with a ring structure and a regular disc (IV + II). Around the center of the nebula, with direct vision, one recognizes a stellar brightening that could be assumed to be the central star. The object's brightness was clearly weaker than the nearby star in the south-south-east. The central star of NGC 2438 has a brightness of 17.5 mag, which is beyond the visual possibilities of an 8-inch telescope. It is one of the innumerable stars of the open star cluster M46. — 1997, Philipp Reza Heck

320 mm aperture: The PN is clearly visible with and without an O-III filter. Without a filter, the view of the beautiful little ring is much more impressive among all the sparkling stars of the open star cluster. With the O-III filter, the stars are weakened too much. — 5. 2. 2022, Ibergeregg, Stefans 320 mm f/5 Dobsonian, Bernd Nies

More Objects Nearby (±15°)


141Strasbourg-ESO Catalogue of Galactic Planetary Nebulae; A. Acker, F. Ochsenbein, B. Stenholm, R. Tylenda, J. Marcout, C. Schohn; European Southern Observatory; ISBN 3-923524-41-2 (1992); (2021-02-18)
145SIMBAD astronomical database;
147Aladin Lite; (2020-12-23)
149SkySafari 6 Pro, Simulation Curriculum;
160The STScI Digitized Sky Survey;
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; (2021-02-17)
397«New Emission Nebulae» R. Minkowski; Publications of the Astronomical Society of the Pacific, Volume 58, Number 344, 1946; DOI:10.1086/125855