One of a group of open star clusters that range through the northern parts of Puppis and westward into neighbouring Canis Major. These clusters are not directly related to one another, and lie at different distances from the Sun on the same line of sight. Some are much more distant than M46, while others (like its bright neighbour M47) are rather closer to the Solar System.
An open star cluster in the northern reaches of the constellationPuppis, falling almost directly eastwards of the brilliantstarSirius in neighbouring Canis Major. In the skies of Earth, M46 appears within about a degree of another open cluster, M47, but this is a line-of-sight effect: at a distance of about 4,900 light years, M46 is more than three times further into space than its apparent neighbour M47.
The distinctive circular shape that seems to lie within the northern parts of M46 is not in fact related to the cluster. This is the planetary nebulaNGC 2438, which lies between the Sun and M46, and so appears on the same line of sight as the cluster beyond. Imagery provided by Aladin sky atlas
M46 is also a much richer cluster than M47: it contains more than three thousand stars, making it some much more populous than M47. The stars of this cluster are spread across a region about thirty light years in diameter, with little obvious concentration towards the centre of the cluster.
A distinctive feature of M46 is a planetary nebula, a colourful double ring of material that appears to fall within the northern edges of the cluster. As with M47, this is another line-of-sight effect: the nebula, NGC 2438, is uncertain, but current estimates suggest that it is about 1,400 light years from the Solar System, placing it far closer than the clusteredstars of M46 that form its backdrop.
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