M46

An open star cluster in the northern reaches of the constellation Puppis, falling almost directly eastwards of the brilliant star Sirius 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.

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 actually only 2,900 light years from the Solar System, placing it far closer than the clustered stars of M46 that form its backdrop.