M46 in Puppis is one of my favorite open clusters and a striking sight through just about any telescope. More than 500 stars are crammed into an area just a Moon's diameter across, creating one of the most jam-packed throngs in the winter sky. 

M46 was discovered by Charles Messier on February 19, 1771, only three days after he had published the first edition of his catalog covering M's 1 through 45. Of his new catch, Messier wrote "A cluster of very small stars, between the head of the Great Dog and the two hind feet of the Unicorn; one cannot see these stars but with a good refractor." But when William Herschel gazed upon M46 through his 18.7-inch reflector on March 19, 1786, he saw something else, something Messier apparently had missed or overlooked, floating among the cluster stars north of the group's center. He probably thought to himself, "That's not a star at all. That's a tiny disk of light." Herschel included his find as H-IV-39, the 39th planetary nebula in his catalog, but today, we know it best as NGC 2438.

While NGC 2438 may look like it belongs to M46, in reality it is much closer to Earth.  The most recent study of the distance to NGC 2438 came last year.  In a paper entitled Searching for central stars of planetary nebulae in Gaia DR2 [Astronomy & Astrophysics. 616], authors N. Chornay and N. A. Walton examined the Gaia spacecraft’s Data Release 2.  Gaia is the European Space Agency’s astrometric spacecraft designed to accurately measure positions, distances and motions of stars with unparalleled accuracy.  Their study pegged the distance to NGC 2438 at 1,376 light years.

In a 2013 study, The Open Cluster NGC 2437 (Messier 46) [Publications of the Astronomical Society of the Pacific, Volume 125, Number 924], author T.J. Davidge placed M46 at 4,660 light years from Earth.

Earlier studies that led to same conclusion of different distances compared the spectra of the planetary with those of stars in M46.  These showed that both M46 and NGC 2438 were moving away from the solar system, but at two different speeds.  Were the planetary and cluster physically associated, they would be moving through space at the same speed.

M46 and NGC 2438 are easiest to find by dropping 5° due south from 4th-magnitude Alpha (α) Monocerotis, the brightest star in Monoceros.  Trying to find that star is its own challenge, especially with less-than-perfect sky conditions.  Fortunately, a line extended from Sirius [Alpha (α) Canis Majoris] through Gamma (γ) Canis Majoris for 11° to the east points right at Alpha Mon.  Use your binoculars or finderscope to trace the line, and then shift southward to find M46 within a slender stellar triangle.

Incidentally, you will also find another open cluster, M47, at the triangle's western tip, just 1½° west of M46.  Both clusters make a spectacular couple in binoculars and rich-field telescopes. Again, however, they are nowhere near each other in space. M47 is 1,624 light years away.

As striking as that low-power view is, NGC 2438 will take at least 150x to tell it apart from just another cluster star. Focus your attention on the stars in the northern part of the cluster, keeping an eye out for a tiny, softly glowing disk of greenish light. That will be the 11th-magnitude planetary.  Through my 8-inch (20cm) reflector at 203x and with an oxygen-III filter in place, the nebula's ring shape is clearly evident and appears very slightly oval.  Removing the filter and using averted vision adds a 13th-magnitude star within the ring, just slightly offset to the northwest of center.  Don't be fooled into thinking that you are seeing the nebula's forbearer, however.  NGC 2438's actual central star barely cracks 18th magnitude.  The dim sun we are seeing is most likely a distant member of M46.  Another of M46's stars, an 11th-magnitude point, appears to just brush the nebula's southeastern edge.

Good luck with this month's challenge! And be sure to post your results in this column's discussion forum.

Remember that half of the fun is the thrill of the chase.  Game on!