DSO of the Month :
Messier 46
AKA: NGC 2437; Collinder 159
Position: 07 hr 41 min 46 sec −14 degrees 48 min 36 sec
Due south at 22:00 (GMT) on 15 February 2022
Messier 46 and NGC 2438
Image: Les Brand, HAS member. Used with permission.
It is always nice to get two DSOs for your money, and Messier 46 in the constellation of Puppis is a good example as it contains the small planetary nebula NGC 2438. If you include Messier 47 which is about a degree to the right or west from Messier 46, that makes it three objects for your money. It takes about five minutes to arrive at Messier 46 if you have Messier 47 in your view and you leave the telescope in the same position. Messier 46 was found by Charles Messier in 1771. It is a splendid bright cluster with a magnitude of 6.1 and it is wide, being about 25 arcminutes in extent or about three-quarters of the size of the full moon. It contains a large number of stars, 500 all told, and is about 250 million years old, which makes it older than Messier 45 and younger than Messier 44. Its distance was rather uncertain until recently but it has now been measured as being almost exactly 5,430 light years away.
The planetary nebula NGC 2438 is just north of the centre of the cluster and looks like a small multi-coloured ring. At magnitude 10.8 and with a diameter of only 1 arcminute, you will need a fairly large telescope and high magnification to see it with the eye, but as the above image demonstrates, it shows up well in images. It was first observed by William Herschel on 19 March 1786, fifteen years after Messier found the cluster. It consists of two shells moving outwards at 37 km/s or about 83,000 mph. The main nebula was formed about 4,000 years ago and the central star is a white dwarf with a temperature of 75,000K, one of the hottest stars known. It is a foreground object to Messier 46 as it is only 1,370 light years distant, barely a quarter of the distance of Messier 46.
To find Messier 46, it forms roughly a right-angle triangle with Procyon above it and Sirius to the right of it. It is also on a line between Sirius and Alphard (Alpha Hydrae), about a third of the way along from Sirius, it is also directly beneath the rather dim (magnitude 3.9) Alpha Monocerotis. It is bright enough to show up well even in small binoculars.
HaRGB image of NGC 2348
Image by Göran Nilsson & the Liverpool Telescope
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