DSO of the Month
Caldwell 39 (NGC 2392)
Due south at 21.53 (GMT) on 14 February
Position: 07 hr 29 min 20 deg 55 min
Image by Martin Gill
Planetary nebulae are often overlooked. They are usually not that bright and they are small compared with star clusters or galaxies so they need a high magnification. However they do have one major advantage, light pollution does not affect them as much as galaxies and you can also use a filter: especially an O III filter to enhance the image. There are a few planetary nebulae that every amateur observer should aim to see. One is M57, the Ring Nebula, in Lyra and other is M27, the Dumbbell Nebula, in Vulpecula which is unusually large for a planetary nebula. However these two nebulae do not explain the puzzling name for this type of DSO. What is planetary about the planetary nebulae? They are not in the solar system nor are they exo-planets. They are the remains of stars similar to our own sun which have blown off their outer gas at the end of their life as an active star. This gas is then excited by ultraviolet radiation given off by the often invisible stellar remnant and glows like a neon lamp. The planetary nebula is short-lived in astronomical terms, usually only surviving for 20,000 years or so, which is why there are so few of them. In the case of fairly regular planetary nebulae, they have the appearance of a small disk. William Herschel who first discovered the planetary nebulae, including Caldwell 39 in 1787, thought they looked like planets. Caldwell 39 is about 6,520 light years away according to the latest Gaia measurements and is about 10,000 years old.
In a small telescope Caldwell 39 looks like a small blue disk with an apparent magnitude of 10 so you need a reasonable-sized telescope, at least four or five inches in aperture. In a larger telescope Caldwell 39 has a feathery or fuzzy edge round it, so it looks like a face in a fur-lined parka hood (if you have a good imagination). Hence it was called the Eskimo Nebula from around the 1950s or so. While this name remains popular (for example it is still used by NASA), the alternative name the Clownface Nebula has now been created. To find Caldwell 39, first find Castor and Pollux, the two bright stars (and “heads”) of the Twins (Gemini) by following a line from Rigel to Betelgeuse upwards (to the north-east). Pollux (mag. 1.1) is the left-hand (or eastmost) of the pair. Going back towards Betelgeuse, you first encounter the relatively dim Nu Geminorum (mag. 4.2) and then about the same distance again from Pollux, the brighter star Delta Geminorum or Wasat (mag. 3.5). If you “see” the Twins as two stick men, Wasat is the left-hand twin’s waist. Caldwell 39 is roughly two degrees to the left of Wasat and slightly lower. There is a 8.2 mag star (HD 59087) just above Caldwell 39 as seen in the above picture. In a small telescope, Caldwell 39 appears as a small oval blue disk.
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