Object of the Week - July 8th, 2018 M 17 - The Swan Nebula
M17
NGC 6618
Sh 2-45
R.A.: 18h20m48.0s
Dec.: -16°11'00"
Size 11.0'
Mag: 6
Somewhat to my surprise M17 has never been covered in the OOTW before so here is its chance. First discovered by Philippe Loys de Cheseaux in 1746 M17 was
independently rediscovered by Messier in 1764, de Cheseaux apparently never published his observation.so Messier did not know about it. De Cheseaux observed from near Lausanne in Switzerland and used a referactor of 4.2m FL and a 60cm FL Gregorian reflector. Unfortunately De Cheseaux suffered from ill health and died at 33 or he might have been better known.
The nebula has many nick names including the Swan nebula, the Omega nebula, the Checkmark nebula, the Horseshoe nebula and the Lobster nebula.
I prefer the Swan nebula nickname as it reminds me of the Swan from Sibelius epic tone poem the Swan of Tuonela which glides on the dark river round the
isle of the dead (somewhat morbid I guess). It was John Herschel with his classical bent that gave it the name the Omega nebula and the Swan nickname
was apparently first coined by George Chambers in his 1889 Celestial Handbook. Easily visible in a small telescope and by naked eye from dark southern
locations M17 is an HII region and cluster lying about 5500 light years away and the core of the nebula spans about 15 light years across. The star cluster (also known as NGC 6618)
contains maybe 35 hot young stars. There are however as many as 800 stars in the centre of the nebula. It is not clear if star formation is still active in the region or recently finished.
M17 is a blister nebula on the side of a massive molecular cloud, not dissimilar to the Orion nebula, to which William Herschel with much prescience compared it. Right from the start there were many historical drawings of M17, some of which even look like the nebula 😊 There is an interesting historical article by Holden on this at https://en.wikisource.org/wiki/Popul...in_Sagittarius. Unfortunately from my northern
location M17 never rises that high, in fact never really gets above 25 degrees altitude so I never get the best views. Perhaps not surprisingly under these
conditions I find that a UHC or even an OIII filter will enhance the views. It does not help either that when M17 is at its highest it never really gets dark
up here either. It is however one of my favourite nebulae. The challenges here are not to see the main nebulae but the large amounts of nebulosity around it.
As well as the main nebula there are two other IC nebulae here (IC 4706 and 4707) discovered by Barnard with a 10” refractor but he appears to have given
Dreyer the wrong positions. He did say he saw two stars involved with nebulosity here which appears, according to Corwin, to be correct if we assume that
what he saw are the stars BD -16 4811 and -16 4812.They are both part of the same nebula so perhaps we should lose the IC 4707 designation and call the
whole thing IC 4706. In the classic Victorian observing guides by Smyth and Webb M17 appears in the constellation of Clypeus Sobieski (now Scutum) rather
than Sagittarius. Both the Herschels did spend quite a lot of time observing it from the UK.
Lassells's drawing of M17. It is unclear to me whether these drawings were done by Lassell or his daughter Caroline as she certainly made at least one attributed to him.
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