Messier 17, the "Swan Nebula"
Messier 17 is a HII region in the northern part of the constellation Sagittarius, right at the border to the constellations Shield and Serpent Bearer. The nebula
was discovered in 1745 by the Swiss astronomer Philippe Loys de Chéseaux.
Despite a declination of -16° Messier 17 still rises 20° to 25° above the southern horizon in central Europe, so that it can be admired in its full splendor even
from our latitudes. Due to its shape the nebula is known by many names: Swan Nebula or Omega Nebula are only two of them. In English speaking countries
it is also called "Checkmark Nebula", "Horseshoe Nebula" or "Lobster Nebula". The beginning was made by Sir John Herschel, when he compared the nebula
complex with the Greek letter "Omega".
The distance to Messier 17 is given as 5,000 to 6,000 light-years. The Omega Nebula itself is only part of a giant molecular cloud (GMC) and combines about
800 solar masses at an extent of about 15 light-years. The GMC, on the other hand, is 40 light-years across and has a mass of 30,000 solar masses. It belongs
to the brightest and most active star forming regions in our Milky Way and has undergone three phases of star formation. In the first phase, about 2,000 stars
were formed between 2 and 5 million years ago. In the process, the gas masses were strongly compressed, and so a second phase was not long in coming,
which produced up to 12,000 stars and has still not come to a complete stop. A third wave of star formation is currently taking place in the outer regions of
Messier 17. There, about 1,000 new stars have been born so far.
During the second phase the massive star cluster NGC 6818 was formed, but unfortunately it is hidden in the dense dust and gas masses of Messier 17. Therefore
it is not visible in the visual spectral range, but stands out mainly on images in the infrared. More than 800 stars are attributed to the cluster, among them 100
stars of spectral type B and 9 stars of spectral type O with surface temperatures above 50,000 Kelvin. These stars excite parts of the molecular cloud to glow.
NGC 6818 is one of the youngest known star clusters with an age of only 1 million years.
South of Messier 17 two bright stars are found (in the drawing at the upper edge). It is currently assumed that both stars are equidistant to Messier 17 and therefore
originated in this nebular complex. The left one of the two stars is named "HD 168607" and belongs to the group of "LBV" - the "Luminous Blue Variables". It
has an absolute brightness of -8.4mag and shines about 250,000 times brighter than our sun. The surface temperature is 9,300 Kelvin. An even brighter example
is "HD 168625", the right one of the two stars. With a temperature of 14,000 Kelvin the star shines 380,000 times as bright as our Sun. Around the star several
nebular arcs were found - almost identical to those of the progenitor star of the supernova SN 1987A. Therefore it can be assumed that HD 168625 will also
become a supernova in the near future.
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On a really dark summer night up on the mountain the observation of Messier 17 with my 20 inch Dobson is hard to beat in beauty. The whole field of view is filled
with luminous nebular masses, extremely structured and reaching far beyond the brightest part of the swan. Again, I don't want to get much into words, because a
drawing simply says much more than words ever could. In any case, the many bright structures in the swan's body and the large, pitch-black dark cloud west of the
neck are exciting. Two smaller dark clouds in the southern areas of the nebula are also visible.
With a little imagination, you can very nicely picture the swan swimming majestically on a lake with parts of its plumage and the flashing sun rays reflected in the
water in the form of the many bright stars. Simply beautiful...
