Birds Of The Mist

The 'Swan Nebula', M17 in Sagittarius is well known and many observers have savoured its delightful swan-like form. The other is the ‘Eagle Nebula’, M16 in Serpens, a faint and large gaseous glow that appears stunning to the camera but often eludes the casual observer. Both these gaseous clouds are associated with young star clusters.

M17 can be found a little over 2° to the south west of gamma Scuti, a magnitude 4.7 A-type giant. It can be seen with the naked eye under the right conditions. Discovered by Philippe Loys de Cheseaux around 1745, he saw it as:

“…another nebula, which has never been observed. It is of a completely different shape than the others; It has perfectly the form of a ray, or of a tail of a comet, of 7’ length and 2’ broadth; its sides are exactly parallel and rather well terminated, as are its two ends. Its middle is whiter than the borders…”

Charles Messier re-discovered it on June 3, 1764, writing in 1771:

“In the same night [June 3 to 4, 1764], I have discovered at little distance of the cluster of stars of which I just have told, a train of light of five or six minutes of arc in extension, in the shape of a spindle, & in almost the same [shape] as that in the girdle of Andromeda [M31]; but of a very faint light, not containing any star; one can see two of them nearby which are telescopic & placed parallel to the Equator: in a good sky one perceives very well that nebula with an ordinary [non-achromatic] refractor of 3 feet & a half [FL]”

Sir William Herschel recorded it well in 1784 as:

“A wonderful nebula. Very much extended, with a hook on the preceding [Western] side; the nebulosity of the milky kind; several stars visible in it, but they seem to have no connection with the nebula, which is far more distant. I saw it only through short intervals of flying clouds and haziness; but the extent of the light including the hook is above 10'. I suspect besides, that on the following [Eastern] side it goes on much farther and diffuses itself towards the north and south. It is not of equal brightness throughout and has one or more places where the milky nebulosity seems to degenerate into the resolvable [mottled] kind; such a one is that just following the hook towards the north.”

M17 is known under a number of names, the ‘Swan’, ‘Horseshoe’, ‘Omega’ and even the ‘Lobster Nebula’ in the southern hemisphere. The brightest region shows as a figure 2, hence the title ‘Swan’ though fainter extensions illuminate the full figures of the other titles above.

M17 is a glorious cloud in any size telescope, its hot ionised gas illuminated by the ultraviolet radiation of very young stars that have just formed or may in fact still be forming. The youngest is around 1,000,000 years old. Around 35 cluster stars have been detected within this cloud, hidden from the eye by the cloaking gas and dust. Each star is around 6 times hotter and 30 times more massive than our Sun.

Dark obscuring material permeates M17, easily seen throughout the body of the ‘Swan’ and large telescopes draw out stunning detail to excite the observer.

The mass of this wonderful cloud has been estimated around 800 solar masses contained in an area spanning 40 light years or more. Lying in the Sagittarius-Carina spiral arm of our Galaxy, M17 resides around 5500 light years distant.

Our next celestial bird lies in the same region as M17 and may be connected. M16 or to give it its affectionate name of the ‘Eagle Nebula’ is a combination of bright open cluster associated with an extensive cloud of faint gas (IC 4703). Discovered in 1745 by Cheseaux who saw the cluster only, it was re-discovered by Charles Messier on June 3, 1764:

“I have discovered a cluster of small stars, mixed with a faint light, near the tail of Serpens, at little distance from the parallel of the star Zeta of that constellation: this cluster may have 8 minutes of arc in extension: with a weak refractor, these stars appear in the form of a nebula; but when employing a good instrument one distinguishes these stars, & one remarkes in addition a nebulosity which contains three of these stars."

M16 resides in the constellation of Serpens where gas and dust has entered an aggressive cycle of star formation. Having formed the open cluster NGC 6611 and its retinue of O6 stars, the faint cloud IC 4703 is still producing stars within its gaseous nursery. The Hubble Space Telescope observed this region a while ago, the images bringing alive this turbulent behaviour in the form of blistered glowing gas and highlighting great pillars of dark hydrogen clouds that are feeding this breeding cycle.

The current cluster is around 5000 to 6000 years old and if the distance of 7000 light years is fairly accurate, the ‘Eagle’ extends over an area 70 x 55 light years. Visually, the gas here is very difficult to see owing to its low surface brightness. The Herschels did not spot it and if indeed Messier saw it, it may be testimony to the value of a small telescope and its low power and wider field of view in spotting these very faint though large gaseous clouds. The use of an O-III nebula filter would help and perhaps dark obscuring dust and gas, including the great pillars, may be seen with care in larger telescopes. Photography does better here, E. Barnard the first to photograph the region in 1895.

M16 lies around 2° north of M17 and as mentioned earlier, may be connected to the same molecular cloud. This region of the sky is a delight to roam in, many open clusters and faint planetary nebulae await to be discovered by the observer.

Clear skies and good hunting ~ CosMos