20 September 2015



The skies have been more cooperative lately and have allowed a few days of imaging. With the official start of Autumn later this week, I decided to image a few objects that I normally would have gotten a couple of months ago, except for the weather. Two of these objects are M 16 (NGC 6611) and M 17 (NGC 6618), located along the southern Milky Way stream in the constellations Sagittatius and Serpens Cauda. These objects are setting early in the evening now, so this was about my last chance to image them this year. I had imaged these objects previously in 2013. Since a number of the components of the SOCO imaging system (particularly the RGB filter set) have changed since then, I thought it would be a good opportunity to re-image these objects. I've previoiusly compared weight factors for use with the old (Astrodon) and new (Baader) filter set derived from G2V star measurements. This would provide an opportunity to compare the weight factors derived from an independent data source.

Figure 1 shows a map of the northern Sagittarius and western Serpens Cauda region that includes M 16 and M 17. In binoculars, these objects can easily be spotted by panning upward from M 8 (NGC 6530/6523), the Lagoon Nebula, which is a naked-eye object. This whole region is filled with interesting deep-sky objects, a fact that adds to my disappointment in not having had clear skies to do more imaging here this summer. But I suppose I shouldn't cry over spilled Milky Way.


Figure 1. The region of northern Sagittarius and western Serpens containing M 16 (NGC 6611) and M 17 (NGC 6618).
Source: Cartes du Ciel.


Both M 16 and M 17 combine open clusters with emission nebulas. My image of M 16 is presented in Figure 2. Technically, the designation M 16 should apply only to the open cluster component of the object, because Charles Messier never really saw the nebular component through his telescopes. While Messier described the cluster in 1764, the nebula was not described until the late nineteenth century where it was found in photographs by E. E. Barnard (1895). Technically, the nebula has the designation IC 4703. However, it is best known by its common name, the Eagle Nebula (due to its resemblence in some exposures to a bird with outstretched wings).


Figure 2. M 16 (NGC 6611), the Eagle Nebula in Serpens Cauda.


The Eagle Nebula is a vast (around 35 LY across) region of glowing Hα and Hβ emissions that Robert Burnham Jr. described as "one of the great masterworks of the heavens". The open cluster of stars embedded in it is very young (only around 1 to 2 million years old) and contains no less than 376 stars packed into a an area approximately 15 arc-min across. The formation of new stars continues in the nebula in regions of dense condensations of gas and dust.

The most prominent of these regions was made famous by images acquired by NASA's Hubble Space Telescope. This is the "Pillars of Creation" region located in the center of the nebula. It is visible in the low-contrast image in Figure 2. I provide a more high-contrast version of this region on the left side of Figure 3. NASA's iconic image is shown on the right side of Figure 3. It should be noted that my version is a "true-color" version, while the NASA image is a pseudocolor version made by combining narrow-band images acquired in the Hα, OIII, and SII emission bands (the so-called "Hubble Palette"). Each of the pillars in this formation is about a LY long. They contain knots or globules of denser gas and dust called "Evaporating Gaseous Globules" (EGGS) that are the sites of new star formation.


Figure 3. The "Pillars of Creation" in M 16, (left) my image, (right) Hubble Space Telescope image.


In his original interpretation of this formation, Burnham gave it the name the "Star Queen". In Volume III of his Celestial Handbook (p. 1787), he says,

Thrusting boldly into the heart of the cloud rises a huge pinnacle like a cosmic mountain, the celestial throne of the Star Queen herself, wonderfully outlined in silhouette against the flowing fire-mist, where, as modern star pilgrims have learned, countless new stars are to be born. In the vast reaches of the Universe, modern telescopes reveal many vistas of unearthly beauty and wonder, but none, perhaps, which so perfectly evokes the very essence of celestial vastness and splendor, indefinable strangeness and mystery, the instinctive recognition of a vast cosmic drama being enacted, of a supreme masterwork of art being shown.

The Wikipedia page describing the Eagle Nebula shows an image of another feature in the nebula located to the north-west of the Pillars of Creation identified as the "Star Queen spire". This should not be confused with the original identification of the Star Queen as the same as the Pillars of Creation, for which there is no doubt in Burnham's book.

Figure 4 shows my image of M 17 (NGC 6618), which lies just across the border from Serpens Cauda in Sagittarius. Like M 16, this object was originally discovered by the Swiss astronomer Phillipe Loys de Cheseaux in 1745 or 1746, and was independently re-discovered by Messier in 1764. Messier described it as a "train of light without stars" and likened it to his observation of the Andromeda Nebula (M 31). In 1783, William Herschel observed it and also proposed a similarity to M 31. Only after a century did astronomers discover that objects like M 31 were far outside our Milky Way.

M 17 is known by three common names. Because some early observers thought that the brightest portion of the nebula resembled a swan resting on the water, it was called the "Swan Nebula". In my image, the brightest portion (light pink in the image) does resemble the profile of a swan (or maybe a duck). Also, the brightest portion of the nebula has a somewhat semi-circular shape (the "neck of the swan"), which has led to the name the "Horse Shoe Nebula". Finally, this portion of the nebula has been likened to the Greek letter omega (Ω), which has led to the third name for the nebula— the "Omega Nebula".


Figure 4. M 17 (NGC 6618), the Swan Nebula in Sagittarius.


The open cluster within the nebula contains 8000 to 10,000 member stars, but practically all are hidden within the dust and gas of the nebula. Only five of them are normally visible. Like M 16, this cluster is very young (around a million years old). Both M 16 and M 17 are located in the Sagittarius Arm of our Milky Way galaxy, with M 16 lying at a distance of around 5600 LY and M 17 at a distance of around 5910 LY. The two objects may have a physical separation of only around 300 LY, so they truly are neighbors in space. The bright yellowish star below the nebula in Figure 4 is HD 168415, a magnitude 5.39 class K3 star lying at a distance of 518 LY.

Each of the images in Figures 2 and 4 were constructed from twenty individual 90-second images acquired in each of the Red, Green and Blue spectral bands.

Now that the Summer stars are sinking into the west, I'm beginning to concentrate on the Fall and early Winter constellations. Luckily, there are lots of interesting objects to image along the sweep of the Northern Milky Way, from Cygnus through Cassiopeia to Auriga. There's also a few interesting galaxies in the southern Fall sky. So, weather permitting, I hope to be busy on moonless nights over the next few months.


The image file for the "Pillars of Creation" image acquired from the Hubble Space Telescope is in the public domain because it was solely created by NASA. NASA copyright policy states that "NASA material is not protected by copyright unless noted". (See Template:PD-USGov, NASA copyright policy page or JPL Image Use Policy.)
Details presented for M 16 and M 17 were primarily taken from: R. Stoyan, S. Binnewies, S. Friedrich, and K. Schroeder, 2008. Atlas of the Messier Objects. Cambridge University Press, Cambridge, UK.



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