Messier 16: The Eagle Nebula...

M16: the Eagle Nebula...

Object: Messier 16 (NGC 6611: nebula catalogued as IC 4703, Sh2-49)
Type: Open cluster with nebula
Constellation: Serpens Cauda
Distance: 6,000 light years

Date: June 22nd. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 12 x 300s Ha, 12 x 300s + 6 x 300s (2x2 binned) OIII, no flats/darks (hot pixel removal in Astroart).

The sixteenth entry in Charles Messier’s famous catalogue of deep sky objects has a long and interesting history. The star cluster itself was first discovered by Swiss astronomer Jean-Philippe de Cheseaux in 1745–46, and was subsequently “rediscovered” on June 3, 1764 by Messier (who listed it as M16 in his own catalogue).

Messier described M16 as a “cluster of faint stars mingled with faint luminosity” although it is not clear that he actually observed the nebula itself, as Messier often accredited star clusters with nebulosity (a product of the poor quality of his telescopes rather than his undoubted ability as an observer).

French astronomer Etienne Trouvelot subsequently observed the nebula and described it in detail (nicknaming it “the Fan”) prior to British astronomer Issac Roberts photographing it in 1894. 

For the IC catalogue, John Dreyer treated M16 as being primarily the cluster itself, listed it as NGC 6611, and added the IC entry for the nebula lit by the cluster.

M16 has had various popular monikers, but the “Eagle nebula” seems to have stuck (although I think Trouvelot’s name is the most sensible). The occasionally-used "Star Queen Nebula" was introduced by Robert Burnham, Jr., in his lyrical description of the object in his classic Celestial Handbook, reflecting his characterization of the central pillar as the "Star Queen", shown in silhouette. 

Location of image field, looking south on June 22nd

Messier 16 can be found in the constellation of Serpens Cauda, low in the southern mid-Summer sky from UK latitudes (see map opposite) where, on rare evenings, the faint glow of the centre of our Milky Way can be glimpsed. M16 is best seen visually in a low-powered, wide field telescope. 4-inch instruments will resolve about 20 stars against several regions of faint nebulosity. The brightest portion of the nebula (which I imaged back in 2006) spans about 20 arc-seconds (a bit less than the diameter of the full moon) although my latest image above shows that it is just the brightest part of a whole expanse of faint nebulosity lurking in the distant Sagittarius arm of our galaxy.

Unfortunately, the general public’s expectations of night sky observing have been raised by the Hubble Space telescope and its iconic images of the Pillars of Creation, and folk are often disappointed by the faint, monochromatic real life views of nebulae such as M16 through amateur telescopes

The pillared, dust-strewn heart of M16 is a stellar nursery, where stars are being born inside dense clouds of cold gas. The detailed Hubble images of several of these natal cocoons led to their naming as the “Pillars of Creation”. The images show these clouds bathed in intense ultraviolet light from M16’s cluster of young, massive stars, with jets of gas streaming off the pillars as the intense radiation heats and evaporates it into space. Denser regions of the pillars are shielding material beneath them from the powerful radiation.

Imaging from the Medway valley with a 4-inch refractor, one is at a slight disadvantage compared to the Hubble telescope. Instead of the vacuum of space, my telescope is attempting to peer through a haze of dust and light-polluted air, particularly as M16 never rises greater than 25 degrees above my southern horizon.  Also, at this time of year, the sky has no true astronomical darkness as the sun never sets more than about 14 degrees below the horizon, although to me the sky looked as dark as it ever does when I started out taking sub-frames at around 23.30 on a warm summer’s evening. 

Auto-guiding wasn’t brilliant given that I was trying to image through warm air low in the sky, but it was good enough.  The Ha data was reasonable and given that I only had about 3 hours of darkness to play with, I swapped over to gathering some OIII frames after I had a dozen Ha frames in the electronic bag. 

Whilst the true colour of deep sky celestial objects is a fairly subjective thing, I am no fan of the “Hubble pallet”, a colour scheme used by NASA to render its images (and which as a result is now fashionable with amateur astro-imagers) whereby H-alpha light is assigned to the green channel and S-II to the red, with OIII as the blue. 

To me, hydrogen emission nebulae should be red, not lurid psychedelic yellows, greens or blues.  However, the plethora of such images of M16 led me to expect a much stronger OIII signal than I was actually getting. Fortunately, I had logged on to Stargazers Lounge and was able to ask the question, with veteran astro-imager Carole Pope suggesting that I take 2x2 “binned” exposures (where four adjacent pixels are bulked together to act as one on the camera chip, increasing sensitivity and reducing background “noise”). I don’t like the way this sometimes makes smaller stars a bit blocky, but it seemed to work OK this time and I was able to grab a few brighter images before an owl sat on my telescope (I think!).

Earlier, when I walked down the garden slope to my observatory to set up, a barn owl had swooped just in front of me, a white flash in my head-torch light that had startled me no end.  I have been visited by the local tawny owls in the past and indeed, one had once flown into the open dome hatch while I was in the observatory, giving me another late-night heart attack experience.

On this occasion, I was keeping an eye on the equipment remotely via a Cat 6 cable link from the observatory lap-top to another indoors. Suddenly, the auto-guiding graph shot off-scale and I thought “cable snag”. I dashed down to the observatory, where everything fortunately seemed normal, although the target was now off-centre. Looking around though, there were a couple of pale feathers on the floor (no other “debris”, luckily…) and I suspect I may have had another avian intruder. Whether the faint light of various LEDs or the gentle ticking of the mount tracking motors attracts them, I don’t know, but I decided to quit while I was ahead. By this time, it was around 3.00 am and the sky was starting to brighten anyway.

It was nice to see Jupiter and Saturn, low together towards the south in the peaceful darkness, with the bright baleful orange of Mars rising over my south-eastern horizon. I toyed with the idea of dragging my C9.25 out for a closer look but thought better of it, and went to bed instead. I am getting old.

Hubble image compared with centre crop from main image, showing "The Pillars of Creation"...

I colour-combined the separate Ha and OIII stacks in both PaintShop Pro and Astroart (with Ha as RED and OIII as blue, with a 60:40 blend of OIII and Ha as a green channel), and got two strikingly different results. The AA version was very red, the PSP a more subdued orange. In the end, I settled for a blend of the two that you see here.

For amusement, I thought it would be interesting to compare the detail in the Hubble image to that obtained by my more modest set-up (see above). Whilst I think it is possible to tell which is which, I was surprised to find that most of the prominent features can be found in my version.

Update:

There is a bit of freeware kicking around on the internet called Starnet. It uses a clever algorithm to rub out stars from astropics.  I hadn't had much luck with getting it to work until my son came home from university and managed to run it on his computer.

Here's what M16 looks like without stars...

Not sure if I like it. It looks like a Turner painting. It does open up possibilities for image processing as you can mess around with selectively processing areas of the image without messing up stars, which can be added back in afterwards.