J.M.W. Turner, Heavy Clouds Above a Landscape
INTRODUCTION: OBSERVING “NOTHING”
This week’s blog entry is about “nothing” seen against a vast star-scape. It might seem ironic to talk about images that eschew light, but bear with me. And, no, I’m not going to get into our contemporary topics of black holes or dark matter this time.
(BTW, next spring, I will indeed talk about how to observe meaningful objects related to these last two topics. I’m asked about black holes and the nature of dark matter all the time. Stay tuned and you’ll see how I try to satisfy their curiosity using my smart telescopes.)
Last week, however, I went to a celebration for the 50th anniversary of the founding of the Naperville Astronomical Association. They are a fantastic club, one of which I feel privileged to be a relatively new member. In many ways, I am an amateur astronomer/stargazer and I am learning much from attending this club’s programs. I brought my Vespera to the event and put my iPad on a tripod, so people could view the output images easily.
In the hour or so of darkness after the many talks, I started going after quick images of a number of objects. Because a large audience was circulating, however, I only stacked images for about 6 minutes on any one object before progressing to the next. Although the sky was clear, the transparency was relatively poor, and the seeing was crummy. I still did okay, however, with the usual suspects: HII regions like M8, the Lagoon; M17, the Omega; and the planetary nebula, M27, aka the Dumbbell.
Although these ionized hydrogen (HII) regions of the inner galaxy may be the most compelling sights, at the end of the session, I took my dual band filter out and started imaging star clusters, including one of my favorite regions — M24, the Small Sagittarius Star Cloud. I was particularly struck with my image (shown below) of M24, because even with this 5-minute image, I clearly saw a famous dark cloud in the NW (upper right hand) corner, and some of my viewers saw it too.
E. E. BARNARD’S DARK PHOTOGRAPHIC MARKINGS This “ink blot” in M24 has an official name. It goes by B92 because it was the 92nd entry in E. E. Barnard’s famous catalog of dark clouds. The catalog’s first group of members was listed in a 1919 Astrophysical Journal article entitled, “On the Dark Markings of the Sky with a Catalogue of 182 Such Objects.” Barnard went on to catalog 369 objects all based on his amazing wide angle deep sky photographic atlas. (The entire atlas has been scanned and is available online here: https://exhibit-archive.library.gatech.edu/barnard/index.html) I think a good way for amateur astronomical photographers to approach Barnard's catalog is to compare it with the Messier Catalog. Both are full of interesting objects that we can hunt down and try to image. But the problem is that many of the “B” objects are hard to see with straight visual telescopes, no matter how large. I think that’s partly because dark clouds are more diffuse than bright objects, and our eyes have a harder time dealing with the contrast; but that is a speculation I hope to explore more in Roger Clark’s fascinating book, Visual Astronomy of the Deep Sky. For the present, however, I suspect that our smart scopes can make quick work of many of Barnard’s dark markings. Object B92 is some 15 arc minutes by 9 arc minutes in size. Barnard hypothesized that what he was seeing were dark clouds and not “black holes” as some were calling them back in the early years of the 20th Century. In another 1913 Astrophysical Journal article (“Dark Regions in the Sky Suggesting an Obscuration of Light”), he specifically talks about B92 and describes how he has come to the conclusion that it is a dark obscuring cloud. Key to unlocking the nature of B92 was a session E. E. had with the Yerkes 40-inch refractor at high magnification to examine the edges of the cloud. Thus it took the world’s largest refractor to establish — at least to Barnard’s satisfaction — that many of these dark spots in his photography are clouds and not “holes” in the heavens. What is also fascinating about the same article is that it is one of the first descriptions by Barnard of what we now call the Horsehead Nebula (B33). It was actually first discovered on a Harvard photographic survey plate by Williamina Fleming in 1888. Ms. Fleming’s story and those of her other female colleagues is well told in Dava Sobel’s book, The Glass Universe.
Here’s one of my slightly processed images of the Horsehead taken with my eVscope from my back alley. I know this nebula is a challenge for regular eyepiece amateur scopes. But if I can do this well in my situation, then I know others can do much better.
Consider the following eVscope image by Stephen Strum.
SMART SCOPES: TUNED INTO THE DARK SIDE
Pondering the opportunities that dark clouds might present to smart telescope users, I decided to crunch a couple of numbers to better understand the angular sizes involved. If you take the Astronomical League’s easiest set of Dark Nebulae to observe*, you find that they average relatively large in angular size — sizes that are ideal for smart scopes too. When I average the sizes, I get 45 arc minutes for the most obvious Barnard catalog objects. Now consider the table of smart telescope fields of view. (All the units are in arc minutes.):
All of these scopes (except maybe the Unistellar eVscope and eQuinox, which have the same imaging chips), are pretty much tuned to be in a sweet spot for dark cloud observing! I really never thought about this at all until working on this blog entry.
WHAT ARE WE REALLY LOOKING AT WHEN WE IMAGE THESE THINGS?
Well, we’re not seeing black holes or dark matter, that’s for sure. We’re seeing cold, interstellar clouds. They appear dark because the dust within them blocks most of the visible light of stars or of the glowing nebular gas behind them. Infrared and microwave telescopes, of course, can see right into and through them. Both the Hubble and the Webb will be turned frequently on these types of objects.
B92, for example, is about 44 light years in its long axis. This is typical of these clouds. On the average, the large clouds probably contain hundreds of solar masses of molecular gas with 1% or so in dust as a “seasoning” that does the light absorbing. The interior cloud temperatures are some of the coldest places in the universe — averaging around 10 degrees Kelvin.
The smallest dark clouds are called Bok Globules. B68, one of the most well-studied, is quite small, just 5x3 arc minutes in size, but well within the capability of our scopes to resolve. It is, in fact, high priority on my observing list. B68’s size is probably in the range of a few solar masses. Its chilly interior is home to complex interstellar molecules like C18O, CS, NH3, H2CO, and C3H2. In these obscured harbors, such molecules are safe from the ionizing glare of the ultraviolet light in interstellar space.
These dark clouds are sometimes called “elephant trunks” because of their slender dark appearance in front of ionized regions. M16’s Pillars of Creation are these types of structures, as is, I suspect, the Horsehead. My astrophysicist friend and colleague, Mordecai-Mark Mac Low, models interstellar clouds. He explains that these trunk-like structures are due to shielding by denser dark clouds as ionization from bright stars in the HII regions races by, taking the less dense route. The HII regions are destined to destroy these features in time. So, make sure and make them a priority.
GESTALT SHIFTS AND ASTROPHOTOGRAPHY
For me, viewing that dark cloud B92 last Saturday represented what might be called a Gestalt Shift — in other words, an experience in which something one has been looking at one way, suddenly is perceived in another way. (What animal do you see above? Now look again.) Dark clouds from Barnard’s catalog, once something I might ignore, are now going on my list of targets along with Messier objects, Caldwell objects, comets, supernovae and other curiosities. They will be about seeing optically invisible clouds where once I saw “nothing.”
The joy of stargazing for me is about squeezing as much wonder out of an observing experience as I can. I hope you too will now find even the darkest interstellar clouds pretty “wonder full” too!
“The flexibility of the human mind - its ability to flip frames, shift gestalts, or reconstruct events – is a wondrous talent.” — Stephen Pinker
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Joseph Mallord William Turner 1775–1851
Heavy Clouds above a Landscape c.1820–40
Stephen Strum’s photography is terrific: https://www.earthtospacescience.com/post/deep-sky-imagery-from-the-evscope
*Astronomical League’s page on observing dark nebulae: https://www.astroleague.org/files/obsclubs/DarkNebula/DN-Optional.pdf
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