Dark Clouds in Taurus: Set yourself a challenge and spend a night or more navigating through nebulae in an under-observed part of the sky.
Dark nebulae are also detectable in much fainter portions of the Milky Way, but while they may look obvious on images, they're fairly challenging at the eyepiece. Since the Milky Way background is too faint visually to silhouette the dust clouds, they can only be detected by the paucity of faint stars in comparison with the rest of the field or surrounding fields. But note that there may not be a marked shortage of brighter stars since many of them will be closer to us than the dust cloud is.
The Millennium Star Atlas (MSA), unfortunately out-of-print, has long been my favorite guide for hunting these absorption nebulae. I find that the dark nebulae boundaries on MSA charts are usually quite accurate. If I star-hop to the correct location for a Barnard dark nebula, and note a marked decrease in the density of faint stars precisely where the atlas says that I should, then I consider that I have detected something that obscures the more distant stars--that is, a dust cloud absorbing the light of stars located behind it. I see no difference between detecting a dark nebula due to a marked absence of faint stars, and detecting a loose open cluster because of a statistically significant increase in star density at the cluster's plotted position.
The Milky Way is certainly faint in Taurus. Nevertheless, my 8-inch f/6 Dobsonian, working at 30x with a 1.25[degrees] field of view, revealed three large winding dust clouds. Spending two nights observing at such low power was very relaxing. On both nights, M33 was visible with the unaided eye, indicating that the transparency was excellent.
The easiest one was B19-B22, the 6[degrees]-long Taurus Dark Cloud. I could follow all of it, although the northwestern finger was just detectable. There were almost no faint stars visible in the 8-inch between RA 4h 28m and 4h 34m, and there was a good southern boundary in that section. East of there the broad dark nebula was still easy to follow since, while there was a smattering of faint stars, there were far more outside of the boundaries plotted in MSA. The eastern end, beyond RA 4h 38m, was nearly empty of faint stars, and this prominent area continued farther than plotted in MSA, all the way to RA 4h 48m. This was one of the very few times that a dark nebula has appeared to be significantly larger than plotted in the MSA, but Antonin Becvar's classic Atlas of the Heavens (1962) shows the dust cloud extending almost as far east as my impression.
Equally long B219 lies a couple of degrees north of the Taurus Dark Cloud and extends into Auriga. Much of the dark nebula is obvious, and my notes on the chart that I used at the eyepiece say "good" at the western end, "no faint stars" around 4h 33m, "a few faint stars" between RA 4h 36m and 4h 40m but "lots" immediately north of the boundary plotted in MSA, and then "sparse" in the main body of the dust cloud up to 4h 47m. The boundary north of unseen B23 and B24 is "less definite than the southern boundary." The thumb that extends south-southeastwards to declination +27.5[degrees] is "all strikingly empty of stars." I found the MSA boundaries to be "pretty exact" at the narrowing northwest of the unseen bright nebula vdB 29.
But beyond that neck, as the dark nebula crosses into Auriga, it was only vaguely visible, and the northernmost extension that lies beside the globular cluster Palomar 2 was not seen. While you're in the area do take a look at Palomar 2, which is one of the easiest of the 15 Palomar globular clusters--my 16-inch revealed this averted vision fuzzy blob at only 65x, so it would certainly be visible with a smaller scope.
South of the Taurus Dark Cloud my 8-inch at 30x showed all of 1.7[degrees]-long B18, looking a little larger than charted, as well as barely discernible B208, by far the thinnest of any of my successes.
The parts of winding B7-B209-B211 that my 8-inch could mostly reveal are 4.5[degrees] long and lie immediately west of the Taurus Dark Cloud. The star fields are sparser outside of the plotted boundaries of B7-B209-B211 than they are adjacent to the other two large dust clouds, so this dark nebula is harder to follow at the eyepiece. The western tip was starless, but the adjacent section immediately south of Psi ([psi]) Tauri was too narrow to detect. The loop southwards around RA 4h 09m was just "slightly sparse in stars," and the northwest-pointing thumb beside it was logged as "uncertain," although it looks prominent on plate 5 in E. E. Barnard's Photographic Atlas of Selected Regions of the Milky Way (https://is.gd/Plate5).
The view improved farther eastward, the faint stars becoming "sparse" between RA 4h 12m and 4h 15m, and then "nearly empty" until B211 passed Phi (<p) Tauri. The thin neck just west of Phi looked wider than it's plotted in MSA. Phi is a wide yellow and blue double star of magnitudes 5.1 and 7.5, easy to split even at 30x. I couldn't follow this complex of Barnard nebulae southeast of Phi with my 8-inch, but nine years earlier I saw little B213 and B216 with my 16-inch at 76x. Both are much smaller dark nebulae than any that the 8-inch could find in this area.
Observing the Taurus Dark Cloud and the other two large winding dust clouds isn't exceptionally difficult, but I haven't read of any other reported observations except for Nova Scotian Paul Gray's detection of the thin neck in B211 near Phi Tauri. Perhaps it was just generally assumed that the Taurus Dark Cloud was not detectable visually? It's certainly been a popular target for astrophotographers--see Chris Schur's image in the Gallery in the April 2015 issue of Sky & Telescope (pp. 78-79).
* Sky & Telescope has been a special part of Contributing Editor ALAN WHITMAN'S life for many decades.
Caption: The dark sooty shapes of the nebulae are clearly seen against the starry backdrop in this wide-field photo. The field of view also encompasses well-known objects such as the Pleiades on the right and Aldebaran at the bottom, as well as the open clusters NGC 1746 and NGC 1647, which are both visible with the unaided eye at true dark sites.
Caption: The Taurus Dark Cloud region offers plenty of opportunity for feasting your eyes on these dark forms, but the nebulae in this section of the cloud complex are challenging to observe.
Barnard's Nebulae in Taurus Object Opacity Size RA Dec. B19 4 360' 04h 33.7m +26[degrees] 16' B22 4 360' 04h 38.7m +26[degrees] 03' B219 3 360' 04h 34.9m +29[degrees] 36' B23 5 5' x 5' 04h 40.6m +29[degrees] 53' B24 5 8' x 8' 04h 42.9m +29[degrees] 44' B18 5 100' 04h 31.2m +24[degrees] 21' B208 -- 60'x 8' 04h 11.5m +25[degrees] 10' B7 5 60' x 60' 04h 17.4m +28[degrees] 34' B209 5 120'x 30' 04h 12.4m +28[degrees] 20' B211 5 60' x 20' 04h 17.2m +27[degrees] 49' B213 5 30' 04h 21.2m +27[degrees] 03' B216 5 -- 04h 24.0m +26[degrees] 38' Angular sizes and separations are from recent catalogs. Visually, an object's size is often smaller than the cataloged value and varies according to the aperture and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0.
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|Title Annotation:||Going Deep|
|Publication:||Sky & Telescope|
|Date:||Dec 29, 2017|
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