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Object of the Week August 2, 2020 – Barnard 92 “Black Hole”
Object of the Week August 2, 2020 – Barnard 92 “Black Hole”
Barnard 92, LDN 323
Constellation: Sagittarius
Type: Dark Nebula
Opacity: 6 (1-6, 6=darkest)
RA: 18h 15m 30s
DEC: -18° 11' 0"
Size: 15’ x 9’
The constellation Sagittarius lies toward the center of our galaxy. When we look toward Sagittarius we look across a relatively vacant inter-arm gap at the next spiral arm in toward the galactic interior, the Sagittarius-Carina Spiral Arm and beyond that the Norma Spiral Arm. It is here where you’ll find most of the well-known and beautiful Messier nebulae and open clusters of Sagittarius. One such cluster is Messier 24, the Small Sagittarius Star Cloud. M24 is a stunning sight in richest-field telescopes and binoculars, and one of the most interesting star clouds in the entire Milky Way. It is here, inside M24, where you’ll find this week’s OOTW, Barnard 92 (B92).
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Image by Roberto Colombari, APOD 2018 June 29
North is down, B92 is toward the bottom left of center
Edward Emerson Barnard, an American astronomer, first announced his hypothsis on the origins of dark regions in 1913 in a paper titled Dark Regions in the Sky Suggesting an Obscuration of Light. He opens with this statement: “The so-called "black holes" in the Milky Way are of very great interest. Some of them are so definite that, possibly, they suggest not vacancies, but rather some kind of obscuring body lying in the Milky Way, or between us and it, which cuts out the light from the stars. This explanation seems to become more and more plausible the more we know of the objects.” He follows with a summary of his observations and images taken at Lick and Yerkes Observatories. He tempers his early observations with this caution that we are all so familiar with. “An observation of this kind, however, requires both good definition and good transparency. A little unsteadiness of the air blurs the light of the many near-by stars into a mistiness of the field, and a want of transparency cuts off any feebly luminous object and readily defeats any effort to see it.”
Following observations, particularly those of July 27 and November 4 1913, under “favorable conditions” and using the Yerkes 40” telescope, Barnard was able to finally confirm his hypothesis. “A glance at the original [image] would show that this is not a perforation in the nebula. It is clearly a dark body projected against, and breaking the continuity of, the bright nebulosity. To me the observation would confirm the supposition of an obscuring medium at this point.”
In 1919 Barnard published his first catalog of these dark regions, On the Dark Markings of the Sky - with a Catalogue of 181 Such Objects. That catalog was later superseded with his 1927 publication of Catalogue of 349 Dark Objects in the Sky.
B92 is one of the densest dark nebulae known which earned it the nickname "Black Hole." Remember before there were black holes as we know of them today1, there were black holes…of a different kind. The latter, Barnard’s "dark nebulae", are believed to be the result of molecular clouds of dust and gas which are present in our line of sight and which absorb the starlight originating behind them, thus providing the earth-based observer working in the visible spectrum the illusion of a black "void". These dark nebulae are NOT associated with dark matter!
Under dark skies and “favorable conditions” you’ll see B92 silhouetted conspicuously on the northwestern side of the M24 star cloud. It runs vertically measuring approximately 15 arc-minutes north-south and 9 arc-minutes east-west. It is estimated to lie 650 light-years away although some sources suggest even 10,000 light-years away. Its companion dark nebula lying less than one-half degree away to the east is Barnard 93 whose apparent size is 15 arc-minutes vertically and only 2 arc-minutes wide.
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North is up, FOV ~3.3°x2.1°
Observing B92 can be done using a range of aperture. 7x50 binoculars will work under the darkest skies with at least good or better seeing and transparency. A richest field telescope3 will provide dramatic views of the M24 star cloud and the dark nebulae contain within it. The lone 12th magnitude foreground star near the center of B92 and the 8th magnitude field star just outside its eastern edge are like islets in a black lake outlined by the rich star swarms of star cloud. The eastern side of B92 is more well defined than its western, which dissipates imperceptibly into the star field west of M24.
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Images by Martin Germano, Rolling Roof Observatory
North is up
Although dark nebulae are frequent targets of astrophotographers, there seems to be a scarcity of visual observation reports. Do you observe dark nebulae, why or why not?
Now it is your turn. Give it a go and let us know!
1Albert Einstein first predicted the existence of black holes in 1916, with his general theory of relativity. The term "black hole" was coined many years later in 1967 by American astronomer John Wheeler.
2Opacity: 6 (1-6, 6=darkest)
3any f/7 or faster instrument with 2” wide angle/low power eyepiece
