NGC/IC Project Restoration Effort

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Basic Information

Location and Magnitude

Right Ascension: 5:36:12.8
Declination: -1:12:5
Constellation: ORI
Visual Magnitude:

Historic Information

Discoverer: Herschel W.
Year of discovery: 1786
Discovery aperture: 18.7


Summary description: !!!, eL, E, ε Orionis inv p
Sub-type: RN

Corwin's Notes

===== NGC 1990. In spite of this nebulosity having been reported by WH, JH, and Dreyer, as well as by several amateurs in recent years, there is no clear evidence of it on photographs of the area. The images that do exist of the nebulosity suggest that it could be a reflection nebula, shining by the light of epsilon Orionis. If so, it will be very blue as epsilon is one of the bluest stars in the sky. Steve Waldee has brought this object back to my attention early in 2011, so I am I reconsidering my earlier conclusion that the nebula is illusory. That is still the best explanation for it, but here are some more thoughts about it. WH says only of this nebula -- in the sweeps on 1 February 1786 -- only, "[Epsilon] passed, and I am pretty sure is involved in Nebulosity, unequally difused [sic]." CH adds the position of epsilon to the note, and that is the one that JH and Dreyer have adopted for their catalogues. JH has a little more detail from sweep 107 on 23 November 1827: "[Epsilon] Orionis. Place by Catal a v brilliant * involved in an immense nebulous atmosphere, whose n and s limits are 91[d] 7[m] 29[s] and 91[d] 31[m] 29[s]. Viewed also and shown to Mr. Dunlop in sweep 110 [16 December 1827]." That much is from his 1833 catalogue in PT. Unfortunately, his notes on the object (it is number 30) in the sweep itself are illegible, but he does have a footnote, some of which I can make out. (JH's penciled notes will have to be examined on his original pages to see what he actually wrote. The high- contrast images in the Herschel Archives are very poor for many pages -- some are essentially blank -- and often words on even the good pages are lost in the photocopying process): [Epsilon] Orionis is involved in a vast milky nebulosity which is something of a lenticular [?] figure and is very much more condensed about the star than elsewhere. [Illegible] to the two limits [illegible] but at the double * it is [illegible] faint. [illegible] see it, he called it a star in a fog. Other large stars have no such appearances[?]. [delta?] Orionis was free from it -- The diffused light of the star (42a[?]) though considerable and giving occasion to a singular [illegible] has[?] quite[?] [illegible] and no way comparable to it. From this, we can make out that he attempted to find the limits of the nebulosity, that a guest observer also saw the nebula, and that other bright stars in the area showed no such nebulosity. In sweep 110, only part of JH's notes are legible: "Viewed [epsilon] Orionis and the neb about it the neb is less[?] bright [illegible] than[?] before[?]." The conclusion is simple: JH saw the nebulosity on more than one night, and it was seen by at least one other person than just him through his telescope. A huge optical corona around the star from its scattered light (in the earth's atmosphere, in virtually any telescope, on virtually any image) is inevitable. This corona could well be hiding a nebulosity that is nevertheless visible, if not easily, to the naked eye. Perhaps an image taken with the star blocked, or carefully removed, would reveal a nebula close to the star similar to NGC 1432, NGC 1435, and IC 349 (all of which see). JH noted that the nebulosity extends at least 12 arcmin north and south of epsilon, while Dreyer (in the NGC Notes) makes it more extensive to the south. On the POSS1 red plate, the star is apparently close to the center of an extended, striated nebulosity. This, however, is not visible on any other photograph or image that I've seen, including several color photos that would certainly show a red nebulosity if it existed. This striation must be a defect on the red plate, apparently caused by imperfections or reflections in the red Plexiglass filter. I certainly do not think that the nebula that the Herschels and Dreyer saw are any of the more scattered nebulosities in the area. Not only are they too far from epsilon Ori, but I doubt that any of them could be seen visually without a nebular filter -- assuming it is emission nebulosity, of course. If it is reflection nebulosity, a filter passing the oxygen and hydrogen lines won't help. So, at the moment, I'm doubtful about the existence of this object. But it has been reported at the eyepiece often enough by reliable observers that it could be easily lost in the light of epsilon Orionis. (In the table, I give epsilon's position for this object.) However, see NGC 7088 for a well-known case of an illusory nebula "seen" by many experienced observers which is almost certainly non-existent.