NGC 6741, Jonckheere 475, PK 033-02 1, PN Sa 2-380, PN G033.8-02.6, Phantom Streak Nebula
Type: Planetary Nebula
Constellation: Aquila
RA: 19h 02m 37.1s
DEC: -00° 26’ 56.7"
Mag(v): 11.4
Size: 0.13’
"The Phantom Streak Nebula. First you see it and then you don’t."
Although fairly bright with a visual magnitude of 11.4, the planetary nebula NGC 6741 is an exceptionally small object with an apparent size of 8 arc seconds. As such it was missed by early surveyors of the skies and wasn’t discovered until August 1882 by American astronomer and physicist Edward Charles Pickering. Pickering discovered this PN using the 15" refractor at the Harvard College Observatory in Cambridge, Massachusetts.
Being just 8 seconds of arc across, NGC 6741 is quite small, rendering it hard to discern features, as witness H. D. Curtis's (Lick Observatory) drawing. He says of it: "No central star...small bright oval...indistinct ring structure...scarcely perceptible protuberance at the western end."
HDCurtisNGC6741.jpeg
Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918
The Phantom Streak nickname was coined by John Mallas in his article, "Visual Atlas of Planetary Nebulae V” (Review of Popular Astronomy, May/June 1963, page 31). Mallas wrote the following about NGC 6741 in this article:
The "Phantom Streak." First you see it and then you don't. It is one of the smallest planetaries in this list. In the 4-inch after it was located. And using high powers. NGC 6741 looks like a broad silver line. Almost uniform in brightness, the ends appear broken and diffused. It is extremely difficult to locate. The color seems very unusual to me. My visual impression agrees with H. D. Curtis description of this object. He states: "It shows some traces of a ring structure, being somewhat fainter along the major axis."
Mallas Phantom Streak Nebula.png
Review of Popular Astronomy, May/June 1963, page 31
Hubble dramatically expands on Curtis' and Mallas' descriptions by showing an interior filled with threads of hot gas (with a high temperature approaching 18,000 Kelvin, much higher than the average of 10,000 K) that terminates in a sharp boundary with no visible outer shell, the central star remaining invisible.
HSTn6741.jpeg
ESA/Hubble and NASA
While the central star is not seen here, it has been detected at magnitude 20.0, and thus is easily lost within the bright nebulosity, its high temperature causing it to glow mostly in the far ultraviolet part of the spectrum where we cannot see it. NGC 6741 is classified as a Peimbert Type I planetary nebula (Manuel Peimbert and Silvia Torres-Peimbert, Instituto de Astronomía, Universidad Nacional Autónoma de México). Most type I PNs are bipolar. Their bipolar nature is considered to be a direct consequence of the large masses and high angular momenta of their progenitor stars. PN of Type I are Helium and Nitrogen rich and show a very pronounced filamentary structure.
You’ll find NGC 6741 located within the Milky Way in southwestern Aquila (not quite five degrees north of Lambda Aquilae). Remember that this PN demands lots of aperture and high power. To whet your appetite here a few observations for your consideration.
George Kepple and Glen Sanner, The Night Sky Observer’s Guide Volume 2: Spring and Summer
8/10" Scopes 250x - NGC 6741 has a tiny greenish-blue disk with on 11th magnitude star touching it on its west edge.
12/14" Scopes 300x - NGC 6741 has a bright 10" disk with a nice greenish-blue color. An 11th magnitude star is on the west edge. A faint star lies just beyond the NW edge.
Steve Coe, using a 13" f/5.6, notes: "Small, pretty bright, little elongated at 135X. Not easy to find at this power, it is just barely larger than the seeing disk at medium power. Installing the Barlow to get 270X makes it much easier to recognize. It is a light green dot in a Milky Way field. I estimate 3 arc seconds in size."
Magda Streicher, using a 12-inch f/10 SCT (218x) notes: At 76x this planetary nebula is completely stellar, and would be undistinguishable from the other stars in the field of view without the aid of an OIII filter, which shows it prominently when blinked. There is a curving line of three bright stars as wide as the field of view, which points to a double star. Within the curve are two faintish stars, once of which is the planetary nebula. At 218x it is visible with averted vision as a hazy, faint stellar-like object. Additionally I think it may be slightly elongated when I use averted vision. Nebula filter help me out. The star in the middle but not sure comes and go.
Christian B. Luginbuhl and Brian A. Skiff, Observing Handbook and Catalogue of Deep-Objects
This small planetary is visible with 25 cm at 200x. A mag. 11 star on the W edge is barely separable from the nebula at 250x, the latter appearing no brighter, but larger than the star. Viewed in 30 cm the greenish-blue nebula is discernable as the southeastern corner of a box of mag. 11-12 stars. At 425x it occasionally appears annular with a bright spot on the E, which looks like the central star at 225x. A mag. 14 star lies 20" NW.
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