Object of the Week October 23, 2022 – NGC 6741 Phantom Streak Nebula

[deepskytraveler]

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.

Now it is your turn.
Give it a go and let us know!

[lamperti]

Three observations:
1) no filter with a 20" at 272x - "Fairly bright star-like object to make an asterism with other field stars."
2) with a 15" at 110x + UHC: "A planetary nebula that was fairly faint and irregular but a filter was needed."
3) with a 13" at 70 & 130x + O-III: "Definitely bright disk with the filter. Blinked with filter and it's easy."

[Raul Leon]

Hi, here's my observation from 8/5/2011: ngc 6741 aka the Phantom Streak is a small planetary nebula in Aquila ; magnitude: 11.4 ; size: .013' ; very small but fairly bright, high magnification needed to confirm. I used a 6mm Ethos at 304x with my 14.5 Starstructure f/4.3ngc 6741.jpg

[KidOrion]

My observation from 8/22/20 (S 7-5; T 8, SQM 21.53-21.63; 12.5-inch Dobsonian @ 112x and 225x):

11:44
NGC 6741 (Aql): Among the objects with popular names in the sky, this nebula deserves a name the least: NGC 6741, the so-called Phantom Streak (it’s not even an appropriate name, really). Jerry showed me this nondescript little nebula two years ago after I had spent some considerable time trying to track it down, not realizing it was so stellar. The nebula’s visible in the 14mm as the brighter of a pair with a 12.5-magnitude star about 0.5’ N very slightly P it. The nebula can and does give glimpses of being non-stellar, but is very tiny nonetheless. It’s especially non-stellar in comparison with the 10.5-magnitude star 2.5 to the N. The field is easily identifiable by a pair of 8.5-mag stars, the P-most of which is slightly brighter than the F; those are separated pretty much P-F by 0.5’ and they lie about 16’ S of the nebula; the one to the P is also a very very very slight bit N of the other; from the P-most N slightly P by 9’ is an 11th-mag star; there’s a 10th-magnitude star N slightly F the more F of the pair by 8’; the 10th- and 11th-magnitude stars are separated by 8’, with the 11th P very very slightly N of the 10th. With the UHC the nebula definitely jumps out; it’s still no more than 2-3”. With the 14mm and the O-III, the nebula’s as bright as the two 8th-magnitude stars S of it; it has a very very small disk to it, just that little bit out-of-focus from the rest of the stars in the field. Almost has an extremely faint light blue color with the filter that no doubt is not a real thing, just an impression of a very pale light blue. With the 7mm, there’s also a difficult star, quite faint, due P the nebula by less than 0.25’. (The seeing has gotten worse, making the nebula more difficult to focus on.) Extra magnification definitely makes it a tiny disk, not a streak by any stretch, so the nickname sucks. Adding the UHC… there’s still no fringe or anything but a tiny disk, no more than 4-5” across, even in averted vision. The O-III adds little but contrast at this magnification.

[Don Pensack]

Hi, here's my observation from 8/5/2011: ngc 6741 aka the Phantom Streak is a small planetary nebula in Aquila ; magnitude: 11.4 ; size: .013' ; very small but fairly bright, high magnification needed to confirm. I used a 6mm Ethos at 304x with my 14.5 Starstructure f/4.3ngc 6741.jpg

My view on Monday night was the same, at the same power, with the same eyepiece, in a 12.5" f/5.75. My sky was bright: 20.90-21.05, but the planetary stood out fine.
That is too low a power to see any details except, perhaps, a trace of ovality. A fairly typical small planetary, and the size of many P-K planetaries I've found.
One note: I went searching for it with 166x, knowing it would have appeared completely stellar at lower powers.

[wvreeven]

I only logged an observation with my old 6" Newtonian telescope. Apparently there was a lot of wind that night so I couldn't use a high magnification. But I confirmed the nebula by using an OIII filter.

[Clear Skies]

Four observations in my log. It only appeared stellar to me when I first observed it using an 8" SCT back in 2008. Even so, I find it a case of "now you see me, now you don't"; easily overlooked when not looking at the exact postion and/or not using AV. That's why I believe the 'phantom' monicker is fitting, even if the nebula is nothing streak-like, at all.

My latest observation was less than two months ago. I wanted see whether or not I could determine the reason why the nebula is also cataloged as components A & B of the double star Jonckheere 475 (WDS19027-0027)... but that has not become any clearer to me.

Per the images below of the observing guide for this week's OOTW (click here to download), here is what I logged. 14" SCT, location French Ardennes, views @ 168x / 29' (17mm) - 316x / 16' (9mm) - 570x / 9' (5mm) - 814x / 6' (3.5mm), filtered and unfiltered.

6741-1.jpg 6741-2.jpg

17mm Nagler T4

A small, round glow, not stellar but easily overlooked when not using AV, quite suddenly brighter in the middle. Slightly larger with use of AV. A nice nebula.

No components or individual parts of the nebula are visible that could form components A and B of Jonckheere 475: AB-C and AB-D split.

To the SSW, ±190°/25" is a mag. 14 star.

17mm Nagler T4, Lumicon UHC

The effect of this filter is better than that of the OIII filter, there is more contrast, the nebula is larger and more even in brightness than it is unfiltered.

17mm Nagler T4, Lumicon OIII

An effect that is comparable to that of the UHC filter but not as strong.

17mm Nagler T4, H-Beta

Using this filter the nebula is faintly visible, the nebula is clearer without use of a filter.

9mm Nagler T6

As with 17mm, no stars or parts of the nebula are visible that could form components A and B of the double star. However, with use of this magnification the nebula is slightly elongated ENE to WSW (almost east-west). Use of filters as with 17mm.

5mm Nagler T6, Lumicon OIII

Using this magnification, this filter and AV a faint knot is visible to WSW of the middle of the nebula (not on the edge).

3.5mm Nagler T6

Using this magnification and AV the nebula is brightest in the ESE part, an offset center, a fainter glow to the west thereof. Without a filter the knot is visible that was visible with use of 5mm and the UHC filter.

3.5mm Nagler T6, Lumicon UHC

Using this filter the nebula is slightly larger and it is clear that the nebula is fainter on the western side in a "half Moon shape" that lies against the eastern part of the nebula.

3.5mm Nagler T6, Lumicon OIII

Using this filter the "half Moon part" that was also visible with use of the UHC filter, is slightly clearer.

Rated it 6/10.

[Uwe Glahn]

This thing is small. I don't have an actual result with the 27-inch but managed to resolve the ring structure with my older 16-inch under very good seeing and higher magnification.

sketch: 16", 720x, no filter, Seeing II, NELM 6m5+
NGC6741.jpg
home

[Steve Gottlieb]

Here are a couple of more observations...

24": Easy found at 200x as a very small, soft bluish glow. At 500x (unfiltered) the planetary appeared fairly bright, small, slightly elongated E-W, ~8"x6", brighter along the rim on the north side. There was no sign of a central star or any superimposed stars. A mag 13 star is 34" NW, a mag 14 star is 18" W and a mag 14.5 star is 25" SSW. Located 16' N of STF 2434, a wide pair of mag 8.5 stars at 27" separation.

18": Picked up at 115x by blinking with an OIII filter. Appears light blue and soft at this magnification, making the identification as a planetary evident. At 323x a small, crisp-edged disc is fairly bright, ~8" diameter. Excellent view at 565x and the planetary appears slightly elongated ~E-W with a faint, very thin outer envelope with total size of ~10"x8".

[Clear Skies]

... I wanted see whether or not I could determine the reason why the nebula is also cataloged as components A & B of the double star Jonckheere 475 (WDS19027-0027)... but that has not become any clearer to me.

A little over 2 years ago I posted the text above in reply to this OotW.

Today, the mystery of components A & B of the "double star" Jonckheere 475 has become clearer to me, hence this post.

This morning I came across this paper by Robert Jonckheere himself (honorable mention for Edward Pickering). In it, Jonckheere describes how he determined the "double star" to in fact be a nebula... followed by him recognizing it as an NGC entry:

With the 14-inch equatorial of the Observatory of the University of Lille, I found in 1911 a double star which I called J475 in the Monthly Notices, Vol. lxxi. p. 753, and identified it as BD-0° 3630. A year later another measure of this object was taken, and published in the Journal Astronomique, vol. i. p. 151.
As this last measure showed an increase both in the position-angle and distance, I determined to reobserve the star with the 28-inch equatorial of the Royal Observatory. This I was able to do on November 8, 1915. The observation was notes "hazy," and the measures showed a continued increase.
Not being satisfied I reobserved it on November 10, and as the definition appeared bad, the lowest power of 450 was again used. I gradually realized, however, that the nebulosity was due to the object itself, and, on referring to Dreyer's General Catalogue, found that is was known under the number 6741.
The original announcement of its nebulous character was made by the discoverer, Prof. E.C. Pickering, in the Observatory, vol. v. p. 295. From this page I quote the following:
The discovery of Huggins, that these objects are also distinguished from most other nebulae by their gaseous spectra, seems to make it expedient to include in the same class all nebulae, however small, which have gaseous spectra and an approximately circular form. A very small nebula of this sort is undistinguishable from a star in an ordinary eyepiece, but retains its original appearance when viewed through a prism, instead of being distorted like ordinary stars under the same circumstances.

Mystery solved. Changed "AB of the double star Jonckheere may be the nebula" to "AB of the double star Jonckheere is the nebula". Components C & D of the double are in fact stars, as noted in my observation in the earlier post.

DSF OotW 2022-43 - NGC6741 Aql_1.jpeg DSF OotW 2022-43 - NGC6741 Aql_2.jpeg DSF OotW 2022-43 - NGC6741 Aql_3.jpeg

Next ;-).