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Object of the Week, December 31, 2023 – M 78, Reflection Nebula
Orion
RA: 5h 46 m 47s
Dec: +0d 0m 50s
Brightest reflection nebula
(Because of the busy holidays, I haven’t had the time to write an OOTW. Below is an abridged version of my January 2024 article in Sky & Telescope magazine – but M78 is a new OOTW!)
M78 was first discovered by Pierre Méchain in early 1780, and Messier confirmed the nebula later that year. Here are Messier’s somewhat confusing notes from December 17, 1780:
"Cluster of stars, with much nebulosity in Orion & on the same parallel as the star Delta in the belt, which has served to determine its position; the cluster follows the star on the hour wire at 3d 41', & the cluster is above the star by 27'7". M. Méchain had seen this cluster at the beginning of 1780, & reported: "On the left side of Orion; 2 to 3 minutes in diameter, one can see two fairly bright nuclei, surrounded by nebulosity"." (diam. 3')”
Cluster of stars? Méchain’s description is right on, but what did Messier see that he saw as a “cluster of stars, with much nebulosity”? That doesn’t sound like M78, but perhaps Messier was thinking the nebulosity would resolve into a cluster of stars? I don’t know, just a thought.
Three years and two days later, William Herschel wrote:
“1783, Dec. 19 (Sw. 59). Two large stars, well defined, within a nebulous glare of light resembling that in Orion's sword. There are also three very small stars just visible in the nebulous part which seem to be component particles thereof. I think there is a faint ray near 1/2 deg long towards the east and another towards the south east less extended, but I am not quite so well assured of the reality of these latter phenomena as I could wish, and would rather ascribe them to some deception. At least I shall suspend my judgement till I have seen it again in very fine weather, tho' the night is far from bad.”
Herschel’s report definitely describes M78, but I am a little surprised he didn’t see more nebulosity given all the faint galaxies he discovered throughout his career. Maybe the sky had poor transparency the night he observed M78.
However, the one night I had great conditions showed me a whole lot more in and around M78. In March 2021 I was able to see way more than I anticipated, and later found out that I’d also seen lots of YSO’s, which are everywhere in and around M78:
NGC 2071
Located just north of M78, this nebulous region is connected to it by the subtle and uneven glow of background nebulosity that suffuses this area, and gives the strong impression that all the brighter nebulae are part of the same complex that are visually separated by dark nebulae. The brightest part of 2071 is punctuated by the star HD 290861, a 10th magnitude YSO.
Attachment 5314 Attachment 5315
Here are two versions of my sketch of the M78 complex, made with my 28-inch f/4 Newtonian using magnifications of 155x to 408x. It was drawn under nearly ideal observing conditions over two nights, and is the only time (so far) that I saw M78 this well.
The nebula surrounding this star is roughly circular, colorless, but with noticeably irregular edges. It gradually dims the further it gets from HD 290861 until it joins the background glow to its south and northwest, where it brightens again around the star HD 290860, which is also 10th magnitude [12]. It quickly dims into background glow of this region.
M78 = NGC 2068
This is the largest and brightest area of nebulosity, and is what Méchain, Messier and Herschel saw. However, the first part of Messier’s description doesn’t really match M78. There’s not only no cluster of stars here, there’s a noticeable lack of them around M78, which has me wondering why he described it as a “cluster of stars”. Where he quotes Méchain is spot on though, and Herschel’s precise notes confirm he was seeing NGC 2068 too.
My sketch shows what I saw. Notice the sharp but slightly uneven northern boundary of NGC 2068 makes with the curved dark nebula, LDN 1627 [16]. I saw one definite notch and perhaps another – to paraphrase Herschel, “I am not quite so well assured of [its] reality” but I did have the impression the northern border of 2068 was somewhat serrated.
To my eye, LDN 1627 helped define the structure of all the bright nebulosities shown in my sketch, and is the dark backbone of this area. It’s as visually important as the bright portions, and I imagine it’s brightly illuminated all along its far side.
The two stars within 2068 stand out as evenly bright, and conjured up a pair of stellar eyes within a ghostly face. The northern most, HD 38563, a B2II-III star [17], is a close double that I saw as a single star, and shines at magnitude 10.8 while the southern star, HD 290862 is a young stellar object (YSO) [18] and is slightly brighter at magnitude 10.4. The third star mentioned by Herschel, just south of the brightest part of NGC 2068, is the 13th magnitude [SSC75] M 78 11 [12].
The three nondescript stars trailing off to the southeast from [SSC75] M 78 11 turn out to be rather interesting. In descending order of brightness, we have a T Tuari star (EM* LkHA 309), a close double (EM* LkHA 312 and EM* LkHA 313A), which I saw as a single star, made up of a YSO and another T Tauri star, and finally a high proper motion star (PM J05472-0000). Many stars in this area are rather interesting if you look up their basic properties [12].
The nebulosity of NGC 2068 is partially defined by LDN 1627 along its northern and northwest border, and also by how it gradually fades to the south and east. It doesn’t fade away completely, because the entire area is dappled with a subtle mix of dark and barely detectable bright nebula, with the most noticeable part straggling off to the south.
NGC 2067
This is the elongated bright nebula on the northwest side of LDN 1627, and follows the curve both to the west and south of NGC 2068. The brightest portion of 2067 is near its northern end. A roughly 10th magnitude star – a close double consisting of a YSO (CXOU J054635.3+000858) and a star of uncertain type (VSS VI-4) [14, 15] – punctuates the brightest part of the nebula as it trails southward down the visual length of 2067.
NGC 2064
Following the curve of 2067 southward, we come to NGC 2064, a subtle knot that’s easy to miss at first glance, but with averted vision it appears more substantial. It’s right about here where LDN 1627 becomes indistinct, and the western side of NGC 2068 begins to merge with NGC 2064. Keep following the overall curve to find a wide double star. I saw a faint halo around the eastern most star, magnitude 14.6 EM* LkHA 301 [8], which is yet another T Tauri star. Its wide companion, V*V2764 Ori is also a T Tauri star. By the way, the location of McNeil’s Nebula is just west of this young stellar pair.
This is a variable nebula discovered photographically in 2004 by Jay McNeil. Looking much like a smaller version of Hubble’s Variable Nebula (NGC 2261) the illuminating star for McNeil’s Nebula is the protostar V1647 Orionis, which is usually impossible to see at magnitude 18.1 – and it seems that no one has seen the nebulosity since 2018.
[B77] 106
Continuing to follow the overall curve of this region southward, I came to a small knot of nebulosity, [B77] 106. Although I only noticed the fairly bright glow of the nebula, there’s a T Tauri star embedded within. YSO’s really are everywhere around M78.
Have you seen all these parts of the M78 complex? How about McNeil’s Nebula - and who will be the first to see its reappearance? Give them a go and let us know - and Happy New Year!
