Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at Cetus A, the bright reflection nebula known as Messier 78!
During the 18th century, famed French astronomer Charles Messier noticed the presence of several “nebulous objects” while surveying the night sky. Originally mistaking these objects for comets, he began to catalog them so that others would not make the same mistake. Today, the resulting list (known as the Messier Catalog) includes over 100 objects and is one of the most influential catalogs of Deep Space Objects.
One of these is the reflection nebula known as Messier 78 located in the direction of the Orion constellation. Located about 1,350 light-years from Earth, M78 is the brightest diffuse reflection nebula that belongs to the Orion B molecular cloud complex, a group of nebulae that includes NGC 2064, NGC 2067 and NGC 2071. It is easily found with small telescopes and appears like a bright, hazy patch in the night sky.
M78 is a cloud of interstellar dust located about 1,600 light years from Earth. It is illuminated over an expanse of four light years by the by the energy of its embedded, bright blue, early B-type stars which emit a continuous spectrum. In the area are 45 low mass stars with hydrogen emission lines – irregular variable stars similar to the star T Tauri – which may very well be at the beginning stages of their stellar life.
As K. M. Flaherty and James Mazerolle said in a 2007 study:
“We study the disk and accretion properties of young stars in the NGC 2068 and NGC 2071 clusters. Using low-resolution optical spectra, we define a membership sample and determine an age for the region of ~2 Myr. Using high-resolution spectra of the H? line we study the accretion activity of these likely members and also examine the disk properties of the likely members using IRAC and MIPS mid-infrared photometry. A substantial fraction (79%) of the 67 members have an infrared excess while all of the stars with significant infrared excess show evidence for active accretion. We find three populations of evolved disks (IRAC weak, MIPS weak, and transition disks) all of which show decreased accretion activity in addition to the evidence for evolution in the dust disk.”
A significant number of dramatic outflow sources are found in the region of M78. Called Herbig-Haro objects, astronomers believe these are jets of matter ejected from neophyte newly formed inside M78 – LBS17. Said Andy Gibb of the University of Kent:
“LBS17 is a dense cloud core which lies close to NGC 2068 in L1630. It was first identified as one of five massive cores by a survey of well-known star-forming complexes. Closer examination of the HCO+ J=3-2 spectra revealed the presence of spatially-separated blue- and red- shifted wing emission, centered on LBS17H. Fifteen years ago, the reaction to this would have been ‘A rotating disc!’; these days the reaction tends to be ‘Outflow!’. The latter initially seemed a better choice, especially as the survey by Fukui (1989) revealed a CO outflow in this region. However, upon calculating the gas parameters and analyzing the energetics it became clear that the data could still be interpreted as a rotationally supported disc. Thus (as ever!) further observations were required to try and decipher exactly what was going on. The apparent dynamical age is low – only 10(4) years or so. If the inclination is 45 degrees then this is equal to the true age indicating that this may be a very young object. The lack of an infrared source supports this interpretation. The compact nature of this source makes it a good target for future interferometric observations. However, despite answering the main question of this project, the data have given rise to several more! What is the nature of the driving source? What is the real distribution of dense gas surrounding the source? Is the second outflow real? The quest continues…”
Another thing we clearly understand about Messier 78 is that its star forming activity seems to be happening in clusters. As D. Johnstone explained in a 2002 study:
“Wide area sub-millimeter mapping of nearby molecular clouds allows for the study of large scale structures such as the Integral Shaped Filament in the Orion A cloud. Examination of these regions suggests that they are not equilibrium isothermal structures but rather require significant, and radially dependent, non-thermal support such as produced by helical magnetic fields Also observed in the large area maps are dense condensations with masses typical for stars. The mass distribution of these clumps is similar to the stellar initial mass function; however, the clumps appear stable against collapse. The clumps are clustered within the cores of molecular clouds and restricted to those locations where the molecular cloud column density is high (Av > 4). As well, the typical sub-millimeter clump reveals little or no emission from isotopes of CO, likely indicating that the combination of high density and low temperatures within the clumps provides an environment in which these molecules freeze-out onto dust grain surfaces.”
One thing is certain – Messier 78 is a pretty incredible star forming region with many mysteries. As P. Andre said in a 2001 study:
“Since the details of the star formation process appear to depend on environmental factors, it is crucial to study a large number of these complexes in order to build a complete observational and theoretical picture. In particular, the typical Jeans mass is likely to differ from cloud to cloud, which may lead to a break in the mass spectrum of pre-stellar condensations at different characteristic masses. Besides cluster forming clouds, more quiescent regions, such as high-latitude starless clouds, should also be mapped in order to investigate the factors that control the efficiency of dense core and star formation.”
History of Observation:
This great nebula was discovered early in the year of 1780 by Pierre Mechain, but wasn’t confirmed and cataloged by Charles Messier until December 12 of the same year. In his records he writes:
“Cluster of stars, with much nebulosity in Orion and on the same parallel as the star Delta in the belt, which has served to determine its position; the cluster follows [is east of] the star on the hour wire at 3d 41′, and the cluster is above the star by 27’7”. M. Mechain had seen this cluster at the beginning of 1780, and reported: “On the left side of Orion; 2 to 3 minutes in diameter, one can see two fairly bright nuclei, surrounded by nebulosity”.
On December 19, 1783, Sir William Herschel would also visit with M78 and make his own private observations:
“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.”
Locating Messier 78:
Finding M78 is as easy as locating Orion’s “Belt” – the famous asterism of three stars. Simply identify Zeta Orionis (Alnitak) the easternmost of the trio and you’ll find it about 2 degrees (less than a thumb length) north and 1 1/2 degrees (less two finger widths) east. However, seeing M78 isn’t as easy as finding it! Because it has a fairly low visual brightness and isn’t particularly large, you’ll need a dark night and good sky conditions.
Messier 78 can be spotted as a small, faint, hazy patch in binoculars as small as 5X30 – but turns nebular with larger aperture binoculars and small telescopes. When telescope size increase, brighter areas are revealed as fueling, light source stars and the visible nebula size itself increases. For larger telescopes, be sure to look for adjoining nebula NGC 2071 to the northeast, NGC 2067 in the northwest and very faint NGC 2064 located southwest. M78 can be spotted under urban skies when using a light pollution filter, but doesn’t hold up well to moonlight conditions.
May your own observation of M78 – and night – be a fine one!
And here are the quick facts on this Messier Object to get you started:
Object Name: Messier 78
Alternative Designations: M78, NGC 2068
Object Type: Reflection Nebula with Open Star Cluster
Right Ascension: 05 : 46.7 (h:m)
Declination: +00 : 03 (deg:m)
Distance: 1.6 (kly)
Visual Brightness: 8.3 (mag)
Apparent Dimension: 8×6 (arc min)\
We have written many interesting articles about Messier Objects and globular clusters here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, M1 – The Crab Nebula, Observing Spotlight – Whatever Happened to Messier 71?, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.