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The GREAT ORION NEBULA : M42 & M43


Early History of the Orion Nebula


Fig.1. The Great Orion Nebula : M42 / M43

Messier 42 is the bowl shaped nebula in the
lowerhalf of the image, while the smaller nebula
of Messier 43 lies in the top-right hand corner.
In the heart of M42 is the multiple star system
commonly called The Trapezium, being quite
easily visible even in small telescopes.


The telescopic history of the Orion Nebula and the M43 region is quite an interesting story and contains one of the greatest conundrums in astronomy, worthy of a decent detective mystery. First realised by Alexander von Humboldt, none of the observers before Galileo had noted the particularly bright nebulosity, although to the naked-eye today, it is obviously bright and definitely not stellar. Why this was not observed has yet to be properly explained, meaning either that the Orion Nebula was simply not noted or the seemingly highly unlikely possibility that the nebula was not as bright in the time of Galileo. More strange is that nobody else prior to him has written down the visibility of this particularly famous Grand Nebula of the entire sky.

The first written observations, like those of Al Sufi, published in 986 A.D. were highly detailed — even noticing the Andromeda Nebula (M31), but he does not mention or show any nebulosity in Sword of Orion. M42 is obviously not a star, even with the naked eye, though the explanation of this discrepancy nebula may not have been as bright during this time. By 1603, Bayer made his highly detailed star chart of the region that also does not show any nebulosity in the Sword of Orion. Yet, Bayer had noted the two stars in the Sword as an optical double, and named them as Theta (θ1) Orionis and Theta (θ2) Orionis.

Amazingly, Galileo never mentions it, even though it was one of the first areas for his telescopic sweeps. In December 1610, he extensively observed the constellation of Orion, merely noting an increase in the number of stars in the region. (See Figure 2.) Added were an additional eighty stars or so within the sword, which Galileo made in the subsequent drawn figure in his Sidereus Nuncius (The Sidereal Messenger).

Nicholas Peiresc in November 26th, 1611 who then made the first known telescopic observations, which continued between December 6th to 10th. The written observations of Peiresc states he was surprised to find a small illuminated cloud. This was followed by Johann Baptist Cysat (Cysatus), a Swiss Jesuit priest, made the next known observation in 1618, while comparing the nebulae with a close-by comet that was visible at the time.

Christian Huygens gave the first detailed description using a Galilean telescope in 1656. Huygens thought he had discovered M42 for the first time, and subsequently published observations in his Systema Saturnium, a few months after the original observation. It contained his full description and drawing, and told of the remarkable star Theta (One) Orionis (theta1 Ori) in the very heart of the nebula. His original sketch, contains seven stars inside the nebula, with five lying just beyond it. The stars of the Trapezium, he curiously only notes as three stars, though for the telescope aperture used, he should have easily seen all four.



Fig. 2. Galileos Telescopic Drawing of Orion

The figure shows the whole asterism of Venuss Mirror — including Orions Belt and Sword. This drawing clearly shows the missing Orion Nebula, which should have been easily seen by the naked-eye but was somehow missed or was not simply noted by him. This is similar to the written document text.


Later History and General Observations
of the Orion Nebula


Perhaps the first detailed descriptions of the Orion Nebula was by the Sicilian amateur scientist Giovanno Batista Hordierna (1597-1660), whose observations have only recently came to light in 1985. In his small catalogue of forty objects, he list and drew the Orion nebulae as No.5. Hordierna describes in De systemate orbis cometici; deque admirandis coeli characteribus [Systematics of the World of Comets, and on the Admirable Objects of the Sky.] (1654)

in the sword of Orion and includes 22 stars as one can see with the telescope. But this Luminosa is more admirable because of some unresolvable luminosity in whose middle can be seen three stars.”,

Another description of importance appear by Edmond Halley in Philosophical Transactions An Account of Several Nebulae or Lucid Spots Like Clouds, lately Discovered Among the Fixt Stars by Help of the Telescope. (Phil.Trans., XXIV, pg.390-391 (1716); here Halley (here partly translated) describes;

The first and most considerable is that in the Middle of Orions Sword, marked with Theta by Bayer in his Uranometria, as a single Star of third Magnitude; and is so accounted by Ptolemy, Tycho Brahe and Hevelius: but is in reality two very contiguous Stars environed with a very large transparent bright Spot, through which they appear with several others. These are curiously described by Hugenius [Christiaan Huygens] in his Systema Saturnium page. 8, who there calls this brightness Portentum, cui certe simile aliud nusquam apud reliquas Fixas potuit animadvertere [a wonderful object, which is certainly unique among the fixed stars]: affirming that he found it by chance in the Year 1656. The Middle of this is at present at [Gemini] 19° 00′, with South lat. 28° ¾.,

In March 1769, Charles Messier placed Orions Great Nebula in his famous catalogue of 103 objects; as M42 and M43. He separated the nebula into two parts because he thought they were different objects, and even made a drawing of them. (See Figure 3.) Messier described the Orion Nebula region in the first version of the Messier Catalogue in the French Mem. Acad. for 1771, pg.450-451 (1772), just two years prior to William Herschel in 1774. Messier says;

Messier Drawing
Fig. 3. Messiers Drawing of M42

I have examined a large number of times the nebula in the sword of Orion, which Huygens discovered in the year 1656, & of which he has given a drawing in the work which he has published in 1659, under the title Systema Saturnium. It has been observed since by different Astronomers. M. Derham, in a Memoir printed in the Philosophical Transactions, no. 428, page 70, speaks of that nebula which he has examined with a reflecting telescope of 8 feet [fl]. Here is the translation of what he has reported in this Memoir.

only that in Orion, hath some Stars in it, visible only with the Telescope, but by no Means sufficient to cause the Light of the Nebulose there. But by these Stars it was, that I first perceived the Distance of the Nebulosae to be greater than that of the Fixd Stars, and put me upon enquiring into the rest of them. Every one of which I could very visibly, and plainly discern, to be at immense Distance beyond the Fixd Stars near them, whether visible to the naked Eye, or Telescopick only; they seemed to be as far beyond the Fixd Stars, as any of those Stars are from Earth.

M. le Gentil also examined this nebula with ordinary refractors of 8, of 15 & of 18 feet length; as well as a Gregorian telescope of 6 feet, which belongs to Mr. Pingré. He has published his observations in a Memoir which can be found printed in the Volumes of the Academy, year 1759, page 453. There is a joint of the drawings which he had made of it at that time, as well as those of Huygens & of Picard; these drawings differ from each other, so that one may suspect that this nebula is subject to sort of variations. Here is what I have reported about that nebula in the Journal of my Observations. On March 4, 1769, the sky was perfectly serene, Orion was going to pass the meridian, I have directed to the nebula of this constellation a Gregorian telescope of 30 pouces [83cm.] focal length, which magnified 104 times; one saw it perfectly well, & I drawed the extension of the nebula, which I compared consequently to the drawings which M. le Gentil has given of it, I found some differences. This nebula contains eleven stars; there are four near its middle, of different magnitudes & strongly compressed to each other; they are of an extraordinary brilliance: here is the position of the brightest of the four stars, which Flamsteed, in his catalog, designated by the greek letter Theta, of fourth magnitude, 80° 59′ 40″ in right ascension, & 5d 34′ 6″ in southern declination : this position has been deduced from that which Flamsteed has given in his catalog.

In March 1774, Sir William Herschel observed M42,and later in the successive years of 1801, 1806 and 1810. His theory, written in the Nebula Hypothesis, realising for the first time the true nature of the Orion nebula. He wrote on M42;

…which would see here an unformed fiery mist, the chaotic material of future suns.

Noting that the observations made by him were different from Huygens, William Herschel stated in December 1810 the belief that the nebula had actually changed. Some thirty-seven years later, he was absolutely convinced that several stars had actually dissipated their nebulosities. By comparing his sketches, he also noted changes in the appearance of the nebulas internal structures. It seems, by our knowledge of the meticulous detail of his observations, it is likely that these changes were real. Later other were to question the same thing about other nebulae like Eta (η) Carinae.

Compared to modern observations, several prominent and obvious stars of the Trapezium are missing. I.e. The E and F stars. Wilhelm Struve, as late as 1826, discovered the fifth E star of the Trapezium. His magnitude estimations, show it has brightened considerably in the last 180 years. Some have speculated that many of these stars are very very young. The 110 stars discovered in the inner portions of the nebulae each show embryonic material, and proto-planetary disks which are being shredded over a short time. In 1936, observations the variable star FU Orionis was found to have brightened by at least six magnitudes over a short period of several months! Ever since this star shows small fluctuations in brightness but has remained at an elevated average of 10.3 magnitude. Another two fainter variables of V1057 Cyg and V1517 Cyg also have shown similar phenomena.

It is very possible that these stars may have switched to a more energetic process, or that the star has suddenly ejected the incipient nebulosity that surrounds them. This may be the case with other stars observed in this region. Observation using infra-red wavelengths and the Hubble Space Telescope (1994-95) have revealed many observational details of the formation and cause of such brightening.

Sir John Herschel in 1826 made observations, whose descriptions remain as the standard, and even today are still quoted in most of the astronomical texts. His most poignant description written in 1837, says;

I know not how to describe it better than by comparing it to a curdling liquid, or to the breaking up of a mackerel sky where the clouds of which it consists beginning to assume a cirrus appearance.

Reverend Thomas Webb in his Celestial Objects for Common Telescopes. (1859)

The Great Nebula, one of the most wonderful objects in the Heavens;…

Richard A. Proctor also in the late 1870s, described it as follows;

…the thought that seemed so impressive, so thrilling, as to surpass even the feeling of awe with which is the solemn darkness of night we see some mighty group of suns sweep into the field of view of the telescope… that here on this tiny square inch of shoreline, with its thin film of chemical sands, has been traversing the solemn depths of space… Here we have mirrored by Nature herself that marvellous round of milky light below* Orion…
(* as observed from the northern hemisphere.)

Richard A. Proctor, in his charming; HALF-HOURS WITH THE TELESCOPE : Being a popular guide to the use of the telescope as a means of amusement and instruction., beautifully describes the finding and observing the Orion Nebula as;

For a change we will now try our telescope on a nebula, selecting the great nebula in the Sword. The place of this object is indicated in Plate 2. There can be no difficulty in finding it since it is clearly visible to the naked eye on a moonless night… [being] the only sort of night on which an observer would care to look at nebulae A low power should be employed.
The nebula is shown in Plate 3 as I have seen it with a 3-inch aperture. We see nothing of those complex streams of light which are portrayed in the drawings of Herschel, Bond, and Lassell, but enough to excite our interest and wonder. What is this marvellous light-cloud? One could almost imagine that there was a strange prophetic meaning in the words which have been translated Canst thou loose the bands of Orion? Telescope after telescope had been turned on this wonderful object with the hope of resolving its light into stars. But it proved intractable to Herschels great reflector, to Lassells 2-feet reflector, to Lord Rosses 3-feet reflector, and even partially to the great 6-feet reflector. Then we hear of its supposed resolution into stars, Lord Rosse himself writing to Professor Nichol, in 1846; I may safely say there can be little, if any, doubt as to the resolvability of the nebula; all about the trapezium is a mass of stars, the rest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability strongly marked.
It was decided, therefore, that assuredly the great nebula is a congeries of stars, and not a mass of nebulous matter as had been surmised by Sir W. Herschel. And therefore astronomers were not a little surprised when it was proved by Mr. Huggins spectrum-analysis that the nebula consists of gaseous matter. How widely extended this gaseous universe may be we cannot say. The general opinion is that the nebulae are removed far beyond the fixed stars. If this were so, the dimensions of the Orion nebula would be indeed enormous, far larger probably than those of the whole system whereof our sun is a member. I believe this view is founded on insufficient evidence, but this would not be the place to discuss the subject. I shall merely point out that the nebula occurs in a region rich in stars, and if it is not, like the great nebula in Argo, clustered around a remarkable star, it is found associated in a manner which I cannot look upon as accidental with a set of small-magnitude stars, and notably with the trapezium which surrounds that very remarkable black gap within the nebula. The fact that the nebula shares the proper motion of the trapezium appears inexplicable if the nebula is really far out in space beyond the trapezium. A very small proper motion of the trapezium (alone) would long since have destroyed the remarkable agreement in the position of the dark gap and the trapezium which has been noticed for so many years.

But whether belonging to our system or far beyond it, the great nebula must have enormous dimensions. A vast gaseous system it is, sustained by what arrangements or forces we cannot tell, nor can we know what purposes it subserves. Mr. Huggins discovery that comets have gaseous nuclei, (so far as the two he has yet examined show) may suggest the speculation that in the Orion nebula we see a vast system of comets travelling in extensive orbits around nuclear stars, and so slowly as to exhibit for long intervals of time an unchanged figure. But of such speculations we may say with Sir J. Herschel there is no end.
To return to our telescopic observations: The trapezium affords a useful test for the light-gathering power of the telescope. Large instruments exhibit nine stars. But our observer may be well satisfied with his instrument and his eye-sight if he can see five with a 3½-inch aperture. A good 3-inch glass shows four distinctly. But with smaller apertures only three are visible. The whole neighbourhood of the great nebula will well repay research. The observer may sweep over it carefully on any dark night with profit. Above the nebula is the star-cluster 362 H. [Herschel]

The star [See Fig.3 Above] below the nebula is involved in a strong nebulosity. And in searching over this region we meet with delicate double, triple, and multiple stars, which made the survey interesting with almost any power that may be applied.

Mary Proctor, daughter of R.A. Proctor, continued with her apt description;

This is an irregular square with a star in each corner, and is situated in the midst of a dark gap in the nebula, within in which the 4 stars gleam brightly… (The Trapezium) The radiant mist surrounding them has a greenish tinge, revealing the vast stellar cloud known as the Nebula in Orion with its Isles of light and silvery streams, and gloomy gulfs of mystic shades.

E.J. Hartung in Astronomical Objects for Southern Telescopes in 1968 describes the Orion Nebula as thus;

One of the most attractive objects in the sky, this great nebula is too well known to need description;…

These possible changes in the whole area are certainly curious. It is hard to reconcile how the observers before Pieresc could have missed such an obvious object, unless there was a sudden brightening of M42/M43 at the time of the introduction of telescopic observations. It is possible, if this hypothesis is correct, that the nebula started rapidly to brighten a few years before the observations of Huygens. An explanation of the mechanism could be the simultaneous throwing off of the embryonic nebulosity of the stars in the Orion Nebula. This would cause the stars to brighten suddenly, like FU Orionis. They would then expose the surrounding nebulosity of the whole region to the intense UV radiations of the stars. Over perhaps a few decades or centuries, the material contained in the nebula begins to shine by its own light, becoming the brightest of all emission nebulae.

It is truly compelling to think that perhaps the Orion Nebula may in fact be a very new object, which has actually has evolved even further since the invention of the telescope just over four-hundred odd years ago.



The Orion Nebula Today


Now at the beginning of the twenty-first century, the Orion Nebula or NGC 1975-1980 is visually the brightest of the nebulae seen in the sky. This magnificent object is a bright emission nebula that is still spectacular even in light-polluted skies. The most modest telescopes can make out the general shape, and its greenish colour. Using even the largest of telescopes, we can observe something new that we have not noticed before. Commonly we call it the Great Nebula in Orion M42 is found lying in the middle of the sword of Orion, or the handle of the saucepan, as seen from the southern hemisphere. In even moderate telescopes it is a truly grand object and words cannot adequately describe this nebula as it truly appears.

My own impression of the Orion Nebula is firmly fixed in my brain. As the one of the first objects that I observed with a small 60mm refractor, I can still recall being fascinated by the faint, but highly delicate nebulosity. A sharp boundary to darker portions contrasts the bowl of the nebula. The most obvious feature is the boundary of light call the Bright Bar. Naturally, when observing it with a much larger telescope, the subtle differences in the surface brightness become obvious, with a mottled surface inter-dispersed with faint filaments.


Orion Nebula M42

Fig. 5. Visual Appearance of the Orion Nebula (M42)

Image of the Orion Nebula as seen in 20cm telescope. The pale green colour is approximately the colour seen in the telescope. Older observers might not seen this colouration at all due to the colour loss under darkened conditions, which is mainly due to the ageing process.
The image clearly shows the curve along the edge of the bowl, with the Trapezium near the centre of the frame — pointed to by the small protuberance of dark nebulosity from the curve of the bowl. Deep colour images reveal this region around the Trapezium has carved out by the hot energetic radiation from the new blue stars, producing the distinct hollow bowl. [See Figure 1.]


One of the first astronomical photographs taken was of this object, allegedly made by American spectroscopist, Henry Draper, at the Harvard College Observatory in 1880. It is obviously a favourite object among those interested in astrophotography or imaging and is truly the most photographed object in the sky! A small telescope, with the short time in guiding, produces truly magnificent results. For the amateur astrophotographer it is likely the first object attempted. It remains a popular photograph in astronomical periodicals, advertising materials and in books. For example, it has been in each issue of the magazine Sky and Telescope for the past seventeen years (2005)!


Illumination of the Orion Nebulosity


Many parts of M42 we see in the bright nebulosity are illuminated by stars associated whose structure making the individual parts of the nebulosity appear so luminescent. In reality, this nebula brightness is because of the molecules and atoms contained within the nebula itself. Excitation of this ionised material is by the energies from the many nearby hot stars that are radiating strong UV radiation — causing the familiar fluorescent glow.

A large proportion of M42 is possibly illuminated by the star known as Becklin-Neugebauer Object (BNO) or sometimes just Becklins Object, which remains visually unseen due to significant absorption of light by the bulk of the nebulosity. Discovered by Eric Becklin and Gerry Neugebauer in 1967, this hidden object appears as the brightest star in the sky at near infrared (NIR) wavelengths (or thermal infrared) at about 2μ to 10μ (microns). Such wavelengths are equivalent to temperatures emitted by the human body. The nature of this star is thought to be a high mass protostar that is in the process of emerging from its embryonic nebulosity from which it recently formed. In more recent times (1993), there was found also another very close object known as IRc2 which seems likely to be a trapezium-like multiple star. Both these stars are some 0.5pc. (1.6 ly.) behind the bright bowl of the Orion Nebula. Origin of the BHO protostar is uncertain and there are several options being considered by astronomers.


Size of the Orion Nebula


In true size of the nebula covers about 1¼ square degrees of the sky, roughly equivalent to be between seven and eight times the apparent area of the Moon. M42 is 66′×60′ in size, while to the north is M43 that measures some 20′× 14′. Combined magnitude of the nebula for M42 is equal to any 2nd magnitude star, while the star known as θ1 Orionis has the combined visual magnitude of 2.9v. This star is also known as the multiple star HJ 4581 / h.4581, which was first catalogued by John Herschel.

Astronomers have estimated that the nebulas total masses, is somewhere between 3,000 ΣM⊙ and 5,000 ΣM⊙. Distance to the nebula has been presently estimated to be between 430±60 pc. (370pc. to 490pc) or roughly 1,400±200 ly. (1,200 ly. to 1,600 ly.) However, most of the literature now often tends towards the higher values. In true size, the bright fan-shaped section of M42 is estimated to be about 8 to 15 ly. across, while the maximum size revealed by the photographic process, measures at about 30 to 35 ly.! We have measured the nebula to approach the Earth at 25 km.s-1 (8 km.s-1 less than stated by Hartung). The Trapezium is travelling at a slower velocity of −11 km.s-1.

Using the distance to the Orion Nebula as 389 parsecs or 1270±76 light-years, means that the trigonometric parallax is 2.571 mas (milliarcsec). If the principal stars in the Trapezium are at this distance, based on the relative position of STF 748A at 6.55V magnitude, then the nature of the system is listed in Tables 1 & 2 (Below.)


No one knows for sure about the future of the nebula of M42, but it seems the stellar creation process is still going on. Some have suggested that the nebulosity will become even more brilliant perhaps sometime within the next million years or so.


The Trapezium Stars


TABLE 1 : TRAPEZIUM DOUBLE STARS
Star System Sep.
arcsec.
mag.
(V)
Spectral
Class
STF 748 / Σ 745 AB08.97.49 O7/B1V
STF 748 / Σ 745 AC13.15.06 O7
STF 748 / Σ 745 AD21.56.38 B0.5V
STF 748 / Σ 745 AE04.5 11.1 O7
Projected distances in AU and light years between the Trapezium components; all from the star A.
TABLE 2 : INDIVIDUAL TRAPEZIUM STARS
Star Dist.
AU
Dist.
l.y.
Mabs Mass
M⊙
A0.0 0.0-1.4 4.5
B3462 0.055-1.4 3.6
C 50950.081 -2.9 6.4
D 8362 0.132 -1.6 4.7
E 1750 0.028 +3.2 1.5

In physical size, therefore for the Trapezium is roughly about 10,000 AU across.

The largest estimated minimum orbital period of these stars is about 250,000 years or about 2½ times that of Alpha Centauri AB to Proxima. This augers well with the fractional movements so far observed with these stars.

You could compare these values with the older ones described in the dated Burnhams Celestial Handbook Vol.2 Orion on Theta Orionis.

Note: If value were between 1400 to 1600 light years, these projected separations would be slightly larger.


Open Note: As stated in the Universe Today site and blog on 2nd April 2009, I have given full permission to my friend under the avatar of Salacious B. Crumb to use this text as he sees fit. I also do appreciate his additional information and discussions on the Trapezium and especially his very useful work on the parameters of the size and shape of this wonderful bright, popular multiple star.



Last Update : 8th June 2012

Southern Astronomical Delights © (2012)

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