M42
Alternate: NGC 1976, the Great Orion Nebula
Orion
RA 5h 35.3 m
Dec -5º 23′
Magnitude 3.7
M43
Alternate: NGC 1982
Orion
RA 5h 35.6 m
Dec -5º 16′
Magnitude 6.8
With the cold winter months upon us, our old cosmic friend Orion the Hunter has returned to our night skies. And with the presence of this most famous constellation is the arrival of its most famous deep sky object, the Great Orion Nebula.
Many readers are probably unaware that when viewing the Great Orion Nebula they are actually observing a cosmic duet. The proximity of M43 to M42 often goes unnoticed due the spectacular nebulosity exhibited by the region overall. The Great Orion Nebula is easily detectable with binoculars. A 4-inch refractor can easily split M42 and M43 at 35x, with more detail revealed when higher powers and larger aperture telescopes are used. No need for wide-angle, super-wide-angle, or ultra-wide-angle eyepieces, a 50º AFOV Plossl will get the job done.
The first discovery of the nebula clouds of the Orion Nebula is generally credited to French astronomer Nicolas-Claude Fabri de Peiresc. Peiresc recorded his observations on November 26, 1610, while observing the nebula through his patron’s refracting telescope.
The first published details of the nebula were provided by the Jesuit mathematician and astronomer Johann Baptist Cysatus in his 1619 monograph on comets. He wrote of observations of the nebula dating back to 1611. Cysatus described the nebula as “one sees how in like manner some stars are compressed into a very narrow space and how round about and between the stars a white light like that of a white cloud is poured out.”
Galileo observed the Trapezium on February 4, 1617, but he did not observe the nebula itself. This oversight was probably caused by the poor optics and the small aperture of his telescope.
All of these discoveries were lost or the knowledge was not well distributed. Historically, Christian Huygens was credited with the discovery in 1656:
“There is one phenomenon among the fixed stars worthy of mention, which as fas as I know, has hitherto been noticed by no one and indeed, cannot be well observed except with large telescopes. In the sword of Orion are three stars quite close together. In 1656 as I changed viewing the middle one of these with the telescope [a 23-foot FL refractor], twelve showed themselves – not an uncommon circumstance, Three of these almost touched each other and, with four others, shone through a nebula so that the space around them seemed brighter than the rest of the heavens which was entirely clear and appeared quite black, the effect being that of an opening in the sky through which a brighter region was visible.”
Christian Huygens was a Dutch mathematician and scientist of the 1600’s whose impact is still felt today in astronomy. Not only is he credited with the discovery of the Orion Nebula, which is the Winter showcase deep sky object for backyard observers and astronomers, but was the originator of the Huygens eyepiece found as standard equipment in many beginner telescopes.
Charles Messier listed the Orion Nebula as Messier 42 in his first catalog, with his comments:
“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[Saturnian System]. 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. Here is the translation [actually here, the text] of what he has reported in this Memoir. ” only that inOrion, hath some Stars in it, visible only with the Telescope, but by no Means sufficient to cause the Light of the Nebulosethere. But by these Stars it was, that I first perceived the Distance of the Nebulosaeto be greater than that of the Fix’d 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 Fix’d Stars near them, whether visible to the naked Eye, or Telescopick only; yea, they seemed to be as far beyond the Fix’d 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 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, 80d 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. [p. 458] 1769.Mar. 4. RA: 80.59.40, Dec: 5.34. 6.A. Position of the star Theta in the Sword of Orion, which is situated in the middle of the nebula in that constellation.”
In 1731, Jean-Jacques Dortous de Mairan was the first to notice M43 as an independent part of the Orion Nebula:
“Finally I will add that close to the luminous space in Orion [M42], one sees the star d of Huygens [NU Orionis] currently (1731) surrounded by a brilliance very similar to that which produces, as I believe, the atmosphere of our Sun, if it were dense enough and extensive enough to be visible in Telescopes at a similar distance. See it in the form and the situation [given by] D, according to what was determined with the Reticule.”
On March 4, 1771, Charles Messier would also come to the same conclusion as he states in his observing notes:
“The star which is above, and has little distance from that nebula, and of which is spoken in the Traite de l’Aurore boreale [Treat of the Northern Light] by M. de Mairan is surrounded, and equally by a very thin light; the star doesn’t have the same brilliance as the four of the great nebula: its light is pale, and it appears covered by fog. I determined its position; its right ascension was 81d 3′ 0″, and its declination 5d 26′ 37″ south.”
M42 is the showpiece deep sky object visible in the Northern Hemisphere. This diffuse nebula is located approximately 1,344 light years away.M42 has a radius of 12 light-years.
The Orion Nebula is part of a much larger cloud of dust and gas, called the Orion Molecular Cloud Complex, which encompasses a large part of the Orion constellation. This nebulous cloud encompasses features such as M43, Barnard’s Loop, the reflection nebula Messier 78, the NGC 2024 Flame Nebula, and the famous B33 Horsehead Nebula.
Both M42 and M43 are emission nebulae, with their light emitted from atoms being excited by the high-energy radiation of massive, very hot young stars within them. Wisps of the Orion Molecular Cloud in the neighborhood of M42 and M43 constitute the fainter reflection nebulae elements. The Orion Nebula is a star nursery, with approximately 700 stars at various stages of early development. These young stars are the main source of energy that illuminates the Orion Nebula.
Backyard astronomers always look for the Trapezium asterism within M42. The Trapezium is group of very young energetic stars, with the appearance in smaller telescopes of four stars. Two of the Trapezium stars can be resolved into double stars on good seeing nights and larger telescope apertures. The Trapezium stars are part of the larger cluster of young stars energizing the HII the Orion Nebula.
Visually, the M42/M43 complex appears as a faintly greenish -white color through the telescope eyepiece. Photographically, the more illuminated regions of the nebula appear red. The green hue is caused by a low probability electron transition in doubly ionized oxygen, known to astrophysicists as the forbidden transition. Normally in astro-spectroscopy, a forbidden transition is a spectral line associated with the absorption or emission of light by H II regions, a result that cannot be observed under laboratory conditions because the gases cannot be rarefied sufficiently. In the laboratory, an excited atom tends to bounce off another particle or the walls of the gas container before it emits a photon, thus interfering with the experiment.
Forbidden is an odd term in this case, since the more accurate description is that the transition is highly improbable. The emissions result from electrons transitioning from an upper energy level to a lower energy level which requires a long time to take place. In an H II region, such as the Orion Nebula, the excited atom remains undisturbed long enough to emit a photon. Adding to this is the transparency of the H II region ionized gases to visible light, which permits the photons given off to contribute to detectable spectral lines. In the case of the Orion Nebula, the O III spectral lines of 495.9 nm and 500.7 nm are highly prominent. This explains the success the using of O III nebula filters for observing M42 and M43.
The red hue seen in astro-images is the result of the hydrogen-alpha recombination line radiation at the wavelength of 656.3 nm. The red hue is not observable to the visual observer because of the human eye is not sensitive to low-level red light. There is a reason why backyard astronomers use red flashlights at night!
The blue-violet hues seen in astro-images are actually reflections originating from O-class stars within the core of the Orion Nebula.
M42 and M43 are one of the most studied and photographed areas of the night sky by both professional and amateur astronomers. Every backyard astronomer with a camera has taken an astro-photograph of M42 and M43.