Right Ascension	05 : 35.4 (h:m)	M 42 Diffuse Nebula M42 (NGC 1976), an emission and reflection nebula, in Orion Orion Nebula
Declination	-05 : 27 (deg:m)
Distance	1.6 (kly)
Visual Brightness	4.0 (mag)
Apparent Dimension	85x60 (arc min)

Discovered 1610 by Nicholas-Claude Fabri de Peiresc.

Located at a distance of about 1,600 (or perhaps 1,500) light years, the Orion Nebula is the brightest diffuse nebula in the sky, visible to the naked eye, and rewarding in telescopes of every size, from the smallest glasses to the greatest Earth-bound observatories and the Hubble Space Telescope.

It is the main part of a much larger cloud of gas and dust which extends over 10 degrees well over half the constellation Orion. The linear extend of this giant cloud is well several hundreds of light years. It can be visualized by long exposure photos (see e.g. Burnham) and contains, besides the Orion nebula near its center, the following objects, often famous on their own: Barnard's Loop, the Horsehead Nebula region (also containing NGC 2024 = Orion B), and the reflection nebulae around M78. Already impressive in deep visible light photographs, the Orion Cloud is particularly gorgeous in the infrared light.

The Orion Nebula itself is still a big object in the sky, extending some 66x60 arc minutes, thus covering more than four times the area of the full Moon. This corresponds to a linear diameter of about 30 light years. It is also one of the brightest Deep Sky objects, well visible to the naked eye, so that many (including the present author) are wondering that its nebulous nature was apparently not documented before 1610, when Nicholas-Claude Fabri de Peiresc (1580-1637), a French lawyer, turned his telescope to this region of the sky. Previously, Ptolemy, as well as later Tycho Brahe and Johann Bayer had cataloged the brightest stars within it as one bright star - the latter cataloging it as Theta Orionis, and Galileo had detected a number of faint stars when first looking at this region with his telescope earlier in 1610. It was independently rediscovered in 1611 by the Jesuit astronomer Johann Baptist Cysatus (1588-1657) of Lucerne who compared it with a comet he had observed in the same year. The first known drawing of the Orion nebula was created by Giovanni Batista Hodierna. All these discoveries apparently got lost for some time, so that eventually Christian Huygens was longly credited for his independent rediscovery in 1656, e.g. by Charles Messier when he added it to his catalog on March 4, 1769.

As the drawings of the Orion Nebula known to him did so poorly represent Messier's impression, he created a fine drawing of this Object, in order to "help to recognize it again, provided that it is not subject to change with time" (as Messier states in the introduction to his catalog).

This gorgeous object continued to influence astronomers since. It was the first deepsky observation by William Herschel with a self-constructed reflecting telescope of 6-foot focal length in 1774. In 1789, with some prophetic touch, he described his observations with his 48-inch aperture, 40-foot FL scope as "an unformed fiery mist, the chaotic material of future suns."

The gaseous nature of the Orion Nebula was revealed in 1865 with the help of spectroscopy by William Huggins. In September 1880, M42 was the first nebula to be successfully photographed, by Henry Draper.

The nebula, on its northern end, is devided by a conspicuous dark lane, well visible in our photograph. This image was obtained David Malin of the Anglo-Australian Observatory. More information on this image is available.

The small northeastern portion was first reported by de Mairan, and was given an extra number by Charles Messier, M43 (see below also). In the very neighborhood, to the north, there are also fainter reflection nebulae, partially reflecting the light of the Great Nebula. They were not notable for Charles Messier, but labeled later with the NGC numbers 1973-5-7. Here we have a collection of more images of M42, M43, and more images of M42, M43 and NGC 1973-5-7.

M42 itself is apparently a very turbulent cloud of gas and dust, full of interesting details, which C.R. O'Dell compares to the rich topography of the Grand Canyon in his HST photo caption. The major features got names on their own by various observers: The dark nebula forming the lane separating M43 from the main nebula extends well into the latter, forming a feature generally nicknamed the "Fish's Mouth". The bright regions to both sides are called the "wings", while at the end of the Fish's Mouth there's a cluster of newly formed stars, called the "Trapezium cluster". The wing extension to the south on the east (lower left in our image) is called "The Sword", the bright nebulosity below the Trapezium "The Thrust" and the fainter western (right) extension "The Sail". Here we have a small collection of Images of detail in M42, including another nomenclature for the brightest region in the nebula by historic visual observers, as well as a pictorial study of the Trapezium cluster and region by Lowell Observatory images.

The Trapezium cluster is among the very youngest clusters known, with new stars still forming in this region. The cluster was first depicted as triple star apparently by Hodierna before 1654 (see his drawing), and first described by Christian Huygens in 1656 when he indepedently rediscovered the Orion nebula. These first three stars are often labelled "A", "B", and "C". At this time, this was apparently the second recognized multiple star (after Mizar in Ursa Major which had been found to be a telescopic double in 1650). The fourth Trapezium star, "D", was first found by Abbe Jean Picard in 1673 (according to de Mairan), and independently by Huygens in 1684. The fifth cluster star "E" was discovered by Friedrich Georg Wilhelm Struve in 1826 with a 9.5-inch refractor in Dorpat, the sixth, "F", by John Herschel on February 13, 1830, the seventh, "G", by Alvan Clark in 1888 when testing his 36-inch refractor of Lick Observatory, and the eighth, "H" by E.E. Barnard later in 1888 with the same telescope. Barnard later found that "H" is double, with two 16th-magnitude components. Today we know that stars "A" and "B" are both eclipsing variables of Algol type: A varies between magnitudes 6.73 and 7.53 with a period of 65.4325 days, while B varies between mag 7.95 and 8.52 in 6.4705 days.

The Orion Nebula is also one of the easiest and most rewarding target for amateur astrophotographers.

The past decades of research on the Orion Nebula have revealed that the visible nebula, M42, the blister of hot, photo-ionized, luminous gas around hot Trapezium stars, is only a thin layer lying on the surface of a much larger cloud of denser matter, the Orion Molecular Cloud 1 (OMC 1). We happen to see this structure approximately face-on. The idea for this model came originally from Münch (1958) and Wurm (1961) and fully elaborated by several authors around 1973-1974 (Zuckerman (1973), Balick et.al. (1974)), soon supported by evidence, and is still studied in detail, see e.g. O'Dell (2001) for a recent review, and references cited therein. The San Diego Supercomputer Center (SDSC)'s VisLab has created a 3-dimensional visualization of the Orion Nebula based on this model (see side-view model image of M42).

The Orion nebula was, continuously since the early times before its refurbishment, a preferred target for the Hubble Space Telescope. One major discovery was that of protoplanetary disks, the socalled "Proplyds" (planetary systems in formation) in these HST images of M42 (these images were used for an animation simulating the approach to a protostar [caption]). HST images of November 1995 have revealed further insight into the complicated process taking place in this "star factory". Hubble investigations of January 1997 have revealed interesting interactions of the young hot Trapezium cluster stars with the protoplanetary disks: Their violent radiation tends to destruct the discs, so that the lower-mass stars forming here may loose the material needed to form planetary systems.

An excellent review of the astrophysics of the Orion Nebula is provided in 2003 with the superb monograph by Charles Robert O'Dell (O'Dell 2003), who summarizes the knowledge of that time, including HST research.

It is very easy to find the Orion Nebula, as it surrounds the Theta Orionis multiple star or cluster, seen to the naked eye in the middle of the sword of Orion. Already under fairly good conditions, the nebula itself can be glimpsed with the naked eye as a faint nebulosity around this star.

It is somewhat unusual that the Orion Nebula has found its way into Messier's list together with the bright star clusters Praesepe M44 and the Pleiades M45; Charles Messier usually only included fainter objects which could be easily taken for comets. But in this one night of March 4, 1769, he determined the positions of these wellknown objects, (to say it with Owen Gingerich) `evidently adding these as "frosting" to bring the list to 45', for its first publication in the Memoires de l'Academie for 1771 (published 1774). One may speculate why he prefered a list of 45 entries over one with 41; a possible reason may be that he wanted to beat Lacaille's 1755 catalog of southern objects, which had 42 entries. Messier measured an extra position for a smaller northeastern portion, reported by de Mairan previously, which therefore has the extra Messier number: M43.