The Orion Nebula, catalogued as Messier 42 or NGC 1976, is one of the brightest nebulae in the sky, and one of the nearest star-forming regions to Earth. It is rewarding in telescopes of every size, and is perhaps the most studied and photographed object in the sky. M 42 is visible to the naked eye as a hazy patch surrounding Theta Orionis, the middle star in the Sword of Orion, just south of Orion's Belt.
Discovery and History
The Mayans of Central America had a folk tale which suggests that they knew of the Orion Nebula. But despite being visible to the naked eye, the Nebula is not mentioned in any known historical records before the invention of the telescope. Neither Ptolemy nor Al Sufi noted the nebula, even though they both listed patches of nebulosity elsewhere in the night sky. Around 130 AD, Ptolemy catalogued the brightest stars within the nebula as one bright star, as did Tycho Brahe in the late 16th century, and Johann Bayer in 1603, who designated them Theta Orionis (θ Ori).
In 1610, Galileo detected a number of faint stars when he first looked at this region with his telescope, but he curiously failed to note the nebula as well. Later, in 1617, Galileo took a closer look at the star, θ Ori, and found it to be triple, but again he failed to perceive the nebula. This has led to speculation that its illuminating stars have flared up since that time, increasing the nebula's brightness.
The discovery of the Orion Nebula is generally credited to Nicholas-Claude Fabri de Peiresc, a French lawyer who turned his telescope to this part of the sky in 1610. His sighting, however, was only reported in his own personal documents and never published. The nebula was found independently in 1611 by the Jesuit astronomer Johann Baptist Cysatus of Lucerne. He was the first to publish note of it, comparing it to a comet he had observed in 1618. The first known drawing of the Orion nebula was created by Giovanni Batista Hodierna.
All of these discoveries were apparently lost for some time. Eventually Christian Huygens, whose sketch was the first to be published, was credited for its rediscovery in 1656 - both by Edmond Halley, who included it in his list of six nebulae in 1716, and by Charles Messier, who added it as the 42nd object to his catalog.
Charles Messier first noted the nebula in 1769, along with three of its central stars. The nebula's smaller northeastern portion, previously reported as a separate object by Jean-Jacques d'Ortous de Mairan in 1731, was also added by Messier as number 43. Messier 42 and 43 were the first deep sky objects observed by William Herschel, who in 1789 described them as "an unformed fiery mist, the chaotic material of future suns."
The Orion Nebula's gaseous nature was revealed in 1865, with spectroscopy done by William Huggins. In 1880, Henry Draper took the first photograph of the Orion Nebula with an 11-inch refractor, marking the first historical instance of deep-sky astrophotography.
Amateur Observation
It is very easy to find the Orion Nebula. M 42 is visible to the naked eye under good conditions as a faint nebulosity surrounding Theta Orionis, the middle star in the sword of Orion. Theta Orionis is an extremely wide (135") binocular double, whose western component, θ1 Ori, is the famous Trapezium multiple star.
M 42 is the finest example of a diffuse emission nebula visible to mid-northern hemisphere observers, and is the brightest diffuse nebula in the sky, shining at 4th magnitude. It is also a large object, extending over 1 degree in diameter, and covers more than four times the area of the full moon.
Messier 43 is a bright nebulous arc to the north, detached from the main body of M 42 by a dark lane. This dark nebula extends well into M 42, forming a feature nicknamed the "Fish's Mouth". The bright regions to both sides are called the "wings". The wing extension to the southeast is called "The Sword"; the bright nebulosity below the Trapezium "The Thrust"; and the fainter western extension "The Sail".
The Orion Nebula contains a very young open cluster, at the end of the Fish's Mouth, known as the Trapezium cluster due to the asterism formed by its four primary stars. Two of these can be resolved into their component binary systems on nights with good seeing, giving a total of six stars.
In the neighborhood to the north, there are also fainter reflection nebulae, partially reflecting the light of the Great Nebula. They were not noted by Charles Messier, but were later labelled with the NGC numbers 1973, 1975, and 1977.
The Orion Nebula is also one of the easiest and most rewarding targets for amateur astrophotographers. Regions of red and areas of blue-violet are apparent in long-exposure photographs. The red hue is emitted by ionized hydrogen, particularly H-α emission at a wavelength of 656.3 nm. The blue-violet coloration is light reflected by fine interstellar dust grains from the massive O-class stars at the core of the nebula.
Visual observers have long noted a distinctive greenish tint to the nebula. This greenish appearance was a puzzle for astronomers in the early part of the 20th century, but is now known to be caused by radiation from doubly ionized oxygen.
Properties and Evolution
M42 is located at a distance of 1,350 light years, with an uncertainty of about 2%, and is estimated to be 24 light years across.
But the Orion Nebula is just a small illuminated blister on the surface of a much larger cloud of gas and dust that extends over 10 degrees, covering half of the constellation Orion. This much larger nebula is known as the Orion Molecular Complex (OMC 1); we happen to see this structure approximately face-on. The Orion Molecular Cloud extends several hundreds of light-years, and includes Barnard's Loop, the Horsehead Nebula, the Flame Nebula, and the reflection nebula M 78.
M 42 is the closest region of massive star formation to Earth. The youngest and brightest stars we now see in the Orion Nebula are thought to be less than 100,000 years old. Some of these newborn stars are particularly massive, and emit large quantities of ionizing ultraviolet radiation. The ultraviolet light of these hot stars causes the nebula to glow by fluorescence.
The Trapezium multiple star complex is among the most recent products of star formation in the Orion Nebula. This small group of class O and B stars is also responsible for most of the ultraviolet radiation that ionizes the nebula. Two million years ago, this cluster may have been the home of the "runaway stars" AE Aurigae, 53 Arietis, and Mu Columbae, which are currently moving away from the nebula at velocities greater than 100 km/s.
Altogether the Orion Nebula contains about 700 stars in various stages of formation. Many of the faint stars around the Trapezium are so young that they are still radiating energy from gravitational contraction, and have not yet settled down as stable main sequence stars. Other studies of the Nebula have revealed the presence of about 150 protoplanetary disks, supporting the view that these objects are common around infant stars.
Stellar winds emitted by newly formed stars creates shock waves when it encounters the gas in the nebula. This complex and turbulent motion shapes the gas clouds, leading to gravitational collapse, and triggering additional star formation. Eventually, most of the Orion Nebula's gas and dust will be ejected. The remaining stars will form an open cluster, which gradually disperse under the gravitational influence of other stars as it travels around the galaxy. The Pleiades are a famous example of such a cluster.