Orion Nebula is one of the most Fascinating objects in the sky. It is a diffuse and brightest nebula. It is naked eye object and closest star formation structure to earth at distance 1344 light years. This nebula is 30 to 40 light years across. Orion nebula revealed much information about the process of star formation. Here stars and planetary systems are formed from collapsing clouds of gas and dust.
Orion nebula is prominent nebula which can even visible in light polluted areas. It can be found in the sword of Orion constellation. The Northern Hemisphere, winter months and in Southern Hemisphere, summer months are perfect time to watch the Orion constellation and nebula. As a general rule, the higher the constellation Orion is in the sky, the easier it is to see the Orion Nebula. Orion constellation is on zenith in the sky around midnight in middle December. Nebula looks like a star encased in a globe of luminescent fog. The nebulosity can be observed by obviously through binoculars or telescope. The new born stars are clustered and known as open cluster.
Orion nebula is also famous as Messier 42, M42, or NGC 1976. It has visual magnitude of 4. Its angular diameter is 85×60 arc-minutes. Co-ordinates of nebula are RA= 5h 35.4m, Dec= -05° 27´. The entire Orion Nebula extends across a 1° region in sky.
The red glow visible part in this nebula are characteristics of ionized hydrogen as it recombines with lost electrons. The bright part of nebula is the glow of many newborn and luminous stars shining on the surrounding gas cloud that they collapsed from. Orion Molecular Cloud is the most important part of Orion nebula. This is a huge clump of very cold gas that has a total mass of about 2000 times the mass of the Sun. The gas from this cloud slowly collapses due to gravity to form stars. The nebula consists of an ionized (H II) region, roughly centered on Theta1 Orionis C, which lies near molecular cloud. The gas in the molecular cloud displays a range of velocities and turbulence, particularly around the core region. There are stellar siblings dominated by a few very massive, very very bright stars called as Trapezium. The Trapezium is made up of just a few stars, but it outshines all the rest of them combined. Astronomers believe that the majority of the glow from the gas in the nebula comes from light from the stars of the Trapezium. Maybe in a few hundred million years, there will be planets like the Earth forming around some of the new stars in the cluster.
Observations by Hubble Space Telescope have yielded the major discovery of proto-planetary disks within the Orion Nebula. Clumps of hydrogen and other gases in H II region gets contract under their own gravity and then star formation takes place. Central clump grows stronger as gas collapses. Then gas heats with huge temperature by converting gravitational potential energy to thermal energy. If the temperature gets high enough, nuclear fusion will ignite and form a proto-star. Then it begins to emit radiative energy to balance out its gravity and halt gravitational collapse. Recent infrared observations showed that dust grains in these proto-planetary disks are growing and this may result in planets like the Earth forming around some of the new stars in the cluster.
As stars form, in colorful and interesting Orion nebula, the blue-violet coloration is the reflected radiation from the massive O-class stars at the core of the nebula. The red hue is a result of the Hα recombination line radiation at a wavelength of 656.3 nm. The green hue was a puzzle for astronomers as none of the known spectral lines at that time could explain it. There was some speculation that the lines were caused by a new element, and the name nebulium was coined for this mysterious material. Later it was later determined that the green spectrum was caused by a low-probability electron transition in doubly ionized oxygen, and it is called as forbidden transition. This radiation was all but impossible to reproduce in the laboratory because it depended on the quiescent and nearly collision-free environment found in deep space.
The history of Orion nebula is quite interesting with a great conundrum in astronomy. Why this was not observed by anyone? Either that the Orion Nebula was simply not noted or nebula was not as bright in the time of Galileo. Nobody else prior to observe and notice this Great Orion Nebula.
The initial deep-sky observations by the Persian astronomer Abd al-Rahman Al Sufi or Azophi, who published his “Kitab suwar al-kawakib” (Book of Fixed Stars) in 986 A.D, were highly defined. He even noticed the extragalactic spiral Andromeda galaxy (M31), “h and Chi” Per (NGC 869/884), M44 (Praesepe, NGC 2632), M7 (NGC 6475), IC 2391 (Omicron Velorum Cluster) he mentioned in his book. Yet he had not mentioned any nebulosity within Sword of Orion. Nebula may not have been as bright during this time.
Johann Bayer (German astronomer) made the highly detailed star chart and published in 1603 at Augsburg, Germany, which is known as “Uranometria”. This also does not show any nebulosity within 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.