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The blue-violet coloration is the reflected radiation from the massive O-class stars at the core of the nebula.The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known spectral lines at that time could explain it.The star appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through binoculars or a small telescope.The peak surface brightness of the central region is about 17 Mag/arcsec (In the photos shown here the brightness, or luminance, is enhanced by a large factor.) The Orion Nebula contains a very young open cluster, known as the Trapezium due to the asterism of its primary four stars.The images have been used to build a detailed model of the nebula in three dimensions.
Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula.
It has a mass of about 2,000 times that of the Sun.
Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.
There was some speculation that the lines were caused by a new element, and the name nebulium was coined for this mysterious material.
With better understanding of atomic physics, however, it was later determined that the green spectrum was caused by a low-probability electron transition in doubly ionized oxygen, a so-called "forbidden transition".
(The first detection of three of the four stars of this cluster is credited to Galileo Galilei in a February 4, 1617 although he did not notice the surrounding nebula — possibly due to the narrow field of vision of his early telescope.) The nebula was independently discovered by several other prominent astronomers in the following years, including by Giovanni Battista Hodierna (whose sketch was the first published in De systemate orbis cometici, deque admirandis coeli characteribus).