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M42 is not only the brightest (its visual magnitude is 4,0) but also the most famous and the most photographed nebulae in the night sky. Approximately 1 500 light years distant, M42 is a very active and turbulent cloud of gas and dust and an important star forming region of particular interest to astronomers. There are many hot young stars (most notably the Trapezium stars) which fuel the dense swathes of surrounding gas, causing it to ionize and produce the red emission glow. M42 has a particularly complex range of emission sources as part of it's spectrum, as well as a strong component of reflected broadband light, which probably accounts for the wide range of visible colours.

It is estimated that the Orion Nebula has enough material within it to form about 10 000 Sun-sized stars. Easily visible to the naked eye, and superb when seen through binoculars or a telescope, the three stars of the Trapezium can often be seen to sparkle on a moonless winter's night.

Here tens of thousands of new stars have formed within the past ten million years or so (a very short span of time in astronomical terms). For comparison: our own Sun is now 4,6 billion years old and has not yet reached half-age. Reduced to a human time-scale, star formation in Orion would have been going on for just one month as compared to the Sun's 40 years.

However, the heart of this nebula also conceals a secret from the casual observer. There are in fact about one thousand very young stars about one million years old within the so-called Trapezium Cluster, crowded into a space less than the distance between the Sun and its nearest neighbour stars. The cluster is very hard to observe in visible light, but is clearly seen in the spectacular image of this area (ESO PR 03a/01), obtained in December 1999 by Mark McCaughrean (ESO Very Large Telescope (VLT) at Paranal (Chile).

The bright part of the nebula is the glow of many luminous, newborn stars shining on the surrounding gas cloud that they collapsed from. The most important part of the Orion Nebula is the part we can't see: the opaque Orion Molecular Cloud. 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. Whenever a bright, new star is formed, its light evaporates the opaque gaseous "womb" it formed from, allowing us to see it. The stars that are being born in the Orion Nebula are part of what astronomers call an "open cluster". When all of the stars are done being born, what will remain is a clump of a few hundred to a thousand stars which are all roughly the same age. These stellar siblings are dominated by a few very massive, very very bright stars called the 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.

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".

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, and first described by Christian Huygens in 1656 when he independently rediscovered the Orion nebula. These first three stars are often labeled "A", "B", and "C". The fourth Trapezium star, "D", was first found by Abbe Jean Picard (according to de Mairan), and independently by Huygens in 1684. The fifth cluster star "E" was discovered by 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.