The A-List Objects in the Constellation
M42 - The Orion Nebula: star formation region
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Probably the best known of all the constellations, Orion dominates the sky in January, February and March.   Perhaps surprisingly, given its closeness to the milky way, it is rather devoid of deep-sky objects that merit inclusion the A-List.  However, the one that does is one of the true highlights of the heavens.
M31 - The Orion Nebula       Star Forming Region         E B L M H
The Orion nebula, seen as a diffuse glow in the sword of Orion, is one of the most beautiful objects in the heavens.  It is a region of star formation 1600 light years from us, the glowing gas excited by ultraviolet light from the young, very hot stars at its heart.  It extends over a region of angular size 1 by 1.5 degrees but is part of a much larger cloud that covers much of the constellation, over 10 degrees across. Photographs show swathes of nebulosity such as 'Barnards Loop' arching around the M42 region.  Seeing that M42 is one of the first deep-sky objects that most astronomers observe – and rightly so – it is surprising that there is no mention of it in ancient records.  Apparently, not even Galileo observed it, even though, at about 4th magnitude, it is visible to the unaided eye under reasonably dark skies.  The gases that make up the nebula are predominantly hydrogen and helium dating from the origin of the Universe along with nitrogen and oxygen produced by nuclear fusion as stars evolve.  The electrons are stripped from their nuclei by the ultraviolet light and, as they re-combine, give off well-defined spectral colours: a pinky red for hydrogen and green and blue for oxygen.  The eye is not sensitive to red so with a reasonably large telescope, the brighter parts of the nebulosity appear greenish in colour.  Observing the nebula under dark and transparent skies with a telescope at low power shows the wonderful looping whirls of dust and gas seen across a wide region.  Using a medium-power eyepiece, the central region of the nebula will be seen to harbour a number of bright stars; three almost in a line and 4 making up what is called the 'trapezium'.  A dark dust cloud can be seen intruding in towards the bright inner core of the nebula surrounding the trapezium. This is called the 'fish's mouth'.  Moving to high power will show the four stars, A to D, of the trapezium.  The brightest is 5th magnitude and provides the vast majority of the ultraviolet light that excites the gas in the nebula.  Two others are 6th magnitude with the faintest 8th magnitude.  Using very high magnification under nights that are both dark and have good seeing (which are rather rare), even a 4-inch telescope will pick out a fifth, 10th magnitude, component called E, making up a flattened triangle with the closest pair of stars.  It lies just outside the trapezium. There is a sixth, 10th to 11th magnitude, star called F on the opposite side of the trapezium but this is much harder to see.
Position 05h 35.4m   -05deg 27m
The Orion Nebula
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