The Orion Nebula | Kitt Peak National Observatory
This picture shows the great nebula in the constellation of Orion the Hunter around 1,500 light years away. On a good clear night, from a dark site well away from the lights of modern civilization, this glowing cloud of gas and dust can be seen with the naked eye as a fuzzy patch surrounding the star Theta Orionis in the Hunter's Sword, below Orion's belt. It is probably the most spectacular of all the objects cataloged by Charles Messier and now called by their `M' numbers. M42 had been known since the beginnings of recorded astronomy as a star, but it is so outstanding that it was first noted as an extended nebula in 1610, only a year after Galileo's first use of the telescope. Detailed descriptions started appearing later in the seventeenth century, and it has been a popular target for anyone with a telescope ever since.
M42 is our closest example of an HII region, being composed mainly of ionized hydrogen. Deep photographs, such as this one, show that it is nearly a degree across, larger than the full Moon (although the Moon is so bright that it looks much larger). The energy to keep the nebula glowing comes from the very hot young stars in a formation called the Trapezium, embedded in the brightest part of the nebula and not visible in this photograph.
Stars are still being born in a dense cloud behind the nebula, but they are hidden from our view by a concentration of dust. This reduces their light to only a million-millionth of its original intensity. Fortunately, astronomers have developed special cameras and other detectors that are sensitive to infra-red radiation, more popularly known as heat. These can penetrate the dust and reveal this stellar nursery to us.
Although M42 is mostly hydrogen, in both neutral and ionized states, with a fair quantity of dust, it does contain significant amounts of other elements, especially oxygen. The green glow of doubly-ionized oxygen is strongest near the intense ultraviolet starlight at the middle of the nebula. To the north-east (the upper left in this picture) is a feature called the Dark Bay. It is a thick cloud of neutral gas that has not yet been ionized.
So many details are visible in even a small telescope that M42 will more than repay the observer that makes it a frequent target, and who will find that it is hard to make a realistic sketch that can capture all of the finer features. This nebula is so impressive in astronomy, it is represented by multiple catalog numbers. M43/NGC1982 is the separated portion to the north-east (top left), surrounding an irregular variable star. Although Messier stopped at only two, other parts of the nebula in this region have received further New General Catalogue (NGC) numbers.
Image Credit: Bill Schoening, KPNO 4m telescope; original Ektachrome color transparency
Image Date: October 1, 1973
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