Celestial News: Behold the Great Orion Nebula
February 1, 2018
Anyone who has ever looked up at the starry winter sky has seen it, although they might not have known what they were seeing. The three bright stars in a neat little row stand out among the other stars like a neon sign.
Some folks mistake this prominent asterism for the Big Dipper. Some call them the Three Marys, others, the Three Wise Men, but officially, these three stars are called the Belt of Orion. From left to right, their names are Alnitak, the Girdle; Alnilam, the String of Pearls; and Mintaka, the Belt.
Three fainter stars beneath Orion's Belt represent his sword, hanging by his side. When comet-hunter Charles Messier aimed his telescope at the middle star in Orion's sword in 1769, he thought he might have discovered a new comet. The object certainly had the fuzzy glow of a comet.
To Messier's dismay, however, the object turned out not to be a comet. Instead, he had made an independent discovery of what we now call the Great Orion Nebula. It became the 42nd entry in Messier's now famous catalogue of comet imposters and is known today as Messier 42, or just M42 for short.
M42 has since become one of the most photographed and carefully studied deep sky objects in all the heavens. It is the closest and best example of an ionized cloud of hydrogen gas in space called an emission nebula.
Ordinary binoculars or any small telescope will reveal the fuzzy appearance of M42. The source of the light illuminating the nebula is a quartet of very hot, luminous stars at its heart, collectively called the Trapezium. It resembles a cluster of streetlights seen through a thick fog. A small telescope at medium power will show the four stars of the Trapezium.
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NASA's Hubble and Spitzer Space Telescopes have helped unravel the mysteries of this colorful cloud of gas. The Great Orion Nebula is a stellar nursery where thousands of new stars and planetary systems are condensing from the interstellar gas. In a few million years, a cluster of young stars will burn where we see the nebula today.
Recent measurements have nailed down the distance to M42 as 1350 light years. The visible portion of the nebula measures about 24 light years across but, like the tip of an iceberg, it is only a very small portion of a much larger dark cloud known as the Orion giant molecular cloud, or GMC. Wherever bright stars illuminate the dark corners of the Orion GMC, we see an emission nebula, like M42. Other regions of the cloud show up as the Horse Head Nebula, the Flame Nebula, M43 and NGC 1977.
No optical aid is required to spot the Great Orion Nebula, although binoculars will enhance the view. Look just south of the three familiar stars of Orion's Belt, at the middle star of the fainter trio that forms the Hunter's sword. Don't expect to see the vibrant reds and blues shown in long-exposure photographs of M42. At night, the human eye's color receptors are not activated, so even the colorful Orion Nebula appears a pale gray.
Jimmy Westlake retired from full time teaching at Colorado Mountain College's Steamboat campus in 2017, after nineteen years as a professor of physical sciences. His Celestial News column appears in the Steamboat Today newspaper monthly. Check out Westlake's astrophotography website at jwestlake.com.