Jimmy Westlake: The Great Nebula in Orion
February 9, 2010
When 18th century French astronomer and comet-hunter Charles Messier aimed his telescope at the middle star in the Sword of Orion, he thought he might have discovered a new comet. The object certainly had the fuzzy appearance of a comet, but to Messier's dismay, it turned out to be a false alarm. He had instead made an independent discovery of what now is known as the Great Orion Nebula. It became the 42nd entry in Messier's now famous catalogue of comet look-alikes and is known 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 H-II nebula. The word "nebula" comes from the Latin word for "cloud." Any small telescope or even ordinary binoculars will reveal the fuzzy nature of M42. It resembles a cluster of streetlights seen through a thick fog.
The source of the light illuminating the nebula is a quartet of four very hot, luminous stars collectively called the Trapezium. A small telescope at medium power should show the four stars of the Trapezium.
NASA's Hubble Space Telescope and Spitzer Space Telescope have helped unravel the mysteries of this colorful cloud of gas. The Great Orion Nebula is a stellar nursery where hundreds, even thousands, of new stars and planetary systems are forming from the interstellar gas. A few million years hence, a cluster of new stars will exist where we now see the nebula.
Recent measurements have nailed down the distance of M42 to be 1,350 light years. The visible portion of the nebula measures about 24 light years across, but it is only a very small portion of a much larger dark cloud known as the Orion Giant Molecular Cloud. Wherever bright stars illuminate the dark corners of the Orion GMC, we see an emission nebula such as M42. Other bright regions of the cloud show up as our Horse Head Nebula, the Flame Nebula, Messier 43, Messier 78, and an enormous structure called Barnard's Loop.
No optical aid is required to spot the Great Orion Nebula, though binoculars will enhance the view. Look just south of the three familiar stars that form 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 green.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. Check out his Web site at http://www.jwestlake.com.