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Winter: bright star clusters and nebulae abound in this season.

THE COLDEST SEASON of the year is a magnificent time for skygazers. The stars seem brighter and closer than any other time of the year, and this is no mere illusion. Our Sun resides in the Milky Way Galaxy, on the inner edge of a spiral-arm fragment called the Orion Spur. During northern winters, Earth's night side faces this arm and its nearby stars, many of them young, blue-white giants.

Orion, the Hunter, dominates the night, boasting more brilliant stars than any other constellation. Our eyes are quickly drawn to Orion's Belt, a line of three equally spaced stars of about the same brightness. Dangling beneath the Belt, a smaller collection of faint stars represents his Sword.

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Under dark skies, you can see that part of the Sword looks a bit fuzzy. This is the great Orion Nebula, a vast star-forming region and the showpiece of the winter sky. Bejeweled with youthful stars that lend it illumination, this diaphanous fan of gas and dust is visible through the smallest of telescopes. Stars are born from the nebula, and they in turn make its gas glow and its dust reflect their light.

The main body of the Orion Nebula is known as Messier 42, or simply M42, signifying its place as the 42nd object in the catalog of the 18th-century French astronomer Charles Messier. A 4-inch telescope at a magnification of 90x reveals the Trapezium, a tiny lopsided box of four stars buried in the brightest part of the nebula. When the atmosphere is steady enough that the stars appear hard and sharp with no twinkling, two dimmer stars join the figure. One lies just outside the Trapezium's shortest side and bears a yellowish hue. More difficult to detect, the other sits near the Trapezium's brightest star and is often lost in its glare.

A dark intrusion, known as the Fish's Mouth, juts into M42 from the east and almost reaches the Trapezium. This is not an empty void, but rather a dark, unlit nebula barring our view of what lies beyond. The outer edges of the Orion Nebula are defined by pale plumes that sweep southwest. If your sky is very dark and haze free, you may see these plumes curve around and meet each other in soft embrace.

Although the dominant color of the Orion Nebula in most photos is pink or reddish, our eyes are not particularly sensitive to this color at low light levels. Instead, the bright region enfolding the Trapezium looks mottled gray or grayish green. The green color becomes more obvious through larger telescopes at high powers, while the texture of the nebula brings to mind the clouds of a mackerel sky. The outer regions of the nebula weave an intricate tapestry of streamers and tendrils that reveals more complexity as you make M42 an oft-visited friend.

A seemingly detached portion of the Orion Nebula carries its own entry in Messier's catalog as M43. At first glance, it looks like a small, roundish glow surrounding a moderately bright star. A closer inspection in a fairly dark sky reveals dimmer regions that turn M43 into a fat, backward comma with a northward-reaching tail.

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At the scale of our galaxy, the Orion Nebula is a next-door neighbor, but even our neighbors are impressively far away. The Orion Nebula is 1,400 light-years distant, and 1 light-year is approximately 6 trillion miles.

Sweeping a bit farther north takes us to the open star cluster NGC 1981. Open clusters are so named because open space is visible between their stars. They reside in the disk of our galaxy, and most of them are relatively young. While our Sun is nearly 5 billion years old, the stars of NGC 1981 formed only 150 million years ago.

Through a 4-inch scope at 50x, the brightest stars of NGC 1981 outline a charming shape that looks much like the path of a bouncing ball. A dozen faint stars join these loosely scattered gems, making a group that spans almost as much sky as a full Moon.

Now plunge southward to the fascinating little nebula NGC 1999. A 5-inch scope at 30x will probably just fit M42 and NGC 1999 in the same field of view, the latter appearing as a faint star wearing a fuzzy fringe. At 160x the nebula starts to betray its secrets, revealing wispy rays and baring an elusive inkblot just west of the star. This sable spot is a Bok globule--a dense, collapsing cloud where low-mass stars may be forming. Subtle details show up much better in a 10-inch scope at 300x, and the globule takes on the shape of a curved wedge or a fat T, as shown below.

The wonders of Orion are among the most glorious spectacles of the sky. Northern observers may shy away from icy winter nights, but those nights offer us some of the most beautiful vistas in the heavens. It's well worth bundling up to spend an evening among them.

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Title Annotation:OBSERVING: Deep-sky Delights
Publication:SkyWatch
Geographic Code:1USA
Date:Jan 1, 2011
Words:835
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