For the week including January 7, 2011

Cone region Hubble's Variable Nebula Red Rectangle Cone location Variable nebula location Red Rectangle Location Larger Illustration

MONOCEROS, THE UNICORN

The constellation of Monoceros, the Unicorn, covers a fairly large area of sky and contains some marvelous deep sky objects, but the fact that its member stars aren’t very bright makes this Unicorn, like most of them, very difficult to find. The origin of the constellation is equally obscure. One historical source attributes its creation to a Dutch mapmaker named Plancius who supposedly drew it up in the late 1600’s. Other sources credit a fellow named Bartschiuse, but it’s also listed in a book that dates back to the 1500’s. Then too, there’s an ancient celestial sphere from Persia that shows the unicorn as well. It will probably take the discovery of evidence in some dusty manuscript to resolve its background.

As shown in our illustration, Monoceros, the Unicorn, occupies the area of space directly up and to the left of Sirius, the brightest star visible in the evening sky. After sunset, Sirius will be very prominent above the southeastern horizon.

One of my favorite objects to see in the constellation of Monoceros, the Unicorn is Hubble’s Variable Nebula. Shown in the Hubble Space Telescope photo at upper right, this is a beautiful, fan-shaped glow of light that in a backyard telescope looks remarkably like a large comet. It’s called the Variable Nebula because it changes its contours and brightness from week to week. This phenomenon so intrigued astronomers that it was the first object photographed by the great Mount Palomar telescope in 1949.

A similar structure is associated with a star in the open cluster NGC 2264. Shown in the National Optical Astronomy Observatory photograph at upper left, the Cone Nebula is a combination of opaque dust clouds and fluorescing hydrogen gas. It takes a strong telescope to find this nebula, but the encompassing cluster of 250 stars is visible in binoculars.

The Hubble Space Telescope photograph at lower left in our illustration is a striking image of an unusual planetary nebula called the Red Rectangle. For those who might be unfamiliar with the term planetary nebula, these individual objects are often stars like our Sun that are in the final stages of their life cycle. As the stars run out of nuclear fuel to burn, they slough off their outer layers, which are in turn illuminated by the radiation of the extremely hot white dwarf star that's left behind. The astronomical term planetary nebula is a little misleading because these dying stars really have nothing to do with planets. The term originated centuries ago when telescopes were not as sharp as the ones available today. Astronomers who saw these dimly glowing, circular clouds were reminded of a fuzzy view of a planet and the name has stuck down through the years. The Red Rectangle was always strange to see in a telescope because it looked much like its name. The first Hubble view of this nebula back in 2004 revealed that the Red Rectangle had a pronounced internal structure that the more recent image in our illustration brings into sharper detail. Although the nebula is still being studied, current thinking is that the nebula is the creation of an aging binary star system (two stars that orbit a common center of gravity). These stars are in the middle of the nebula behind an obscuring disk of thick dust. The disk constricts the outflow of material from the dying star and so we see an hourglass-shaped nebula instead of a spherical one. The gravity of the orbiting companion star influences the outflow as well, causing it to be released at fairly regular intervals which have produced the ladder-like effect inside the cloud.

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