D e s e r t E x p o s u r e
February
2008
Constellation of the Month: Monoceros, the Unicorn
Representing a mythical creature, Monoceros, the Unicorn, should have a long and colorful mythological history behind it. It does not. This constellation is composed of fourth-magnitude and fainter stars, so it did not stand out in the ancient sky. It has been found on an ancient Persian star-sphere, but the first time Monoceros's existence was recorded in modern times is in a 1624 star chart by Jakob Bartsch, a German mathematician, under the name Unicornus. But there have been references to it as early as 1564. The name was changed to Monoceros, from the Greek, by Johannes Hevelius in his 1690 star charts called Prodromus Astronomiae, and has stuck since.
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Click the Map to Enlarge It
Monoceros hides among the bright
winter constellations of Orion, Canis Major and Gemini. We are looking
toward the southeast about halfway up in the evening sky. The Milky Way
cuts right through Monoceros. Many of the stars in this area belong to
the same arm of the Milky Way as we do, the Orion Arm. Many of the nebulae
we see in this area of the sky also belong to the Orion Arm, including
the famous Orion Nebula.
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The most interesting object in this constellation is Hubble's Variable Nebula and the associated variable star R Monocerotis (abbreviated R Mon). The nebula was first discovered in 1783 by William Herschel, but it was not until Edwin Hubble observed it in 1916 that the variability of the nebula itself was discovered. This is almost a unique object, with its brightness and shape changing in as short a time as a few weeks. This is a reflection nebula, meaning the light from the star at the southern tip of this triangular-shaped object is reflected off the dust grains of the nebula, lighting it up for us to see. The star at the southern tip is the variable star R Mon. But the star itself has never been seen since it is wrapped in a cocoon of dust and gas associated with the nebula.
R Mon varies by as much as two magnitudes in an irregular pattern. It is a young star that is still in the process of forming. R Mon's central gravity pulls matter inward from the nebula toward a disc around the star's equator. Most of the material gets thrown back out, but along the equator the material is blocked by the disc. The poles of the star are not blocked and two jets flow outward, one from each pole. One jet flows out into the nebula.
The nebula itself is an open-ended shell shaped somewhat like a child's top or the bottom half of an acorn. The jet from the star flows into this relatively hollow area and starts to get twisted around by the star's magnetic field, throwing off threads of gas and dust. It is the shadows of these threads that darken the shell. These threads are not far from the star; some are as close to R Mon as the Earth is from the Sun. The shadows from these threads are cast far out onto the shell of the nebula. These shadows move slowly near the star. The projections of these shadows on the distant shell appear to move quickly across the face of the nebula. This makes the brightness and the shape of the nebula appear to change in short time scales.
Another object in Monoceros is the Rosetta Nebula, a large doughnut-shaped ring of glowing gas. This is formed from a cloud of gas that at one end has been compressed by nearby giant stars. This caused this end of the nebula to collapse and form the open star cluster NGC 2244. This cluster's solar wind has cleared the area around itself of dust and gas, so we can see it in the middle of the Rosetta. The ultraviolet light from the cluster hits the atoms of the gas, causing them to glow like a red neon tube. The entire complex is some 130 light-years across at a distance of 5,200 light years.
The cluster itself is visible in binoculars or a small telescope. A larger telescope is needed to see the nebula itself, and it is photography that brings out the stunning red of glowing hydrogen gas. Less dramatic is the green from oxygen and blue from sulfur that soften the brilliant red. Superimposed on this scene are dark knots of dust that form threads running through the entire nebula. There are even blobs of simple molecules that are traveling at a high speed through the nebula. The source of these molecular blobs is still to be discovered.
The Planets for February 2008
Mercury, which graced our evening sky last month, has disappeared back into the Sun's glare this month, leaving only the Red Planet Mars in our evening sky. Staying in the far eastern end of Taurus all month, Mars is moving slowly eastward among the stars of the Bull's horns. Now starting to shrink more rapidly, it is only 10.4 seconds-of-arc across at mid-month. The disc is 79 percent illuminated and the God of War shines at magnitude -0.2. The features on the Martian disc will become harder to discern as it becomes smaller, so take a telescopic look at the Red Planet this month.
Saturn spends February in Leo, between the fore paws and rear paws of the Lion. Rising about 6:30 p.m., the Ringed Planet is magnitude 0.2 in the middle of the month. The Rings are tilted up 8.0 degrees to our line of sight with the southern face showing. They are 45.1 seconds-of-arc across. The ball of the planet is only 19.9 seconds-of-arc across.
Nothing much happens in this month's skies until about 4:30 a.m., when Jupiter pops over the east-southeastern horizon. Still moving eastward among the stars in Sagittarius, Jupiter is magnitude -2.0 and 32.6 seconds-of-arc across. Jupiter is becoming better placed for telescopic observing, but is still relatively low as morning twilight begins.
On the morning of Feb. 1, Jupiter will have Venus as a companion. The pair will be just slightly more than half a degree apart that morning. For the rest of the month, Venus will drop below Jupiter. Starting the month with Jupiter in Sagittarius, Venus moves eastward into Capricornus on the 17th, where it spends the rest of the month. At mid-month Venus is magnitude -3.9 and 11.8 seconds-of-arc across, slowly shrinking as the 88-percent sunlit disk becomes increasingly full.
As if one close pairing of planets were not enough, Venus will be 1.2 degrees south of Mercury on the morning of Feb. 26. Mercury will be magnitude 0.3, and they will both be fairly low on the east-southeastern horizon with the Sun coming up soon after. Mercury begins the month in Capricornus, moving westward into Aquarius and then back into Capricornus for its rendezvous with Venus. This will not be a good apparition for Mercury.
In 2008, February is in the middle of one of the two "eclipse seasons" we have every year. The first eclipse this month, visible only in Antarctica, is an annular solar eclipse on Feb. 6. The second eclipse will be visible from the desert Southwest — a total lunar eclipse on the evening of Feb. 20. The eclipse will already be in progress as the Full Moon rises at 5:48 p.m., but it will be hard to detect since all parts of the Moon will still be getting at least some sunlight. Slowly, you will notice the eastern edge of the Moon become increasingly dark as it gets less sun. The Moon will have just passed Regulus and will be heading toward Saturn, but the Moon will not pass Saturn until after the eclipse ends. By 6:43 p.m., that edge of the Moon will be bereft of sunlight and will appear very dark as the Moon enters the dark central core of the Earth's shadow. The Moon continues moving into the shadow and is completely in the core by 8:01 p.m.
The Moon will now get only light from the Sun that has passed through the Earth's atmosphere, just as we see around sunrise or sunset — giving the Moon a ruddy complexion during the total phase of the eclipse. The Moon is at the greatest eclipse at 8:26 p.m. and starts to move out of the core of the shadow at 8:51 p.m. By 10:09 p.m., the Moon is completely out of the core. By 11:16 p.m., the Moon is completely back in full sunlight and the eclipse is over. So enjoy this month's spectacles and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.
Watch the Sky
(all times MST)
Feb. 1, 5 a.m. — Jupiter 0.6 degrees south of Venus
Feb. 4, morning — Moon near Jupiter and Venus
Feb. 6, 8:44 p.m. — New Moon (annular eclipse in Antarctica)
Feb. 13, 8:33 p.m. — First Quarter Moon
Feb. 16, 1 a.m. — Mars near the Moon
Feb. 20, 8:30 p.m. — Full Moon (total lunar eclipse)
Feb. 24, 3 a.m. — Saturn at opposition
Feb. 26, morning — Mercury near Venus
Feb. 28, 7:18 p.m. — Last Quarter Moon