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Galaxy group: December 18 - December 24, 2006.


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Pinwheel.
The Pinwheel - Credit: NOAO/AURA/NSF.

For most of us, the longest night of the year is fast approaching Why not enjoy solstice with an in-depth look at an amazing galaxy? For those of you who enjoyed the great display the Geminids produced last week, there will also be two more meteor showers to add to the celestial show! So grab your binoculars, set up your scopes and lets head out into the night because

Heres whats up!

Monday, December 18 - Readers of this year-long tour know that the most splendid examples in our Local Group of Galaxies are our own Milky Way, the Magellanic Clouds and the Great Galaxy in Andromeda. But a distant contestant is the object of tonights exploration - the 3 million light-year distant Triangulum galaxy, M33.

To locate M33, start by sweeping with binoculars between Alpha Triangulum and Beta Andromedae. You will pick up a faint, large and very round contrast change. Since M33 is so large, youll find this Galaxy best at low power in all scopes. Small aperture will see its misty appearance, while most mid-sized scopes reveal spiral structure - giving rise to its nickname - the Pinwheel. For those with large aperture, prepare yourself to visit with NGC objects outside our own galaxy. One of the most easily noted is NGC 604, a nebulous region in the northeast section.

Tuesday, December 19 - Tonight we continue our study of the Triangulum Galaxy. Possibly seen by Hodierna in the mid-seventeenth century, Charles Messier logged it the night of August 25, 1764. Although M33 is not much more than 50,000 light-years in diameter, it is about average in physical size for a spiral. Astronomers have determined that all three major members of the Local Group are being inexorably drawn together, but its likely that M33 and M31 will converge long before the resulting super-galaxy joins with our own. Meanwhile, much of M33 will be thrown off into space by gravitational action and both Galaxies will undergo fantastic explosions of star birth as gas and dust rapidly condenses.

Return again tonight to study and look for subtle details - with attention to NGC 604.

Like most spiral galaxies, M33 consists mostly of stars, gas, dust, and exotic dark matter. Obscuring dust is most easily found in great dark lanes that absorb visible light and put out cold heat (far infrared). Gas interpenetrates the dust, but depending on how excited the gas is, it can also put out energy. Some of that energy is detected as radio waves - emanating from neutral hydrogen, while other energy is seen as visible light caused by ultraviolet stimulation from nearby stars. Dark matter is totally undetectable because it appears to have nothing to do with light whatsoever - neither blocking it, nor radiating it. Dark energy, however, can be felt as negative gravity and such matter may actually comprise 90% of all substance present in the Universe. Interestingly, there is far more Dark matter outside the visible borders of Galaxies than within them.

Tonight were going to track down an intense region of starburst activity within a vast 1500 light-year HII region on the outskirts of M33. This region is so large and bright it can even be detected in small telescopes. NGC 604 can be seen as a condensed luminous spot with a surface brightness equal to that of the core of the M33 galaxy. As you look for it, keep in mind that it is some 35 arc-minutes northeast of M33s nucleus at the tip of the galaxys east-sweeping spiral arm. That locale lies near the frontier where Dark matter begins to influence the observed mass of the galaxy.

Wednesday, December 20 - Today in 1904, the Mt. Wilson Solar Observatory officially opened its doors. Today also marks the birth in 1876 of Walter S. Adams. Observing from Mt Wilson, Adams uncovered the true nature of Sirius companion. First seen by Alvan Clark in 1862, Sirius B is an aged White Dwarf whose expansive red giant phase ended long ago.

If you are up early, take a few minutes to enjoy the peak of the Coma Berenicid meteor shower. Although meteor activity is low (with an average fall rate of about seven per hour), the Coma Berenicids still warrant study.

Noted first in 1959, the stream was traced in 1973 to another minor shower bearing the same orbit known as the December Leo Minorids. As we know, meteoroid streams are often associated with comets - but in this case no Comet was confirmed. Observed in 1912 by amateur Astronomer B. Lowe, the Comet was officially designated 1913 I and was only subsequently seen four times before it was lost to sunrise. Using Lowes observations, independent researchers computed the comets orbit. The stream however, was later forgotten until 1954. At that time, Fred Whipple made a connection between photographic studies and the enigmatic Comet Lowe. Continued observations determined that the orbital period of the parent Comet was around 75 years - but the two major meteor streams occurred 27 and 157 years apart! Thanks to the uneven dispersion of material, it may be another decade before we see some real activity from this shower, but even one meteor can make your day.

And if you want to make your night an early one, why not try looking for another odd shower? Tonight is also the peak of the Delta Arietids. These unusual meteors bear something in common with last weeks Geminids - their source appears to be the sun-grazing asteroid Icarus. Around twelve fast, bright shooting stars per hour should be visible early in the evening due to the high sky position of the radiant - constellation Aries. Tonights New Moon favors observation!

Thursday, December 21 - Tonight the slender crescent Moon will slip below the horizon well before skydark. Autumn will soon give way to winter and our 2006 tour of the heavens will later come to an end. The holiday season now is before us and the Sun is about to turn the corner and take up its northern swing just as the north temperate winter gets especially cold. Tonight we encourage you to set up your largest scope and return to visit the night sky. Before launching on a final tour, take a moment after sky dark to follow the grand sweep of our own Galaxy from the west-northwest - where its faint sheen is most luminous - to the south-southeast, where it can only be noted by the profusion of fainter unaided eye 4th through 6th magnitude stars and numerous galactic clusters.

With this expansive view held before you, turn to your scope and revisit the Andromeda family of Galaxies now hanging overhead as Alpheratz and Algenib of the eastern Great Square point the time to Zero hour at skydark on the longest night of the year

Start at the Great Andromeda Galaxy itself and note round M32 due south of M31s splendid core. Head one degree north-northwest from M32 to M110s smoothly graduated light disappearing softly into the night. Push less than 7 degrees due north from M110 to Omicron Cassiopeiae for the just this side of the edge dwarf elliptical NGC 184. Slew a degree west to on-the-edge NGC 147. Return to M31 and look less than a degree due south of M110 for NGC 206 - a difficult but small-scope achievable HII star forming region 100 times larger than M42? in another galaxy. Remembering that view, make the leap across Mirach to M33 and its HII region - NGC 604, a region that eclipses even M31s NGC 206. Go low power and follow the face-on Triangulum galaxys northeast and southwest sweeping spiral arms. Keep in mind that all we see of it - even through the largest professional instruments - is but a tiny fraction of all the gravitational influence making such grand spiral structure possible.

Happy solstice!

Friday, December 22 - Get up early because the pre-dawn hours have a treat in store - the Ursid meteor shower. Cruising around the Sun every thirteen and a half years, Comet 8P/Tuttle shed some skin and although the Comet never passes inside Earth orbit, six years later we burrow through its debris trail. Not so unusual? Think again, because it took as much as six centuries before the comets debris trail was deflected enough by Jupiters gravitation to pass into our atmosphere!

With no interference from the Moon, this circumpolar meteor shower could see early dawn activity of up to 12 per hour. By keeping watch on the constellation Ursa Major, you just might spot one of these slow moving, 600 year old travelers burning up the sky.

With dark skies tonight, why not stop to take a good look at a pair of Galaxies just on the other side of Zero Hour. The first is 12th magnitude NGC 16, lying a little less than a finger-width south of Alpha Pegasi. Due to small size, its compact elliptical core can just be distinguished by mid-sized scopes with good sky conditions. If you have large aperture, be sure to power up and try to get a sense of the core regions fuzzy frontier.

Continue around another finger-width south of NGC 16 to locate another 12th mag Galaxy - NGC 23. As a tilted spiral of similar size to NGC 16, this one doesnt take well to higher powers. Dont confuse the faint star along its frontier for a supernova! Large scope users might also look for a neighboring 13th magnitude companion - NGC 26.

Saturday, December 23 - Today in 1672, Astronomer Giovanni Cassini discovered Saturns Moon Rhea. Saturn can now be seen well above the eastern horizon by midnight. At magnitude 1.4, it rivals slightly brighter Regulus which trails it slightly to the south across the sky. Now some 1.4 million kilometers from Earth, Saturns apparent size is 19.3 arc seconds. Making up for its modest size, is the 40 arc second apparent diameter ring system which is still well aspected.

Tonight after sunset, be on hand as the Moon makes a very brief appearance on the western horizon. Get out those binoculars and have a look! About mid-point on the southeastern quadrant, see if you can pick out the dark grey expanse of Mare Australe. Around central near the limb, look for another elongated deep grey oval - Mare Smythii. Just north of Smythii you might spy crater Neper on the edge, or Mare Marginus north of it. Crater Hubble awaits you just a bit further north, and at the extreme north along the limb is Humboldtianum.

When the Moon sets, take advantage of continuing dark skies and have a look at NGC 7793 around 4 degrees south-southeast of Delta Sculptor. Although southern hemisphere observers are favored, some viewers in the north can still get a glimpse of this 9.1 magnitude galaxy. This fairly large Galaxy is moderately inclined, and as a result may appear quite misty at the edges. Look for fainter and more challenging NGC 7755 about two degrees to the northwest.

Sunday, December 24 - Today in 1968, Apollo 8 became the first manned spacecraft to orbit the Moon. Lets celebrate that event by having a look at the lunar surface.

While the slender crescent will show few details, head south of emerging Mare Crisium and look for a patchy grey area near the center called Mare Undarum. The large grey oval you see to the west is crater Firmicus.

Lets honor our southern friends once again as we head toward the incomparable NGC 55. Located about two finger-widths north-northwest of Alpha Phoenicis, this large, near edge-on Galaxy is truly a southern gem. At magnitude 7.8, this bright member of the Sculptor Galaxy group can easily be spotted in binoculars. Mid-sized scopes will begin resolution of mottling in the structure, while large aperture will show individual stars, nebulous areas, and dark dust clouds - with a very prominent one east of the nucleus. Rock on

May all your journeys be at light speed Tammy Plotner.





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