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Lagoon Nebula: August 21 - August 27, 2006.


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Lagoon Nebula.
Lagoon Nebula. Image credit: NOAO.

The night is alive this week with everything from bright planets to the first sliver of the returning Moon. This is a great time to explore with binoculars or get out your telescope to pick up some challenging studies.

Monday, August 21 - If you're up early, be sure to look for the Moon and Venus gracing the pre-dawn skies.

Have you seen any Kappa Cygnid meteors yet? Tonight the fall rate drops off, but be sure to follow the trails of any you do see with binoculars. Now let's head towards more unusual open clusters - this time in Cygnus. Starting with Gamma Cygni, locate a loose cluster involving Gamma, Do (Dolidze) 43. Now shift two degrees southwest to pick up Do 42 as well. Don't confuse Do 42 with nearby M29 though, for the two look very similar. For fans of the “Double Cluster” in Perseus, you'll like the next pairing! Shift another half degree southwest along the body of Cygnus to pick out Do 40 and Do 41. This pretty pair can be placed in the same low power field. By moving another half degree due west, you'll find highly populated Do 39 and that, too, is a double treat. The brighter clump of stars in the same low power field is IC 4996.

Now for two bright open clusters. The first, Ruprecht 173 is about a degree northwest of Epsilon Cygni. You'll truly appreciate this heavily populated star cluster! The next is as easy as identifying the constellation of Lyra. Just southeast of bright Vega is a wonderful double for binoculars, Delta 1 and 2 - the easternmost most two stars in the lyre. This bright pair is part of an open cluster known as Stephenson 1.

Tuesday, August 22 - Just before dawn, watch as the last sliver of the Moon slips past both Mercury and Saturn.

With very dark skies tonight, let's take this opportunity to visit a deepsky study that's great in Telescopes and binoculars. Are you ready for another swim in the “Lagoon?”

Easily located about three finger-widths above the tip of the teapot's spout (Al Nasl), M8 is one of Sagittarius' premier objects. This combination of emission/reflection and dark nebula only gets better as you add an open cluster. Spanning a half a degree of sky, this study is loaded with features. One of the most prominent is a curving dark channel dividing the area nearly in half. On its leading (western) side you will note two bright stars. The southernmost of this pair (9 Sagittarii) is thought to be the illuminating source of the nebula. On the trailing (eastern) side, is brightly scattered cluster NGC 6530 containing 18 erratically changing variables known as “flare stars.” For large scopes, and those with filters, look for small patches of dark nebulae called “globules.” These are thought to be “protostar” regions - areas where new stars undergo rapid formation. Return again to 9 Sagittarii and look carefully at a concentrated portion of the nebula west-southwest. This is known as the “Hourglass” and is a source of strong radio emission.

Wednesday, August 23 - Tonight is New Moon and time for us to have a look at one of the summer's most curious galaxies - NGC 6822. This study is a telescopic challenge even for skilled observers. Set your sights roughly 2 degrees northeast of easy double 54 Sagittarii, and have a look at this distant dwarf Galaxy bound to our own Milky Way by invisible gravitational attraction...

Named after its discoverer (E. E. Barnard - 1884), “Barnard's Galaxy” is a not-so-nearby member of our local Galaxy group. Discovered with a 6″ refractor, this 1.7 million light-year distant Galaxy is not easily found, but can be seen with very dark sky conditions and at the lowest possible power. Due to large apparent size, and overall faintness (magnitude 9), low power is essential in larger Telescopes to give a better sense of the galaxy's frontier. Observers using large scopes will see faint regions of glowing gas (HII regions) and unresolved concentrations of bright stars. To distinguish them, try a nebula filter to enhance the HII and downplay the star fields. Barnard's Galaxy appears like a very faint open cluster overlaid with a sheen of nebulosity, but the practiced eye using the above technique will clearly see that the “shine” behind the stars is extragalactic in nature.

Now look less than a degree north-northwest to turn up pale blue-green NGC 6818 - the “Little Gem” planetary. Easily found in any size scope, this bright and condensed nebula reveals its annular nature in larger scopes but hints at it in scopes as small as 6″. Use a super wide field long-focus eyepiece to frame them both!

Thursday, August 24 - With little or no Moon to interfere tonight, let's try for another quest - M20.

Located a finger-width above earlier study M8, the “Trifid” nebula appears initially as two widely spaced stars - one of which is a low power double - each caught in its own faint lobe of nebulosity. Keen eyed observers will find that the double star - HN 40 - is actually a superb triple star system of striking colors! The 7.6 magnitude primary appears blue. Southwest is a reddish 10.7 magnitude secondary while a third companion of magnitude 8.7 is northwest of the primary.

Described as “trifid” by William Herschel in 1784, this tri-lobed pattern of faint luminosity broken by a dark nebula - Barnard 85 - is associated with the southern triple. This region is more brightly illuminated due to the presence of the star cluster and is suffused with a brighter, redder reflection nebula of Hydrogen gas. The northern part of the Trifid (surrounding the solitary star) is fainter and bluer. It shines by excitation and is composed primarily of doubly ionized oxygen gas. The entire area lies roughly 5000 light-years away.

What makes M20 the “Trifid” nebula, are the series of dark, dissecting dust lanes meeting at the nebula's east and west edges, while the southernmost dust lane ends in the brightest portion of the nebula. With much larger scopes, M20 shows differences in concentration in each of the lobes along with other embedded stars. It requires a dark night, but the Trifid is worth the hunt. On excellent nights of seeing, larger scopes will show the Trifid much as it appears in black and white photographs!

Friday, August 25 - Somewhere out there, the Moon is furthest from Earth, but not so far away from Mars. Be sure to check IOTA for occultation events.

On this date in 1981, Voyager 2 made a fly-by of Saturn. Eight years later in 1989, Voyager 2 went on to fly by Neptune on this same date. Why don't we make a “date” tonight to have a look at this distant world? Tonight you'll find Neptune's 7.8 magnitude blue disk a little more than a degree northeast of Iota Capricorni.

With little Moon to interfere, let's have a look at a great binocular target and treasure trove for the telescope - M24. To locate M24, head about four finger-widths north of Lambda Sagittarii. Often referred to as the “Small Sagittarius star Cloud,” this vast region is easily seen unaided from dark sky sites and displays a profusion of faint stars in binoculars. Telescopes will find a dense, but unresolvable galactic cluster - NGC 6603 - embedded near its northeastern border. For those seeking a challenge, look for the Barnard Dark Nebula - B92 - just north of the central region.

Saturday, August 26 - If you're up before dawn this morning, be sure to look for Venus and Saturn very close to one another.

Did you spot the slender crescent Moon tonight just after sunset? Then be glad it's gone early as we say farewell to the favorites of the southern Milky Way. Start at G Scorpii and neighboring 7.4 magnitude globular cluster NGC 6441. Head a little more than 2 degrees due north to large and brightly scattered open cluster M7. Shift slightly northwest and include neighboring 10th magnitude globular NGC 6453 in the same low power field. Be sure to visit less than four degrees northwest for the lovely arching loops of “Butterfly” cluster - M6!

Centering on Gamma Sagittarii, go north 6 degrees to find the billowing lobes of the Lagoon Nebula and the “Strawberry Cluster” - NGC 6530. Less than 2 degrees north-northwest brings you to the subtle lobes of the Trifid Nebula - M20. From M20 shift a little less than 6 degrees north-northeast to enter the open magical window of the Small Sagittarian star Cloud and be sure to catch the faint sheen of open cluster NGC 6602 within it. Less than three degrees north-northeast reveals the gentle “Swan Nebula” floating effortlessly on the ocean of deep space. Perhaps the “Swan” sees the subtle “Eagle” gliding overhead less than three degrees north. Look for its attendant open cluster of stars - M16.

And for last, the densely populated open cluster M11. The “Wild Duck” cluster soars about a fist's width northeast of M16. Dominated by a single 8th magnitude star, this conically-shaped 3,000 member assembly of stardust easily resolves into innumerable stars with any significant amount of magnification. Through intermediate aperture, this 6000 light-year distant, 250 million year old cluster takes on a new form as several hundred 13th and 14th magnitude members begin to spill outside its V-shaped bounds! Discovered by Gottfried Kirch of Berlin observatory in 1681, the cluster was first noted as stellar by William Derham in the first third of the 18th century. Charles Messier added it to his catalog May 30, 1764.

Sunday, August 27 - Heads up for an occultation event! At 5:30 a.m. PDT asteroid Delia will occult a magnitude 6.9 star on a transcontinental path from SE Canada to the SF Bay area, including Minnesota, Reno and Sacramento. There is always uncertainty in predicted asteroid path locations, but at the moment it seems that the path will favor the northern portion of the Bay Area. In any event, it will be useful for many to observe from their homes, and for some to travel. Expect more information after there is a path update. If there is a possibility that you will be able to observe - mobile or from a fixed location - please respond to IOTA so that a reasonable attempt can be made to distribute observers across the path.

Tonight the Moon sets by skydark, but if you're looking for a lunar challenge, return to crater Petavius about one-third the way up from the southern cusp just after sunset. This ancient crater is a wonderland of detail when lying on the terminator. Look for its rugged walls interrupted by crater Wrottesley to the northwest and elongated Palitzsch southeast. If conditions are stable, power up to look for a massive, multi-peaked central mountain region, along with a deep scar - Rima Petavius - cutting diagonally across the wavelike floor.

When the Moon has set, look for the southern Crown - corona Australis. Its hidden jewel is 7.3 magnitude, 28,000 light-year distant globular cluster NGC 6723. Discovered on June 3, 1826 by James Dunlop of New South Wales, Australia, NGC 6723 can be best found by heading less than 7 degrees due south of Zeta Sagittarii. This mid-sized cluster gives a surprising view, but if you're more north, best catch it at its highest.

May all your journeys be at light speed... Tammy Plotner with Jeff Barbour.


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