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Nebula: June 26 - July 2, 2006.


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Lagoon Nebula.
Here’s what’s up! M8: The Lagoon Nebula. Credit: NOAO/AURA/NSF

As warm nights and later sky dark hours descend on the northern hemisphere, it’s time to begin our studies along the incredible arm of the Milky Way galaxy. What better way to begin than with a swim in the "Lagoon Nebula"! Enjoy the beautiful nights and get out your binoculars and telescopes, because...

Before you read this week’s What’s Up, I just wanted to remind you that What’s Up - 365 Days of Skywatching now has a blog of its own. You can access it by going to http://www.astrowhatsup.com
We’ll be adding many more features, with cool photographs for every day, so come check it out.

Now, on to the week.

Monday, June 26 - On this day in 1949, sun-grazing asteroid Icarus was discovered on a photographic plate. It was made using the 48-inch Schmidt nine months after that telescope went into operation, and just prior to the multi-year National Geographic - Palomar Sky Survey. The asteroid was found to have a highly eccentric orbit and a perihelion distance of just 17 million miles (closer to the Sun than Mercury), giving it its unusual name. Icarus was just four million miles from Earth at the time of discovery, and the peculiarities of its orbit have been used to determine Mercury’s mass and test Einstein’s theory of general relativity.

But, today is even more special. It is the birthday of none other than Charles Messier - the famed French comet hunter. Born in 1730, Messier cataloged the 100 or so bright nebulae and star clusters we now refer to as the Messier objects - a list intended to keep observers from confusing fixed objects in space with possible new comets. Despite his discovery of over a dozen such comets, he is best remembered for the nebulae that didn’t stray, but stayed right where he and his associate Pierre Méchain found them!

Look after skydark for two of the most easily recognizable constellations in the night sky. Scorpius looks much like its namesake, "the Scorpion," while Sagittarius resembles a "teapot." The brightest star in this constellation is Kaus Australis (Epsilon). Kaus - and two other eastern stars of Sagittarius, Al Nasl ("the nose") and Kaus Media form the "spout." Rising upwards is the "steam" of the Milky Way. If you follow the "steam" north from Al Nasl you will arrive at M8 - the "Lagoon" nebula!

Tuesday, June 27 - While the Moon is still so close to new and tender, it won’t hamper studies as we follow the Milky Way north. Tonight we’ll take a more detailed look at seven studies all within half a fist width north of Al Nasl (Gamma.)

Begin with Gamma and look less than one degree north-northwest for a pair, 9.5 magnitude NGC 6528 and 8.6 magnitude NGC 6522, Class V and VI respectively. From NGC 6522, continue a little more than 2 degrees north to capture a view of the 8.0 magnitude open cluster NGC 6520. While looking at this fairly large cluster of two dozen 9th to 12th magnitude stars, walk back the magnification and see if you can also include the "C"-shaped obscuration nebula Barnard 86 northeast.

Two and a half degrees further north leads you to 8.0 magnitude globular cluster NGC 6553. This bright, Class IX study was discovered by William Herschel and initially mis-identified as a planetary nebula in 1784. Despite its relative brightness, this 20,000 light-year distant globular requires a larger scope and higher magnifications to resolve. Continuing north another degree we encounter the 8.3 magnitude, mid-sized globular cluster NGC 6544. Another Herschel discovery, this irregular class study is far more likely to show some resolution than NGC 6553, but still needs a mid-sized scope to make out individual stars. Another degree further north leads us to open cluster NGC 6530 - "the Strawberry Cluster." At magnitude 4.6, this cluster is part of the "Lagoon Nebula."

Dark skies later tonight also mean great success at spotting a handful of meteors originating near the constellation of Corvus. The Corvid meteor shower is not well documented, but you might spot as many as ten per hour.

Wednesday, June 28 - Tonight the Moon is a slender crescent low to the western horizon. Have a look at the northern edge for the smooth grey sands of Mare Humboldtianum and crater Endymion to its west. Once it sets, let’s head out once again to venture along the Milky Way!

Tonight, we’ll begin again with M8 and its attendant open cluster NGC 6530, which was first noted by Flamsteed in the late 17th century. Charles Messier emphasized this cluster’s appearance in his notes: "A cluster which appears like a nebula in an ordinary telescope of 3 feet [focal length] but with an excellent instrument, one perceives nothing but a large number of small stars." The Lagoon Nebula - M8 - was first reported by Le Gentil in 1747, who also made reference to the star cluster.

To the northeast a little more than a degree is 8.0 magnitude open cluster NGC 6546. Unrecognizable as a cluster in small scopes, several dozen faint stars can be resolved in modest instruments at higher magnifications. Another degree north and slightly west leads to open cluster M21. An original discovery of Messier on June 5, 1764, it is estimated to be around 5 million years old - rather young for a cluster with around 50 members.

Moving less than a degree southwest of M21 is the faint nebulosity of M20 - the Trifid Nebula. Located some 5,000 light-years away, this object is spectacular in photos and gives a recognizable view in larger scopes. Two 8th magnitude stars dominate the cluster - one of which is a superb triple system.

Thursday, June 29 - If you’re lucky enough to catch the very beginnings of the tender crescent Moon just after sunset, be sure to look for Regulus nearby!

Today we celebrate the birthday of George Ellery Hale. Born in 1868, Hale was the founding father of the Mt. Wilson Observatory. Although he had no education beyond a baccalaureate in physics, he became the leading Astronomer of his day. He invented the spectroheliograph, coined the word astrophysics, and founded the Astrophysical Journal as well as Yerkes Observatory. At the time, Mt. Wilson dominated the world of astronomy, confirming the nature of galaxies as "island universes" and verifying an expanding universe cosmology. Later Hale went on to found Palomar Observatory and the 5-meter (200″) telescope (named for him) was dedicated on June 3, 1948. It continues to be the largest telescope in the continental United States.

Although Mt Palomar Observatory’s largest instrument was dedicated to exploring the extragalactic realm, the smaller 48-inch Samuel Oschin telescope was one of the first Telescopes to survey the entire northern hemisphere sky. This fine instrument captured six degree wide fields of the sky. Tonight, let’s explore an area just that size.

Start three finger-widths northeast of the "teapot’s lid," Lambda, for open cluster M25. Added to Messier’s catalog on June 20, 1764, it was first noted by Philippe Loys de Chéseaux in the mid 1740s. In modest telescopes, this 2,000 light-year distant cluster shows around four dozen various magnitude stars and larger aperture reveals many finer stars across the field. Two and a half degrees further north-northeast reveals challenging open cluster NGC 6645. Somewhat small, this 8.5 magnitude cluster is condensed enough to distinguish itself among the rich star fields of the Milky Way. Appearing like a nebulous patch to smaller scopes, mid-apertures reveal two dozen faint members.

Around three degrees west is much brighter open cluster M18. Its members begin at about magnitude 8.5 and perhaps two dozen stars are visible to magnitude 12. Messier discovered and catalogued this cluster June 3, 1764. One degree north-northeast of M18 is the evening’s most spectacular study: the graceful and beautiful "Swan Nebula" - M17. Even through a small scope, this one is quite impressive. Just the amount of gas in this 6,000 light-year distant area could condense to form as many as 800 suns!

A short hop north will bring us to M16 - the "Eagle Nebula." Unlike the "Swan" the "Eagle" is not easily seen. Although detectable through most telescopes, it is low surface brightness and requires a nebula filter to really stand out.

Friday, June 30 - Let’s return to the Moon and look toward the southern shore of Mare Fecunditatis and previous study crater Petavius. Just to the southwest you will see a smaller, but very prominent pair - Snellius and Stevinus. So close to the terminator, this duo of Class I craters show their sharp, younger outlines very well.

When the Moon has begun to set, find a comfortable seat, relax, and enjoy the June Draconid meteor shower. The radiant from this shower is near the handle of the Big Dipper. The fall rate varies from 10 to 100 per hour, but tonight’s darker skies will offer a better than usual chance to spot what are now known to be the offspring of comet Pons-Winnecke. On a curious note, today in 1908 the great Tunguska impact happened in Siberia. A fragment of the comet, perhaps?

Saturday, July 1 - Today in 1917, Astronomers at Mt. Wilson celebrated the arrival of the 100″ primary mirror for the Hooker Telescope. The mirror was cast by the Saint Gobrain Glassworks of France using the same type of glass as wine bottles. Funds for casting, shaping, and silvering the mirror were provided by Los Angeles businessman John D. Hooker. The 100 inch telescope ultimately proved to usher in a new age of astrophysical investigation and expanded human thinking to include a Universe of innumerable galaxies beyond our own.

Although the Moon is furthest from the Earth tonight, it most definitely won’t stop us from exploring. Let’s have another look at a previous study crater on the lunar surface tonight as we locate shallow crater Cleomides just north of Mare Crisium. With binoculars, or a telescope at low power, follow the rings to the north as you encounter Burckhardt, Geminus, and the faded old Messala. For a telescopic challenge, look for crater Delmotte at the eastern edge of Cleomides’ rim. Shift to the northwest for Trailes and Debes on its western edge.

Since the distant Moon is not overpowering, wait for it to wester, then head off to explore a trio of star clusters. Just a little more than a fist width south of Antares is large open cluster NGC 6124. At magnitude 5.8, this mixed variety of bright stars resides almost precisely between Zeta Scorpii and Eta Lupi. NGC 6124 is easily recognized and contains a visibly condensed core region.

Now head a little more than 5 degrees due east between Zeta and Mu Scorpii to have a look at 6.4 magnitude NGC 6242. With an expanse one-third the size of the previous cluster, this 4,000 light-year distant, compact gathering of more than two dozen stars was first noted (along with earlier study NGC 6124) by Abbe Lacaille during his mid-eighteenth century trip to South Africa. Now return to Zeta and look less than one degree north for brilliant NGC 6231. This magnitude 2.6 study is around 6000 light-years away. Binocular users can collect these trophies!

Sunday, July 2 - On this day in 1967, the Vela gamma-ray satellite was launched. Originally designed to detect nuclear explosions, Vela made a much more important contribution by detecting gamma-ray bursts in space. These very short-lived, highly energetic explosions can happen from almost any direction and are caused by events as cataclysmic as the collapse of two neutron stars to form a black hole.

Tonight we’re going to look on the lunar surface for a crater so old that it’s almost extinct. Start by identifying the three rings of Theophilus, Cyrillus and Catharina. To the south you will see the broad, bright wall of the Altai Scarp and further south a huge shallow crater on the terminator. This crater can only be seen during this particular phase of lunar sunrise and has become so dilapidated it is unnamed. Younger craters, Lindenau and Rothman invade its northern wall and you will see a small collection of craters to the south that resemble a "paw print." Enjoy it tonight, for it will be gone tomorrow.

Wait until the Moon begins to set and return to Mu Scorpii. About a finger-width east you will find large open cluster NGC 6281. At magnitude 5.4, you’ll find this sky gem punctuated by a wide pair of 6th magnitude stars. This brightly scattered cluster of three dozen members shows no real nucleus but is easily recognized at low magnifications.




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