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Star clusters: July 17 - July 23, 2006.


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Nebula.
Nebula - Credit: Steve Mandel.

As the Moon slowly leaves the scene the observing heats up as we take a look at some of the Summer's finest objects as we tour star clusters and nebula. Get out those Telescopes and binoculars. The “Veil”

Just a reminder, What's Up this Week has its own website at http://www.astrowhatsup.com.

Monday, July 17 - Today in 1963, the Nuclear Test Ban Treaty was signed. The treaty prohibited the detonation of nuclear devices in our atmosphere. To be sure all countries were in compliance, the United States later launched the first gamma ray detectors into orbit. In 1967 these detectors picked up a new discovery - the first of many cosmic gamma ray sources.

As we know most stars begin life in stellar nurseries and end life either alone or in very small groups as doubles or multiple stars. Tonight we can have a look at a group of young stars beginning their stellar evolution and end with an old solitary elder preparing to move on to an even “higher realm.” Open cluster IC 4665 is easily detected with just about any optical aid about a finger-width north-northeast of Beta Ophiuchi. Discovered by Philippe Loys de Chéseaux in the mid-1700s, this 1400 light-year distant cluster consists of about 30 mixed magnitude stars all less than 40 million years of age. Despite its early discovery, the cluster did not achieve broad enough recognition for Dreyer to include it in the late 19th century New General Catalog and it was later added as a supplement to the NGC in the Index Catalog of 1908. Be sure to use low power to so see all of this large group.

About three finger-widths north-northeast of IC 4665 is a study that did make Dreyer's catalogue - NGC 6572. This 9th magnitude planetary is very small - but intense. Like the “Cat's Eye” in Draco, and NGC 6210 in Hercules, this planetary can take a lot of magnification. Those with large scopes should look for a small, round, blue inner core encased is a faint shell. A challenge to find? You bet. Worth the work? Sometimes working for something makes it all the more fun!

Tuesday, July 18 - 26 years ago today, India launched its first satellite - Rohini 1 - from the Satish Dhawan Space Center (SDSC), located on Sriharikota Island in the Bay of Bengal. Tonight we'll launch our imaginations as we revisit a lineup of globular clusters now well placed in southern Ophiuchus.

Let's begin with Antares and head around a fist width southeast to locate the first. Viewable in binoculars, M62 is known to be one of a small group of globular clusters having undergone a core collapse. Possibly caused by a central black hole, this mysterious process means that the center of the cluster is far more densely populated than it should be. Look for this influence as you note the peculiar way this irregular Class IV globular brightens toward the core.

Now head about two finger-widths north. Also viewable in binoculars, M19 is intensely blue and displays an extremely elongated core. Like many of its nearby globulars, this Class VIII flattens to the west as a result of tidal actions from the galactic core. This cluster is known to have a large number of “blue stragglers” - bright stars that should have became red giants in the 12 billion since the cluster formed. Although these stars appear to live in perpetual youth they remain blue as a result of a “cosmic face lift” - their outer red atmospheres have been stripped away through gravitational interactions with other nearby stars!

To complete our tour, head 1.5 degrees north to Class IX globular NGC 6284 - then 2 degrees further north for Class VII NGC 6287. Only larger scopes will resolve any stars in this pair, but the faint “mounding” of their glow is quite enchanting to the eye.

Wednesday, July 19 - Today in 1846, Edward Pickering was born. Although his name is not well known, he became a pioneer in the field of spectroscopy. Pickering was the Harvard College Observatory's Director from 1876 to 1919, and it was during his time there that photography and astronomy began to merge. The archive known as the Harvard Plate Collection still remains a valuable source of data.

Tonight let's look at a faint study in Cygnus, part of which was first recognized in 1904 by Willamina Fleming of Harvard College and named for Edmond Pickering. Possessed of three main components - NGC 6960, NGC 6979, and NGC 6992 - these regions are also called the Eastern, Northern, and Western Veil Nebulae. Of the three, the Northern Veil is also distinguished by the name “Pickering's Wedge.” At one time these components of the “Cygnus Loop” looked a lot like M1 - the Crab Nebula - but due to greater antiquity have expanded and separated to encompass three degrees of the night sky. Like the Crab Nebula, the Cygnus Loop came out of a supernova explosion. This one was located around 1,500 light-years away, and occurred about 20,000 years ago. Such an explosion would have emitted as much light as all the stars in our galaxy!

To study the Cygnus Loop is a serious challenge. Very dark skies, or filters, are essential. Through moonless rural skies, binoculars and small scopes can detect the brighter regions of all three Veils - including the faint Pickering's Wedge. Start at 52 Cygni - located about two finger-widths south of the star marking the eastern wing tip of Cygnus. Look for a broad thread of nebulosity running north of 52 - this is the handle of the “Witches Broom” (NGC 6960). To locate Pickering's Wedge look north-northwest. Look for a faintly glowing triangle of light. Continue east to the brightest portion of the Loop - NGC 6992. At low power, larger scopes will see numerous filaments and projections in this expansive crescent-shaped nebula.

Thursday, July 20 - In the early morning hours, the Moon and the Pleiades are going to be very close. This means a possible grazing event, so be sure to check IOTA for details.

Today was a busy day in astronomical history. In 1969, the world held its breath as Apollo 11's Eagle landed and Neil Armstrong became the first human to leave a footprint on the Moon. In celebration of our very humanity, Armstrong was so moved that he momentarily forgot his speech. The famous words were meant to be “One small step for a man. One giant leap for mankind.” That's nothing more than one small error for a man, and a huge success for mankind! Seven years later in 1976, Viking 1 landed on Mars - sending back the first images ever taken from that planet's surface.

While we can't walk on the Moon or land on Mars, we can journey to the furthest planet. Tonight is your last chance to place 13.9 magnitude Pluto in the same field with 5.9 magnitude SAO 160701. So grab your field sketch and check to see which 14th magnitude “star” is no longer visible south-southeast of our marker. Pluto moves surprisingly quick for a 4.7 million kilometer distant world, doesn't it?!

Friday, July 21 - Tonight's late Moon means that we have a chance to revisit the Cygnus Loop then track down the less elusive but equally expansive region of neighboring nebulosity - the North American Nebula.

Under superb conditions, even unaided vision can detect NGC 7000 as a glowing patch in the Milky Way around two finger-widths east-southeast of Deneb. Turn binoculars its way and the true shape of this huge region of luminous gas and dust begins to reveal itself. To fully explore the North American Nebula requires no more than a very small scope... but very dark skies.

Saturday, July 22 - Celestial Scenery Alert! Get up early on a weekend? When you see bright Venus so near to a waning Moon... It's worth it!

Tonight we honor the work of Friedrich Bessel, born on this day in 1784. Bessel was a German Astronomer and mathematician whose mathematical functions still carry his name in physics. But put away your calculator, because Bessel was also the first person to measure a star's parallax. In 1837, he chose 61 Cygni and the resulting measurement proved to be no more than a third of an arc second. His work ended a debate that stretched back two millennia to Aristotle's time and Greek theories concerning stellar distances.

Named the “Flying Star” in 1792 by Giuseppe Piazzi, 61 Cygni is actually a Binary star of magnitudes 5.3 and 6.0 separated by 30 arc seconds. Located roughly between Deneb and eastern wingtip Zeta, look for bright Tau and 61 is a finger-width northwest. Of the visible stars in the night sky, 61 is fourth closest to Earth, with only Alpha Centauri, Sirius, and Epsilon Eridani closer. Just how close is it? Right around 11 light-years. As you observe this double star, keep in mind that Bessel had to somehow detect a shift of 1/100th the pair's separation as the Earth moved from one side of the solar system to another!

Sunday, July 23 - With continuing dark skies ahead, now is a good time to do some serious study in Scorpius. Starting at Theta, move one finger-width south to pick out bright, 6.9 magnitude, 32,000 light-year distant globular cluster NGC 6388. Like many globulars in this region, it's within 10,400 light-years of the galactic core. Easily seen in binoculars, a telescope shows Class III NGC 6388 to be similar to Class II M80. This one is a jewel in the crown of the southern sky!

Return to Theta and head two finger-widths west for the small, but bright, open cluster NGC 6322. Through a small scope or binoculars, this mixed magnitude cluster appears loose, and consists of a triangle of bright stars plus a smattering of dim ones. Larger scopes can resolve several dozen members.

Now for a challenge. Start at Lambda Scorpii and head due east toward 3.3 magnitude G Scorpii. See anything unusual? NGC 6441 is a faint, but intense Class III globular...one that accompanies a bright star!

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


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