What’s Up this Week: July 24 – July 30, 2006

Greetings, fellow SkyWatchers! Welcome to a land of all sight and no sound… A land where shadow meets substance. Welcome to the “X” zone! Caught in a time warp? Not this week. Be sure to enjoy some of the best observing of the year as we head towards the center of our galaxy, walk on the lunar surface and chase shooting stars. So head out into the twilight, because….

Here’s what’s up!

Monday, July 24 – Tonight let’s go to the other extreme – from the Tail of the Scorpion to the Handle of the Little Dipper. We’re going to start answering the question – What’s up there anyway?

As we have already explored, our northern Pole Star is not alone. It is mildly variable and the brightest component of a complex multiple star system. Now, aim your binoculars that way for they will reveal a wonderful little nearby asterism that turns this 2nd magnitude star into a “diamond ring.” Look for a full-moon sized circlet of 7th and 8th magnitude stars just south of Polaris.

What’s the most commonly accessible deep sky study near Polaris? Let’s say the long-lived open cluster NGC 188 in Cepheus. Due to its smaller size, and low cumulative brightness (magnitude 8.1), this more than 10 billion year-old “Methuselah Cluster,” requires dark skies and at least a mid-sized scope to detect. Look for a faint sheen of light punctuated by a half dozen 12th and 13th magnitude stars some 4 degrees from Polaris in the direction of Alpha Cassiopeia.

Tuesday, July 25 – It’s New Moon and dark skies are ours to explore the summer Milky Way. Tonight would be a good time to find a dark site and just look! Aptly described as “milky,” our view of the great galactic pinwheel we call home is based on a location some 26,000 light-years from its very heart. Our solar system is one tiny light within the Orion Spur between the Centaurus and Perseus Spiral arms. When we look toward our galaxy’s core, we peer through a great disk at its bulging core region. The center of that expanse is now known to lie about one degree due south of 4.6 magnitude X Sagittarii. Hidden inside is a black hole with the combined mass of some 3.6 million suns – acting as the pivot around which all things galactic turn. To locate X Sagittarii visually, use the tip star of the Sagittarian “teapot” spout (Gamma) and look for it around three finger-widths northwest. Not sure? Try placing the little finger of your right hand on Antares. X is just about a hand span away towards your thumb to the east.

Does “X” mark the spot of anything visible? Two finger-widths south-southwest is 7.2 magnitude open cluster NGC 6425. Large binoculars reveal the cluster as a slightly condensed arc of stars, while scopes resolve a dozen members. Even closer to the galactic core is 8.0 magnitude NGC 6451 between our last study and X. This small open cluster is more densely populated but its brightest members are fainter. Capturable in a small scope, it will require aperture to begin resolution. As you observe this cluster tonight, remember you are looking within 17,000 light-years of the very center of the Milky Way galaxy!

Wednesday, July 26 – While we still have dark skies, let’s resume our tour of studies near the galactic core. Return to X and head slightly more than two finger-widths north-northwest for 34,000 light-year distant globular cluster NGC 6401. First seen by William Herschel on May 21, 1784, this “Uncertain Class” globular proved unresolvable and was thought by him to be gaseous – rather than starry – in nature.

A finger-width due west is tonight’s challenge – 12th magnitude planetary nebula NGC 6369. Its pinkish color is due to the red filtering of its light by gas and dust in the direction of the galactic core. Visible in small scopes, it will take careful inspection of stars around it to detect this faint annular “Little Ghost Planetary.”

Now locate Theta Ophiuchi and head south-southeast less than a finger-width. There you will find small, 9.5 magnitude globular NGC 6355. Discovered by Herschel three days after NGC 6401, this Uncertain Class globular lies within 6,000 light-years of the galactic core and 31,000 light-years from Earth.

Thursday, July 27 – Tonight the tender crescent of the Moon will join the western horizon quickly. Let’s take this opportunity to enjoy a warm summer evening and have a look at the lunar edge near the center and see what we can identify. Do you see the beginnings of Langrenus? If so, power up and continue east to look for three rings that get progressively larger as you move toward the limb. These are tiny Barkla, larger Kapteyn with central peak, and La Pérouse. If lunar libration is in our favor, you might even spot diminutive Elmer on the edge!

Now let’s head off once more toward the galactic core and continue our tour where “X” marks the spot.

Two globular clusters are found 4 degrees southeast of X Sagittarii but there’s a far simpler way to find them. Both are less than a finger-width north-northwest of Gamma Sagittarii – Al Nasl. Do you recognize this pair? The brightest and largest is NGC 6522. Located 25,000 light-years away, this globular is within 2,000 light-years of the center of our galaxy. Its companion – NGC 6528 – is slightly less than 26,000 light-years distant – but is equally close to our galaxy’s black hole. This pair was discovered simultaneously by William Herschel on the night of June 24, 1784.

To finish our “X” tour of studies, locate diminutive, but densely packed open cluster NGC 6520. You’ll find it less than two finger-widths north of Al Nasl. At 6300 light-years distant, this 8th magnitude open cluster is far closer to us than any globular of comparable magnitude. Look for NGC 6520 and the accompanying dark nebula – Barnard 86. Déjà vu?

Friday, July 28 – Tonight the young Moon sets early, so we’ll hustle off to revisit a single small globular – M80. Found about two finger-widths northwest of Antares, this little globular cluster is a powerpunch at Class II. Located in a region heavily obscured by dark dust, M80 shines like an unresolvable star in small binoculars and reveals itself to be among the most heavily concentrated globulars through the telescope. Discovered within days of each other by Messier and Méchain in 1781, this intense cluster is around 36,000 light-years distant.

In 1860, M80 became the first globular cluster to contain a nova. As surprised astronomers watched, a centrally located star brightened to magnitude 7 over a period of days to become known as T Scorpii. The event then dimmed more rapidly than expected, making observers wonder exactly what they had seen. Since most globular clusters contain stars of about the same age, the hypothesis was put forward that perhaps they had witnessed an actual collision of stellar members. Given that the cluster contains more than a million stars, the probability remains that some 2700 collisions of this type may have occurred over M80’s lifetime.

Now grab a comfortable seat because the Delta Aquarid meteor shower reaches its peak tonight. Not considered especially prolific, the average fall rate is about 25 per hour – but who wouldn’t want to catch a meteor every 4 to 5 minutes? These travelers are considered to be quite slow, with speeds around 24 kilometers per second, and they leave yellow trails. One of the most endearing qualities of this annual shower is its broad stream of around 20 days before and 20 days after peak. This allows it to overlap with the beginning stages of the famous Perseids.

Saturday, July 29 – Tonight the Moon will be high enough for us to have a great look at the area just a little southwest of Petavius to identify a small pair of Class I craters, known as Snellius and Stevinus. Power up and notice how these younger craters display much sharper and well defined outlines. If skies are steady, you may spy an odd looking series of confluent craters between the pair and the limb. Showing itself as an odd looking black streak, take pleasure in the fact that you have just identified Class II Hase!

After the Moon sets, revisit the Great Hercules Cluster – M13 – while it remains well placed. Those with very small scopes should avert their vision to detect a dozen or so of the cluster’s brightest outlying members well away from its luminous core region. For large aperture, take a very careful look at M13’s edge for a dark obscuration known as the “Propeller.” Rather than actually being part of this pure population II cluster, it is believed that this dark nebula is between us and the cluster.

Sunday, July 30 – If you are exploring the lunar surface tonight, be sure to look closely along the eastern boundary of Mare Nectaris. The bright cliffs seen there are the Pyrenees Mountains which hold crater Gutenberg in their grasp. Gutenberg has been filled with lava and terribly eroded over its lifetime. Its northeast wall was broken before the lava flow by an impact known as Gutenberg E. The southern edge contains a very unusual mountain-walled enclosure.

Now, relax and enjoy the peak of the Capricornid meteor shower. Although it is hard for the casual observer to distinguish these meteors from the Delta Aquarids, no one minds. Again, face the general direction of southeast and enjoy! The fall rate for this shower is around 10 to 35 per hour, but unlike the Aquarids, this stream produces those great “fireballs” known as bolides. Enjoy…

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