What’s Up this Week: August 7 – August 13, 2006

Greetings, fellow SkyWatchers! The week begins with the Moon and ends up going globular as we have a look at some of the finest the Summer has to offer. Although skies will be bright, don’t forget the most reliable meteor shower of all – the Perseids! Hang around to explore multiple star systems because…

Here’s what’s up!

Monday, August 7 – Today in 1959, Explorer 6 became the first satellite to return photos of the Earth from orbit.

Tonight let’s explore the Earth’s natural satellite with binoculars as we view the areas of all the historic Apollo missions. Starting with Apollo 11, you will find its landing site on the southwest corner of Mare Tranquillitatis where it meets with Mare Nectaris. Apollo 12 was near the terminator to the west and just north of the small, bright punctuation of Euclid. Apollo 14 landed due east on the border of Mare Cognitum. Look to the north for shallow Archimedes and the Apennine Mountain range where you will find Apollo 15’s site in Palus Putredinus. Look southeast of Apollo 11’s site in the rugged terrain west of Theophilus for Apollo 16, and Apollo 17 ends our tour on the southeastern shore of Mare Serenitatis where it joins Mare Nectaris.

This evening, let’s round out our exploration of the Summer Triangle with a look at Deneb. Seemingly the faintest of the three bright stars of summer, this one star puts out more light than some globular clusters. With an absolute magnitude of -7.1, Deneb would be among the very first stars resolved out of the combined light of the entire Milky Way by astronomers living in the Great Andromeda Galaxy. Among earthly astronomers, it is widely believed Deneb is the source of the bulk of the light illuminating the neighboring North American Nebula (NGC 7000). But, Deneb won’t last long. With a mass 25 times that of our Sun, its voracious nuclear engine will burn through most of its available hydrogen and helium within 50,000 years, while the intense heat of its core will cause the star’s outer shroud to expand hundreds of millions of kilometers into the surrounding space. When this one eventually goes supernova, even 1600 light-years may not be an absolutely safe distance. Afterwards, a second truly superb supernova remnant will join the Veil Complex in Cygnus and future generations of astronomers will contemplate it in great wonder!

Tuesday, August 8 – Although the Moon will be very nearly full tonight, we can still look for a lunar feature. On the south limb near the terminator, look for a well defined black ellipse with a highlighted southwest wall. This is Inghirami, and its northeastern section will be quite dark. This high wall will cast a shadow across the grey crater floor towards the terminator.

Now let’s revisit 52 Cygni. Located due south of Epsilon, this foreground star to the Western Veil Nebula is also a pretty double whose 8.7 magnitude disparate companion lies some 6 arc seconds east-northeast of the 4.2 magnitude primary. As a challenge for small scopes during the lunacy, try different eyepieces to strike a balance between darkening the sky, and condensing the light of the faint companion.

Wednesday, August 9 – Tonight is the Full Moon. During years when the Harvest Moon occurs late in September, this was often referred to as the “Fruit” or “Barley” Moon – a time when both are ripe. While we can trace its bright rays and features tonight, why not try to look for something a bit different?

We often overlook the simple beauty of practicing astronomy without a telescope. This evening as the Sun sets and the Moon rises opposite it, take advantage of some quiet time and really stop to look at the eastern horizon. If you are lucky enough to have clear skies, you will see the Earth’s shadow rising – like a dark, sometimes blue band – that stretches 90 degrees north and south. Look just above the horizon for a Rayleigh scattering effect known by some as the “Belt of Venus.” This beautiful pinkish glow is caused by backscattering of sunlight. As the Sun continues to move west, the boundary between Earth’s shadow and the Venus’ Belt rises higher in the sky and gently blends into the coming night. What you are seeing is the shadow of the Earth’s translucent atmosphere, casting a shadow back on itself.

Thursday, August 10 – Tonight the sky remains bright all evening – but that won’t keep us from challenging ourselves on a fine double now high to the south in Sagittarius at skydark. Tight, disparate double 21 Sagittarii is less than five degrees due north of Kaus Borealis – Lambda Sagittarii. Because of bright skies, it will take a finderscope to track this 5th magnitude star down. To help single it out, look for Mu Sagittarii 3 degrees to 21’s west. Resolution of this 1.9 arc second pair is possible in just about any size instrument, but like other pairs we’ve visited, this one is disparate. Look for the 5.1 magnitude blue A star’s 7.6 magnitude orange companion leading it across the sky.

How about another? Simply swing 3 degrees west to a very challenging multiple star system – Mu Sagittarii! Under the circumstances, this four-star group will need a bit of aperture to see all three faint companions. Ranging in magnitude from 9.3 to 13.5, all members are easily resolved at low magnifications.

If you choose to look at the lunar surface tonight, look for several bright rays extending away from crater Copernicus. Traveling hundreds of kilometers across the surface, each ray is unique.

Friday, August 11 – On this date 129 years ago, Asaph Hall of the U.S. Naval Observatory unwittingly prepared for an evening of planetary discovery. That night in 1877 would prove to be the first time anyone saw a satellite of Mars! Six nights later, Asaph went on to observe Deimos’ partner, Phobos, as well. Since then, we humans have added a few “satellites” to Mars’ skies, but Phobos (“Fear”) and Deimos (“Terror”) are the Red Planet’s only two known natural moons. When Hall discovered these two small, irregularly shaped satellites, Mars was riding high in Aquarius less than one month before opposition. However tonight you can only catch Mars trailing the Sun to the west less than an hour after sunset. Look for it as a much diminished 1.8 magnitude “star” in Leo just above the western horizon.

Tonight is the peak of the Perseid meteor shower, but observations will be hampered by the Moon. Despite this, you can improve your chances by putting a tree or building between yourself and the Moon. Let’s sit back and talk about the Perseids while we watch…

The Perseids are the most famous of all meteor showers and never fail to provide an impressive display. Records of Perseid activity go back to 36 AD. In 1839, Eduard Heis was the first observer to take a meteor count and discovered the Perseids had a maximum rate of around 160 per hour. Other observers have since continued these studies to find the fall rate varies considerably.

Astronomer Giovanni Schiaparelli was the first to relate the orbit of the Perseids to periodic comet Swift-Tuttle (1862 III). We now know there are four individual streams from the comet’s 120 year orbital period. Their peaks occur on different nights around this time – but tonight’s stream is the heaviest. This debris enters the Earth’s atmosphere at 60 km/sec from the border of Perseus and Cassiopeia. Watch as the radiant swings an arc around Polaris northeast to northwest. The Perseids will be around for a few more days yet, so keep up the watch and make some counts of your own!

Saturday, August 12 – Tonight the Moon rises just as the sky gets dark, but that doesn’t mean we can’t take scope to sky. Returning to Gamma Sagittarii, use low power to look for 9.2 magnitude globular cluster NGC 6638 to the east-southeast. Like all 9th magnitude globulars, there is little hope of resolving more than a few of the brighter members without using a large telescope.

How about another? If you shift Gamma almost a degree in the direction of NGC 6638 and look north in the low power field you will see something a bit more impressive – mid-sized 6.9 magnitude globular M28. Discovered by Messier on July 27, 1764, this 19,000 light-year distant globular cluster is 60 light-years in diameter. First resolved as a “star cloud” by William Herschel, tonight you might be able to pick out a few of its brightest members.

Sunday, August 13 – Don’t forget to keep watch for more of the Perseids!

Tonight we’ll have about a half an hour of no Moon to use to real advantage. Let’s return to Gamma Sagittarii and head less than 3 degrees east-northeast to the spectacular 5.1 magnitude globular cluster M22.

At 10,400 light-years distance, M22 is one of the closest globular clusters to Earth and glows with the combined light of 100,000 suns. Its 100 light-year diameter globe spreads across a full moon’s size in professional telescopes. Like most globular clusters, only about half its true apparent size is visible through common equipment, but the view is as expansive as the true apparent size of many other clusters – including the Great Hercules Cluster – M13!

Consisting of numerous stars bright as 11th magnitude, M22 is easily resolved through dark skies into dozens of stars even through the smallest scopes. Like M13, this superb globular appears to have regions where stellar density varies within its form – a mottling effect that can be seen through modest telescopes. Among globulars, M22 is the third brightest in the sky – following Omega Centauri and 47 Tucanae and trailed distantly by nearly sixth magnitude M13 and M5. Southern observers will have little trouble seeing M22 unaided.

After viewing M22 head one degree northwest to visit 8.8 magnitude globular cluster NGC 6642. NGC 6642 is about three times more distant than M22 and lies some 5500 light-years from the galactic core. Intrinsically, NGC 6642 is every bit as luminous as M22!

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