The Sky This Week, July 7 – 14, 2015 !

The Sky This Week, 2015 July 7 – 14

Summertime is Globular season.
Messier 4, Antares, and NGC 6144
Imaged from Morattico, VA, 2014 July 5

The Moon wanes in the morning sky this week slimming through her crescent phases before New Moon, which occurs on the 15th at 9:24 pm Eastern Daylight Time. By the week’s end Luna joins the rising constellations of early winter in the pre-dawn sky. Look for her just west of the bright star Aldebaran, the fiery “eye” of Taurus, the Bull.

The absence of the Moon from the evening sky allows us to track down some of the most interesting objects that preferentially inhabit the summer heavens. Known as globular star clusters, these objects appear as round fuzzy patches of light scattered throughout the southern half of the sky and more or less centered on the center of the Milky Way galaxy. These clusters have an astonishing range of forms that gradually reveal themselves as you increase telescope aperture. Most of them have a bright suffused core surrounded by scattered pinpoints of faint stars in varying degrees of condensation. Some 150 of these clusters are associated with our galaxy, and they contain some of the oldest stars known in the universe. The typical globular cluster contains tens to hundreds of thousands of stars and may be the cores of ancient dwarf galaxies that formed with the Milky Way a billion or so years after the Big Bang. As they plunged through the plane of the Milky Way on their looping orbits their star-forming gas clouds were stripped away, leaving just their cores of ancient stars. Some of the most prominent are easy targets for binoculars, and a good six-inch telescope will resolve dozens of them into their component stars. One of the best for northern hemisphere observers is Messier 13, which passes very close to the zenith at around 10:30 pm EDT this week. It is located along the western side of the “keystone” asterism of Hercules. Another easily located globular may be found just a degree to the west of the bright star Antares in Scorpius. Messier 4 basks in the ruddy star’s glow, and shows a prominent vertical band of stars across its face. A more distant cluster, NGC 6144, lies in the same field just northwest of Antares. My favorite summertime globular cluster, Messier 22, may be found just two degrees east of the star Kaus Borealis, the top of the “Teapot” asterism in Sagittarius. This cluster begins to resolve nicely in a four-inch telescope. It lies in a fantastically dense Milky Way star cloud, with the smaller globular Messier 28 nearby, just north of Kaus Borealis.

Venus and Jupiter continue to put on a spectacular evening show in the western twilight sky. While the gap between them slowly grows this week, they are still the dominant sight in the western sky. Over the course of the week both planets will slowly drift closer to the bright star Regulus in Leo. By the end of the week Venus will be just over two degrees from the star. Venus halts her eastward progress at this time and begins a rapid three-week plunge toward solar conjunction.

Saturn remains ideally placed for observing in the evening sky, crossing the meridian as evening twilight ends. The planet’s distinctive rings can be seen with almost any kind of optical aid, but the view through a modest astronomical telescope is one of the most striking sights in all of Nature.

Tempting as the view of Saturn may be, all eyes will be on a much more distant world on July 14th. That’s when the New Horizons space probe will fly by far-flung Pluto and its five known moons after nearly a decade of travel from Earth. Here at the Observatory we’ll be most interested in the appearance of Pluto’s largest moon Charon, which was discovered by staff astronomer Jim Christy back in 1978. It’s going to be an amazing sight!


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