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The Sky This Week, 2019 July 23 - 30

Hanging out with Hercules.
Messier 13, the Hercules globular cluster, imaged 2013 August 5 from Fishers Island, New York.
Messier 13, the Hercules globular cluster, imaged 2013 August 5
with an 80 mm (3-inch) f/6 Antares Sentinel refractor and a Canon EOS Rebel T2i DSLR
from Fishers Island, New York.

The Moon wanes in the pre-dawn sky this week, climbing northward along the ecliptic, joining the first rising stars of the winter sky.  New Moon occurs on July 31st at 11:2 pm Eastern daylight Time.  Early risers can watch Luna’s dwindling crescent pass by the bright star Aldebaran in the pre-dawn skies of the 27th and 28th.

The Moon’s absence from the sky offers us another opportunity to take in the wonders of the summertime sky.  It also gives you a chance to do a little “citizen science”.  The Globe at Night program returns for its July installment this week, inviting skywatchers to look for stars in prominent constellations to gauge the brightness of the night sky.  This month’s featured constellation is Hercules, which you will find nearly overhead as evening twilight ends.  Hercules is the second-largest of the classical constellations delineated by Ptolemy, and his most prominent feature is a four star asterism popularly known as “The Keystone”.  You’ll find this distinctive pattern about one-third of the distance between the bright blue star Vega in the east and the rose-tinted springtime star Arcturus in the west.  If we imagine the Keystone as the hero’s torso, his extremities straggle off to the north and south.  To help with the Globe at Night project, simply go to their website and follow their directions to record an observation.  When we look just east of the stars of the Keystone, we are looking along the direction that the Sun and its gaggle of planets, moons, and asteroids are headed as we drift through our local spiral arm of the Milky Way galaxy.  Referenced to the more distant stars we are moving at a velocity of around 48,000 kilometers (30,000 miles) per hour.  From a dark-sky site Hercules offers one of the most spectacular “deep sky” objects in all the sky.  If you use a pair of binoculars, look between the two stars that form the western side of the Keystone for a fuzzy knot of light.  A telescope of four inches aperture will begin to resolve this knot into a swarm of faint stars, and the more you increase the telescope aperture the more it resolves into a myriad of luminous pinpoints.  The view through my 14-inch reflector is simply breathtaking, and I will often spend an hour or more poring over the object’s fine details.  Known as Messier 13, it is a fine example of a globular star cluster.  Globular clusters surround the central bulges of many spiral and elliptical galaxies, and they may be the remnant cores of dwarf galaxies that swept through the planes of their larger galactic hosts.  We find many such objects in the summer sky, a time when our galactic center is in full view.

The summer Milky Way takes center stage in the overnight hours.  Our home galaxy is a huge flattened disc of stars that spans some 200,000 light-years and contains roughly 200 billion stars.  Our Sun is in the galactic “suburbs”, some 30,000 light-years from its center.  While its visual appearance is that of a more or less continuous band of amorphous light, exploration with radio telescopes reveals its structure of spiral arms wrapping around its central bulge.  As we look toward the southern horizon we are looking in the direction of the galactic center, and you can see a noticeable widening of the glowing band just above the “teapot” asterism of Sagittarius.  However, we can’t actually see the center in visible light; a huge wall of opaque gas and dust obscures our direct view of this feature.  You can see some of this obscuring material in the form of the “Great Rift” that runs from the center of the Summer Triangle down to the horizon.  Fortunately astronomers have developed instruments that can observe the center in any different wavelengths, and the structures that they have revealed are truly mind-boggling.  X-ray and radio telescopes have revealed a very compact, highly energetic energy source which we now believe to be a super-massive black hole.  This object contains the equivalent mass of over 4 million Suns!

Jupiter shines brightly in the south during the evening and overnight hours.  The giant planet crosses the meridian at around 10:00 pm, so you’ll have ample time to give him a good look with binoculars or telescopes.  Low power scopes and binoculars are sufficient to follow the motions of his four bright Galilean moons, while instruments of four or more inches of aperture will reveal details on the planet’s gaseous face.  The prominent equatorial cloud belts will break up into a myriad of swirls and eddies in large-aperture instruments, and these features will change dramatically from night to night.

Jupiter is trailed by Saturn, which doesn’t glow as brightly as Jupiter but is still easy to spot as darkness falls.  The ringed planet is located just east of the “teapot” of Sagittarius, and anyone who points a telescope in his direction will be in for a treat.  The planets rings, composed of billions of ice particles, are as wide-open to our line of sight as they can get and will reveal themselves in a simple spotting scope.  Put a larger aperture on Saturn and the rings will begin to show detail.  You’ll also see more of the planet’s many icy moons.  Titan, the planet’s largest and brightest moon, is one of the most interesting bodies in the solar system.  It has a substantial atmosphere of nitrogen, methane, and other complex carbohydrates.  The Cassini orbiter, which circled Saturn for over 13 years, revealed that Titan has oceans of liquid methane on a surface of hard water ice.  What a strange world to stoke the imagination!

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