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The Sky This Week, 2018 May 8 - 15

A look into the deep...
Markarian's Chain
"Markarian's Chain", the heart of the Virgo Galaxy Cluster, 2013 April 14,
imaged from Vaucluse, Virginia with a 80-mm (3.1-inch) f/6 Antares Sentinel refractor and a Canon EOS Rebel T2i DSLR

The Moon wanes in the pre-dawn sky this week, passing through the dim constellations as she slips through her crescent phases before disappearing in the glow of twilight.  New Moon occurs on the 15th at 7:48 am Eastern Daylight Time.  

The nights of May grow shorter as the Sun continues to climb northward along the ecliptic.  Sunset now occurs well after 8:00 pm, which means that the end of evening astronomical twilight now falls at around 10:00 pm.  Morning astronomical twilight begins shortly after 4:00 am, so we now have just six hours to enjoy a fully dark sky.  Fortunately May evenings can be quite mild, and with the Moon moving into the early morning sky it’s a great time to explore the “deep sky” well beyond the bounds of our Milky Way galaxy.  In fact, if you were to look for the luminous band that defines the plane of the Milky Way you will find it confined to the horizon.  This means that as we look out into the space between the bright stars Arcturus, Spica, and Denebola (the “tail” of Leo, the Lion) we are looking through a thin veil of nearby stars across a vast gulf of intergalactic space several tens of millions of light-years deep.  Populating this region beyond the Milky Way’s stars brings us to hundreds of faint fuzzy swatches of light that betray entire galaxies like ours sprinkled across an enormous volume of space.  A modest telescope of 8-inches will show dozens of these misty patches within the three-star boundary, and larger instruments will reveal hundreds more.  All of these galaxies are associated with each other in a huge gravitationally-bound grouping known as the Virgo Cluster.  Astronomers recognize about 1500 galaxies in the cluster, most of which lie at a distance of over 50 million light-years.  Several of the galaxies near the center of the cluster are truly gigantic, with masses estimated at trillions of Suns!  The influence of the cluster is far-reaching, and its gravity even influences the motions of the galaxies in our “Local Group”, including our Milky Way!

Closer to home, the last of the winter stars are setting at 10:00 pm.  The last to settle below the horizon are Castor and Pollux, the Twin Stars of Gemini.  They are now replaced by lesser lights: where the Great Winter Circle contains nine first-magnitude stars, our current sky only hosts three.  These three stars, Arcturus, Spica, and Regulus, lead large scattered groupings of fainter stars that make up the constellations Boötes, Virgo, and Leo.  Of these, only Leo bears a passing resemblance to its namesake.  The most prominent springtime asterism, the “Big Dipper” portion of Ursa Major, contains no first-magnitude stars at all, but fortunately all but one of its seven stars are second-magnitude.    

Venus is steadily moving eastward through the setting stars of Taurus, the Bull.  This week she passes between the Bull’s “horns”, marked by the stars Zeta Tauri and El Nath, a star “shared” with Auriga, the Charioteer.  The dazzling planet is running a pace that keeps her even with the advances of the Sun, setting about 45 minutes after the end of evening twilight.

Jupiter is now visible in the sky all night long after reaching opposition on the 8th at 9:00 pm.  This is the time when Old Jove presents his largest apparent disc diameter, and owners of small to modest telescopes should be able to glimpse his main cloud belts and the Great Red Spot when it is facing us.  His four bright Galilean moons will offer a different configuration around the planet each night and are easily visible in binoculars and small telescopes.

Saturn sits just above the top of the “Teapot” asterism of Sagittarius.  The ringed planet sits in the middle of a dense Milky Way star cloud.  He is slowly drifting westward as he approaches his opposition in late June.  Take advantage of the moonless sky to spot the bright globular star cluster Messier 22, just over a degree south of Saturn.

Ruddy Mars crosses into the faint constellation of Capricornus as he continues to move eastward along the ecliptic.  His distinctive pink hue is unmistakable in this barren part of the sky.  His apparent disc is steadily growing as is his brightness.  When he reaches opposition in late July his disc will be twice as big as it appears now and he will be seven times brighter.

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