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The Sky This Week, 2017 November 7 - 14

Gems in the autumn sky
M31_171021_04small.jpg
Messier 31, the Andromeda Galaxy
imaged 2017 October 20 from Great Meadow, VA by Geoff Chester
with an Explore Scientific AR102 4-inch f/6.5 refractor
and Canon EOS Rebel T2i DSLR.

The Moon wanes in the late evening and morning skies this week, passing through the rising constellations of winter and spring.  Last Quarter occurs on the 10th at 3:36 pm Eastern Standard Time.  Look for Luna near the bright star Regulus in the constellation Leo on the morning of the 11th.  By the end of the week the Moon will be near ruddy Mars.  She’ll be just over three degrees east of the red planet before sunrise on the 15th. 

The change back to Standard Time now pushes the onset of night an hour earlier.  You can still see the stars of the Summer Triangle in the hours after sunset, and from a dark location the summer Milky Way is still prominent as evening twilight ends.  The densest part of the home galaxy is now setting on the southwest, but the prominent star clouds and dark rifts still split the three stars Vega, Deneb, and Altair.  Its brightness slowly wanes as you look toward the northeast, but the number of bright stars begins to concentrate with the rising constellations of Cassiopeia and Perseus.  These two constellations are wonderful places to explore with binoculars or a small telescope, with dozens of star clusters scattered among their brighter stars.  One of my favorite objects in this area lies in the middle of the Milky Way between the two constellations.  The “Double Cluster” can be seen with the naked eye from a dark location, and its stellar nature is revealed with only slight optical aid.  My favorite views come with my 3- and 4-inch refractors using a low-power eyepiece.  These clusters each contain some of the most intrinsically bright stars in the sky, with luminosities over 100,000 times that of our Sun.  Looking south at around 8:00 pm, you’ll find a relatively sparse sky that’s dominated by a large square of second-magnitude stars that marks the constellation of Pegasus.  The upper left star of this square is Apheratz, shared by Pegasus and Andromeda.  Under dark skies you’ll notice two diverging “chains” of stars pointing toward Perseus.  Go to the second star in the brighter chain, then hop up two fainter stars to spot a small, fuzzy “cloud” that looks like a detached piece of the Milky Way.  Binoculars will reveal an elongated streak of light that refuses to resolve into stars in all but the largest telescopes.  This is the famous Andromeda Galaxy, Messier 31, the largest member of our Local Group of island universes.  At a distance of 2.5 million light years, this is the most distant object you can see with your unaided eye.  It’s estimated that this galaxy contains some 400 billion stars.  The Milky Way and M31 are approaching each other at a speed of about 110 kilometers (68 miles) per second.  It will collide with us in about 4 billion years!

Saturn still lingers in the southwestern sky, visible for a short time after the end of evening twilight before setting at around 7:00 pm.  His low altitude means that detailed views of his rings will be quite distorted by turbulence in our atmosphere.  

You’ll find Mars rising in the hours before dawn.  The red planet begins the week just under two degrees south of the second-magnitude close double star Porrima in Virgo.  He continues to trek eastward toward the bright star Spica and is joined by the waning crescent Moon as the week ends. 

Venus is now only visible in the brightening morning twilight, rising about an hour before the Sun.  You should be able to spot her if you have a low eastern horizon.  If you’re lucky and have a clear sky on the morning of the 12th look for Jupiter just a quarter of a degree from Venus in the brightening sky.  They will be in bright twilight but should be easy to spot in binoculars half an hour before sunrise.

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