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The Sky This Week, 2020 September 22 - 29

Messier 27, planetary nebula in Vulpecula, imaged 2020 SEP 19 from Blackwater Falls State Park, Davis, WV.
Messier 27, planetary nebula in Vulpecula, imaged 2020 SEP 19 from Blackwater Falls State Park, Davis, WV
with an Explore Scientific AR102 4-inch (10.2-cm) f/6.5 refractor and a Canon EOS Rebel SL2 DSLR

The Moon skims the southern horizon this week, passing through the southern summer constellations before entering the barren star fields of the autumnal sky.  First Quarter occurs on the 23rd at 9:55 pm Eastern Daylight Time.  You will find Luna near the bright planet Jupiter on the evening of the 24th.  On the following night she will be just a few degrees southeast of Saturn.

With the passing of the equinox we are now officially in the astronomical season of Autumn.  Each day now sees the length of daylight change at its maximum rate, which is 2 minutes 30 seconds per day at the latitude of Washington.  As we mentioned last week, after the 26th our nights will be longer than our days, with daylight reaching its minimum on the winter solstice on December 21st, when we’ll have just 9 hours 26 minutes between sunrise and sunset.

As the Moon waxes towards the full phase her light washes out the all but the brightest stars in our lengthening night skies.  Fortunately we still have many of summer’s bright stars to enjoy during the evening hours.  As summer’s splendid Milky Way dissolves into the brightening sky we still have many of the bright stars that are sprinkled along its path to keep us looking up.  As twilight ends we find the three bright stars of the Summer Triangle asterism crossing the meridian almost straight overhead from our mid-northern latitude.  Vega, Deneb, and Altair can be seen from almost any location, although sadly sometimes they are the only stars easily seen from city centers.  Their outward appearance might lead one to believe that they are physically similar.  Each has a bluish-white tint and similar apparent brightness.  We perceive stars as small bright dots scattered on an inverted dark bowl with no indication of their true locations in the vastness of space.  Vega, the brightest of the trio, and Altair, the southernmost apex of the triangle, are both considered to be “ordinary” stars that are located relatively close to the solar system, at 25 and 17 light-years, respectively.  Deneb, however, is a very different beast.  It is much farther away, some 100 times the distance to Altair, but it appears to be almost as bright to the naked eye.  This means that it must be very luminous, generating the equivalent light of some 200,000 Suns!  It is a star that lives a fast and furious life.  The Sun has existed in its current state for about 4.5 billion years and should maintain its steadfastness for a few billion more before exhausting the hydrogen “fuel” in its core.  Deneb has used up all of the hydrogen in its core and is now fusing hydrogen into helium in a shell around the core.  This is the first step into evolving into a “red supergiant” star like Betelgeuse in Orion or Antares in Scorpius.  Deneb will reach this stage in a few million years.  Shortly (on the cosmic scale) after that it will suffer a catastrophic core collapse and become a supernova.  If our distant descendants see it explode, they will be treated to seeing a star that would be visible in broad daylight!  In contrast, when the Sun reaches the end of its life, it will go out with more of a “whoosh” than a “bang” gradually shedding its outer layers to form a so-called “planetary nebula”, an expanding shell of rarefied luminous gas.  Planetary nebulae abound along the plane of the Milky Way, and one of my favorites may be found near the center of the Summer Triangle.  Looking like a soft puff of smoke, Messier 27 gives us a snapshot of what our fate will ultimately be a few billion years from now.

Jupiter has slowly begun to resume direct eastward motion across the sky and is gradually inching toward nearby Saturn.  Old Jove will pass Saturn on the night of the winter solstice in a so-called “great conjunction”.  Right now, though, he is well-placed for early evening viewing.  His generous disc will reveal details in his cloud belts to modest telescopes, and his four bright Galilean moons can be glimpsed in a pair of binoculars.

Saturn follows Jupiter across the meridian and offers his famous ring system for owners of small telescopes.  His disc is about half the apparent size of Jupiter’s, but Saturn is also almost twice as far away.  This week, if you hopped a starship capable of travel at light speed, you’d still need 80 minutes for the one-way trip.

Mars is ready to inherit Jupiter’s prominence in the later night sky.  He is well up in the east by 10:00 pm, and his apparent brightness is now the same as that of Old Jove.  However, there’s no mistaking Mars for Jupiter.  His ruddy tint stands out in the sparse star field that surrounds him, and he will continue to brighten as he approaches opposition in early October. 

Venus greets early risers with her dazzle as morning twilight begins to brighten the sky.  This week she moves into the constellation of Leo, the Lion, drawing a bead on the constellation’s brightest star, Regulus.  She will pass the star next week.