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The Sky This Week, 2020 September 1 - 8

Rings and things
Messier 57, the Ring Nebula in Lyra, imaged from Fishers Island, NY, 2016 August 25.
Messier 57, the Ring Nebula in Lyra, imaged from Fishers Island, NY, 2016 August 25
with an Antares Sentinel 80-mm (3-inch) f/6 refractor and a Canon EOS Rebel T2i DSLR

The Moon wanes from her full phase as she climbs northward along the ecliptic, passing through the star-poor reaches of the autumnal constellations.  She ends the week among the rising stars of the bright winter constellations, delighting early risers.  Last Quarter occurs on the 10th at 5:26 am Eastern Daylight Time.  Look for our fair satellite just east of ruddy Mars before dawn on the 6th.  On the morning of the 9th you’ll find her among the stars of Taurus, the Bull, set between the bright star Aldebaran and the Pleiades star cluster.

September usually brings an end to the hazy hot nights of July and August as the first hints of autumn add a bit of crispness to the air.  Cooler airmasses generally bring more transparent skies, and the end result is clearer views of many of summer’s celestial showpieces.  By the end of evening twilight the bright star Vega, the westernmost star in the Summer Triangle asterism, lies directly overhead.  Vega is located about 25 light years from the solar system, and its relative proximity and luminosity of about 40 Suns make it the fifth brightest star in the sky and second brightest in the northern sky.  It was the first star other than the Sun to be photographed and have its spectrum measured.  For decades it was used as a “standard” star for a number of parameters, but recent measurements show that it has its quirks.  Its light output varies slightly over time, and it is now known to rotate very rapidly, spinning once every 12.5 hours.  For comparison the Sun rotates once in about 25 days.  Vega is the lead star in the diminutive constellation of Lyra, the Harp, which contains a number of interesting targets well-known to amateur astronomers.  Northeast of Vega sharp-eyed skywatchers may see a close pair of faint stars that easily resolve in binoculars.  Stepping up to the eyepiece of a three-inch telescope reveals that each component is itself a close double star that’s popularly known as the “Double-double”.  Located some 160 light years away, the four stars form a true quadruple system, and recent observations indicate that there may be a fifth companion.  

After Vega the brightest stars of Lyra form as small parallelogram that “points” southward.  Between the two southern stars of this figure a telescope will reveal one of the most famous “deep sky” objects known, Messier 57, also known as the Ring Nebula.  Appearing as a small hazy patch of light in small telescopes, under higher magnification you’ll see what appears to be a tiny smoke ring set in a background of shimmering stars.  It is the first example of a type of object known as a “planetary nebula” to be discovered, and while it outwardly resembles a ghostly planetary disc it is a far different kind of beast.  Located some 2500 light years away, it represents the ultimate fate of our Sun when it exhausts all of its nuclear fuel.  The radiation pressure from the fusion of hydrogen into helium will disappear, allowing gravity to squeeze the Sun’s mass into a super-dense, Earth-sized body called a “white dwarf”.  As the star’s core shrinks it gets very hot, illuminating the surrounding diffuse cloud of gas with powerful ultraviolet radiation, causing the gas to glow.  Eventually the white dwarf’s thermal energy runs out and its surrounding nebula will fade.  Its core then crystallizes into an even more bizarre object, a black dwarf.  Such stellar remnants may litter the Galaxy, but they cannot be detected except by their gravity.

Jupiter is now in prime viewing position in the evening sky, crossing the meridian at around 9:30 pm.  Due to his far southerly declination you should take advantage of this time since this is when Old Jove is highest above the southern horizon.  This is when you can expect the least turbulence from our own atmosphere, affording you time to look for the planet’s interesting atmospheric features.  Small telescopes should easily show the four bright Galilean moons, and an aperture of four inches should show the famous Great Red Spot.  This feature will be prominent in “prime time” on the evenings of the 3rd, 5th, and 8th.

Saturn follows Jupiter by half an hour across the meridian.  While not as bright as his giant companion, Saturn is still worth your attention.  The planet’s most famous feature, the system of concentric rings, is always a delight to see in just about any telescope.  Larger instruments will start to bring out subtle details in the rings, and under dark skies the planet seems to float among its many small icy moons.

Not to be outdone, ruddy Mars now rises before 10:00 pm and is prominent in the east an hour later.  There is no mistaking this glowing jewel as he rises into the night.  His warm pink-orange tint is unlike that of any other planet, and he will continue to brighten until he outshines Jupiter by the month’s end.

Venus moves from the environs of the bright stars of Gemini into the obscure constellation of Cancer, the Crab.  She continues to be a fixture in the pre-dawn sky, where she will remain for the rest of the year.