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The Sky This Week, 2013 April 9 - 16

Farewell, Orion!

The "Great Nebula" in Orion, Messier 42, 2013 March 30

Imaged from Vaucluse Spring near Stephens City, VA
with an 80mm (3.1-inch) f/6 Antares Sentinel refractor
and a Canon EOS T2i DSLR camera,
aggregate exposure time 5m 20s @ ISO 1600.

The Moon returns to the evening sky this week, vaulting into the evening sky after new Moon, which occurs on the 10th at 5:35 am Eastern Daylight Time.  Look for the slender 1.5-day-old crescent in twilight on the 11th.  On the evening of the 13th Luna lies between the bright star Aldebaran and the Pleiades star cluster in the constellation of Taurus, the Bull.  The next evening she is just three degrees away from the bright planet Jupiter, giving us a nice “photo opportunity”.  While watching the Moon this week, look for the ghostly illumination of “Earthshine” on the dark portion of the Moon’s disc.  That phenomenon is literally a reflection on us as it is caused by sunlight reflecting from our home planet and bathing the part of Luna that’s not directly illuminated by sunlight.  It’s quite easy to spot during the Moon’s crescent phases, but it becomes more difficult to see by the time the Moon reaches First Quarter.  When do you lose sight of it?

Before the Moon climbs back into the sky you have one last chance to bid a wintertime friend goodbye for the season.  The bright stars of Orion, the Hunter are now dipping to the southwest as twilight ends, affording you about an hour to explore his bright stars and famous Great Nebula.  Easily recognized by his three “Belt Stars”, the ice-blue supergiant stars Alnitak, Alnilam, and Mintaka, Orion also hosts the prototype red supergiant star Betelgeuse and the incredibly luminous Rigel.  Betelgeuse is a highly-evolved star; having exhausted the nuclear fuel in its core, its fusion reactions now take place in concentric shells around a core of heavy elements.  This has forced the star to bloat out to huge proportions.  If it occupied the position of the Sun in our solar system, its outer layers would approach the orbit of Mars!  Rigel and the three belt stars will soon follow in Betelgeuse’s wake.  They burn hydrogen in their cores at a voracious rate, pumping out between 75,000 to 150,000 times the energy of the Sun.  This is why they appear so bright in our sky.  Even though they are between 1500 to 2000 light-years away from us, they are among the 30 brightest stars in the sky!  Most of Orion’s stars originated in a region that forms the middle “star” in the Hunter’s “sword”, which dangles below the left-side belt star Alnitak.  Look at this region with binoculars and you will see a soft swirl of light surrounding several stars.  Through a telescope this swirl takes on an almost three-dimensional structure that becomes more intricate with each increase in aperture.  The “Great Nebula” is a stellar nursery in which hot, energetic protostars are forming all the time.  There is enough material in the visible parts of the nebula to forge some 10,000 stars of the Sun’s mass or greater.  The visible part of the nebula is just the tip of the proverbial iceberg in this part of the sky.  Wide-angle, deep-sky images of Orion and his surroundings show a pervasive background haze of nebulosity, making it one of the largest star factories in our galaxy.  Now’s the time to give it a final look before the Hunter returns to the evening sky in late fall.

Bright Jupiter follows Orion toward the western horizon in the early evening sky.  The giant planet is still the brightest object in the evening after the Moon, and it’s hard to miss him once the sky darkens during the twilight hour.  He still presents an impressive sight through the telescope eyepiece, surrounded by his four bright Galilean moons.  The best time to view him now is during deep evening twilight.  By 8:30 pm he’s only 40 degrees above the western horizon.  By 10:00 pm he’s just 20 degrees up and subject to the swirling turbulence of the air above the horizon.  Old Jove presents an interesting view on the evening of the 11th, when only his two outermost moons, Ganymede and Callisto, are visible.  Io and Europa will be hiding behind the planet’s vast bulk.

Golden Saturn now rises in the southeast at around 9:00 pm.  The ringed planet will reach opposition on April 28th, so we are now entering prime observing season for the second-largest planet.  Features on Saturn are much more subtle than those on Jupiter, but most of us are completely distracted by the planet’s rings, which have puzzled and delighted astronomers since their true nature was first deduced by the Dutch astronomer Christiaan Huygens 1655.  Huygens also discovered Saturn’s largest moon Titan, which we now know is the only moon in the solar system to possess an atmosphere.  The Huygens probe, which landed on Titan in January, 2005, found titan to be a very dynamic place despite a surface temperature that keeps methane in a liquid state.  Indeed, methane on Titan is similar to water on Earth; it exists in three states and has an evaporation cycle that modifies the moon’s surface.  Titan is easily seen in a telescope of three inches aperture or more.  Watch its progress this week as it moves from west of the planet to eastern elongation.

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