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The Sky This Week, 2019 February 19 - 26

The distant stars of Orion
Orion and friends, imaged 2019 February 16 from Mollusk, Virginia
Orion and friends, imaged 2019 February 16 from Mollusk, Virginia
with a Canon EOS Rebel SL2 DSLR.

The Moon heads into the morning sky this week, turning southward along the ecliptic as she wanes to Last Quarter, which will occur on the 26th at 6:28 am Eastern Standard Time.  Luna begins the week among the rising stars of Leo, the Lion, one of the signature constellations of spring.  During the early morning of the 22nd she passes just over a degree north of the star Porrima in the constellation of Virgo.  Porrima is a very close double star and a good indicator of stability in Earth’s atmosphere.  On the morning of the 23rd the Moon will be well north of the first-magnitude star Spica in Virgo.  By the end of the week Luna is scudding over the southeast horizon during the wee hours, and closes in on bright Jupiter.

The waning Moon gradually brings dark skies back to evening skywatchers, and with each passing night you’ll gain an hour of “dark-sky” time to enjoy winter’s bright bounty of stars.  The stars of the Great Winter Circle are on display through the early evening hours, with the familiar figure of Orion straddling the meridian at 8:00 pm EST.  Orion is bright enough to be seen easily from the heart of major urban areas.  His principle stars are all of first- or second-magnitude, and his three “belt stars” can be seen from every inhabited part of the globe.  Unlike many of the first-magnitude stars in the sky, Orion’s luminaries are bright in our sky because they are very luminous, emitting thousands of times the energy of our nearby Sun.  Compare the visual appearance of Orion’s brightest star, Rigel, with nearby Sirius, which follows the Hunter across the sky.  Sirius is bright because it is a mere 8.5 light-years away.  Rigel, on the other hand, is located 100 times farther away!  This means that Rigel shines with the luminosity of well over 100,000 Suns.  Rigel and the three “belt stars” are members of a loose association of hot, energetic stars of similar characteristics.  From left to right across the sky the belt stars are Alnitak, Alnilam, and Mintaka.  Alnilam is the most distant of these stars, estimated to be nearly 2000 light-years distant.  If this is the case Alnilam must have a luminosity of some 800,000 Suns.  By tracing the long-term “proper motions” of these stars we find that they all share a common origin in the small clump of stars that can be seen from suburban areas hanging under the eastern part of Orion’s belt.  This small asterism is known as “The Sword”, and if you point a small telescope at the middle star you’ll see a softly glowing glow surrounding a tight group of blue stars.  This is the heart of the Great Nebula in Orion, and it is a crucible that is continually forging new stars.  From suburban skies you can only see the bright central portion of the nebula, but from a dark-sky site on a moonless night this nebulous glow fills the field of a low-power eyepiece.  In fact, long-exposure photographs of the entire constellation reveal that it is full of this luminous gas, and it is estimated that there is enough material in this cloud to form well over 10,000 stars as massive as the Sun.  Even if you don’t have a telescope, Orion is still a treat for the naked eye.  Nowhere else in the sky will you find such a distinctive collection of bright stars.

The evening twilight sky now hosts the fleet planet Mercury, which may be found low in the western sky about half an hour after sunset.  The elusive world will steadily climb higher as the week progresses, and by the week’s end he should be easy to locate in the darker sky at around 7:00 pm.  While he is nowhere near as bright as Venus, he will be the brightest object in this part of the sky.  

Mars is now crossing through the outlying stars of the small constellation of Aires, the Ram.  He will spend the next month within the bounds of this constellation before moving into the brighter starfields of Taurus, The Bull  

Jupiter rises at around 2:30 am and should be easy to spot in the southeastern sky as morning twilight begins to brighten the sky.  He is located just over 10 degrees northeast of Antares, the red-tinted star that marks the heart of Scorpius, the Scorpion.  If you’re up early, he should be well-placed for telescopic viewing until he fades in the brightening twilight sky.

Venus starts the week just east of Saturn in morning twilight, pulling away from the ringed planet over the succeeding mornings.  Venus will continue to move closer to the Sun over the next few weeks, then linger in twilight through the months of early summer.  Her bright glow should be easy to find up until a few minutes before sunrise, but she will remain low in the southeast for early risers. 

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