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The Sky This Week, 2019 May 28 - June 4

Bright stars of spring and summer.
Vega and the summer Milky Way, imaged 2018 June 20 in Desolation Canyon, Utah
Vega and the rising summer Milky Way, imaged 2018 June 20
with a Canon EOS Rebel T2i DSLR from Desolation Canyon, Utah.

A waning crescent Moon passes through the dim stars of the autumnal constellations this week as morning twilight brightens the eastern sky.  New Moon occurs on June 3rd at 6:02 am Eastern Daylight Time.  She will return to the evening sky next week. 

As we move into June and close in on the summer solstice we are now experiencing the shortest nights of the year.  The Sun now stays above the horizon for well over 14.5 hours every day through mid-July, and if you include the times of astronomical twilight a fully dark sky won’t occur until 10:30 pm and will last just under four hours here in the Washington, DC area.  The farther north you live the shorter your times of full darkness will be.  Nevertheless, the late spring sky still has much to offer.  By 10:30 pm the bright star Arcturus is close to meridian passage, leading the kite-shaped constellation of Boötes, the Herdsman.  Arcturus is the brightest star in the northern hemisphere sky and the fourth-brightest overall.  It is an evolved star that gives us a clue to the fate of our Sun.  It’s estimated to be about 7 billion years old and is fusing hydrogen in an expanding shell around an inert helium core.  This has caused the star’s outer layers to expand and cool, giving it a decidedly warm rosy tint.  It’s the closest example of its class to the solar system at a distance of 36.7 light-years.  It is also moving quite rapidly through space, and its mobility was first described by Edmond Halley of comet fame, who noted a large discrepancy between its position as plotted by the ancient Greek astronomer Hipparchus and his observed values.  Arcturus is moving in the general direction of the bright star Spica to the southwest; it will move the apparent diameter of the Full Moon over the next 1800 years.

Spica is also close to the meridian at the end of twilight, about 30 degrees south of Arcturus.  The brightest star in the sprawling constellation of Virgo, Spica offers a nice color contrast to the latter star, glowing with a distinctive bluish tint.  Spica is about 250 light-years away and is a very close binary system whose components orbit each other every four days.  The two stars together shine with a luminosity of about 25,000 suns!

Between Arcturus, Spica, and Regulus, the brightest star in the constellation of Leo, lies a rather bland area of the sky that is peppered by hundreds of distant external galaxies that can be seen with modest telescopes from dark-sky locations.  Located at an average distance of about 50 million light-years, our Milky Way is a far-flung outlying member, slowly wheeling around three gigantic elliptical galaxies at the cluster’s center as the eons pass.

Our last bright star in the late evening sky is now climbing in the northeast.  Vega leads the small constellation of Lyra, the Harp, one of the signature constellations of summer.  The fifth-brightest star in the sky, it shines from a relatively nearby 25 light-years.  It has about twice the mass of the Sun, so it will burn through its nuclear fuel some 10 times faster that Old Sol.  Thanks to the slow wobble of the Earth’s axis known as precession, Vega was the “pole star” for the Northern Hemisphere about 12,000 years ago and will be again about 13,000 years from now.  It has long been used as a calibration standard for photo-sensitive instruments, and it is arguably the most-studied star after the Sun.

Ruddy Mars hangs tough in the early evening sky, best seen about an hour after sunset in the west.  This week the red planet moves through the stars of Gemini, and on the evening of May 31st he may be found less than a degree from the third-magnitude star Mebsuta, providing a nice binocular view. 

Jupiter is now just two weeks away from opposition, and you should be able to spot him low in the southeastern sky at the end of evening twilight.  Despite his far southerly placement in the sky, Old Jove remains the best planetary target in the sky for the small telescope.  He should be high enough by midnight to offer views of his cloud belts and four bright moons.  Wait for moments of steady air to get the best views.

Saturn now rises before 11:30 pm and should be high enough for telescopic perusal by 1:30 am.  You may still prefer to look for him before sunrise when you’ll find him just west of the meridian.

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