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The Sky This Week, 2017 August 1 - 8

Stars of the Summer Triangle
NGC 7000, the "North America Nebula" in Cygnus,
Imaged on 2011 August 8 from Fishers Island, NY

The Moon waxes in the southern half of the sky this week, skirting the southern horizon as she travels through the bright star clouds of the summer Milky Way before trekking across the sparser star fields of the rising autumn constellations. Full Moon occurs on the 7th at 2:11 pam Eastern Daylight Time. August’s Full Moon is variously known as the Corn Moon, Grain Moon, and Sturgeon Moon and marks the beginning of harvest seasons for far northern climes. This year people in regions of the western Pacific Ocean, Indian Ocean, Australia, and most of Asia will see a partial lunar eclipse at the time of Full Moon. At maximum eclipse about 25 percent of the Moon’s southern limb will pass through the umbral shadow of the Earth. This will be a fitting prelude to the total solar eclipse that will cross the United States two weeks later. Luna forms an attractive triangle with the ruddy star Antares and the planet Saturn on the evening of the 1st. On the following night she may be found just northwest of Saturn.

The Moon’s bright glare continues to wash out the soft glow of the Milky Way’s star clouds, but there are still plenty of interesting objects to ponder in the summer sky. Let’s start with the most obvious, the three stars that form the Summer Triangle. Vega, Deneb, and Altair are each the brightest stars in their respective constellations of Lyra, Cygnus, and Aquila. At first glance they seem about the same in appearance, with blue-white tints and about the same apparent brightness. Vega, the brightest of the trio, is located some 25 light-years from the solar system and for many years was used as a "standard" star for calibrating various types of astronomical sensors. Recent study of the star reveals that it is anything but "standard", however. Spectroscopic studies have revealed that it rotates very rapidly, spinning once in just over half a day. By contrast the Sun rotates once every 27 days. Vega also seems to have an extensive disc of dust surrounding it, which makes it an anomalous emitter of infra-red radiation. Altair, southernmost and second-brightest of the trio, is a mere 16 light-years away. It is also a fast spinner, turning once in just under 9 hours! If we could see the discs of these stars, they would be much flattened at their poles thanks to their speedy rotation. The final star in the trio, Deneb, has a similar naked-eye appearance to its companions, but here the similarities end. Where Vega and Altair are considered to be "normal" stars in terms of luminosity and mass, Deneb is considerably different. Its distance is over 100 times that of Altair. Light that we see today from Deneb left its surface at about the time of the fall of the Roman Empire, which means its luminosity is well over 100,000 times that of the Sun! If Deneb were to switch places with Altair our concept of "night" would be very different from today, as it would shine with the apparent brightness of the Moon at First Quarter. It is far and away the most luminous of the 30 brightest stars in the sky, and its distance exceeds the next most-luminous star in the top 30 by a factor of two. It is fortunate that it is so far away; when it "dies" it will go out as a supernova, and if it were only 16 light years away the gamma rays from that event would sterilize the surface of the Earth.

Much closer to home we find bright Jupiter lingering in the southwestern sky as dusk settles in the early evening. He’s still a worthwhile target for the small telescope as soon as he becomes visible, and you should be able to follow him for an hour or so before he drops into the turbulent air above the horizon. He now sets at around 11:00 pm.

Saturn takes over where Jupiter leaves off. The ringed planet is near the meridian at 9:30 pm, and he is well-placed for viewing until the first hours of the early morning. A small telescope should easily show his widely-tipped rings and his brightest moon, Titan. Larger instruments will reveal more of his fainter moons. Watch the website for the NASA Cassini probe over the next few weeks. It will execute a couple of dazzling dashes between the planet and the rings before it takes a suicide plunge into Saturn’s atmosphere next month.

Venus greets early risers in the northeastern sky as morning twilight gathers. She is now coursing her way through the stars of Gemini as she continues her fast-paced circuit of the sky. She will reach her most northerly declination for the year on the morning of the 6th.

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