SKYLIGHTS

Skylights featured on Astronomy Picture of the Day

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Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9

Sunset clouds

Photo of the Week. Peaceful sunset.


Astronomy news for the two weeks starting Friday, July 29, 2016.

Phone: (217) 333-8789
Prepared by Jim Kaler.

Clear skies and thanks to Skylights' blogger visitor reader.


Go to STARS for previous stars of the week. Last week's Skylights is still available. Access Skylights' Archive and photo gallery. From the Sun to the Stars: the OLLI Lectures provides a linked, illustrated introduction to astronomy.
The Constellations has a linked list with locations and brightest stars. Constellation Maps show the locations of the constellations. The 170 Brightest Stars lists them through magnitude 3.00. For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way. Watch a total eclipse of the Moon and an annular eclipse of the Sun. Moon Light presents scenic photos of the Moon. Go to MoonScapes for labelled telescopic images of the Moon and other lunar information.
See the Moon move and pass just below Nu Virginis. Watch planets move against the background stars. See a classic proof of the curvature of the Earth with a "hull down" series. Visit Measuring the Sky to learn about the celestial sphere.
Admire sunsets, rainbows, and other sky phenomena in Sunlight. Read the illustrated Day Into Night on the phenomena of the sky See the The Aurora and the Midnight Sun. See and understand the ocean tides.
Enjoy Our Complex Universe: A Human Understanding through Art, with 12 illustrations. Advances in Astronomy, 1989-2011. Take a ride aboard Asteroid 17851 Kaler (1998 JK). Look for Books about the sky and stars.

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ASPSupport science literacy by joining the Astronomical Society of the Pacific, an international organization that is among the world's premier providers of astro education. Get Mercury and a variety of other benefits.


Presenting three audio courses with 70 to 100-page study guides, narrated and written by Jim Kaler.
Heavens Above: Stars, Constellations, and the Sky from Recorded Books. Astronomy: Earth, Sky, and Planets, is available from Recorded Books. Astronomy: Stars, Galaxies, and the Universe, is also now available from Recorded Books.
Astronomy: Earth, Sky, and Planets is published as Vault of the Heavens: Exploring the Solar System's Place in the Universe by Barnes and Noble.

Enjoy Our Complex Universe:A Human Understanding through Art, with 12 illustrations.

Read "Heaven's Touch: From Killer Stars to Seeds of Life, How We Are Connected to the Universe," Princeton University Press, now in Chinese translation.

SSTo learn about stellar spectra, read STARS AND THEIR SPECTRA: An Introduction to the Spectral Sequence, Second Ed., with two new chapters and 140 new illustrations, Cambridge University Press (UK or North America), 2011.


Read From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

SSNEWEST! FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. Read the interview with Jim Kaler.


NEW! Read Dust to Dust in Stellar Stories.



The next skylights will appear August 12.

We begin with a wisp of a waning crescent Moon, which terminates at new Moon on Tuesday, August 2. Your last view of it will be in eastern twilight the morning of Monday the 1st. We then turn in the other direction to see the waxing crescent, which will appear the twilit evening of Thursday the 4th immediately to the left of Mercury, with Venus a bit farther down and much farther to the right, the bright planet hard to see. The following evening, that of Wednesday the 5th, will be better, with the Moon just to the west of Jupiter. The Moon will occult, or cover, the planet after the two have set. Look then the next evening to see the Moon well up and to the left of the giant planet. After passing first quarter on Wednesday the 10th, it enters the waxing gibbous phase as it heads towards full on August 18. The evening of Thursday the 11th, the Moon will appear directly above Mars, while to the right Saturn will be just above the star Antares, the four making a nifty quadrangle. The Moon passes apogee, where it is farthest from Earth, on Tuesday the 9th, less than a day before first quarter.

After giving us a lovely show over the past few months, Jupiter is disappearing from the nighttime sky, now setting before evening twilight draws to a close. It'll be back in the east in October. And as augured above, Mercury is making a bit of an appearance in western twilight. So is Venus. Climbing only slowly from night to night, Venus does not get into a dark sky until mid-October. But we still have Mars and Venus. With Mars moving to the east against the stellar background, and Saturn (just to the east and north of Mars) very slowly still moving
retrograde (to the west), the two are on course to pass each other. But before then the chase will be on, as Saturn ceases retrograde about as our fortnight ends,and slowly reverses to move easterly in direct mode. But it's no match for speedy Mars, which will catch it on August 25. All this action plays out against the bright stars of Scorpius, in particular Antares, which is almost due south of the ringed planet. Mars is to the west of bright second magnitude Delta Scorpii (the middle star in the trio that makes the Scorpion's head) and will pass just south of it on Tuesday the 9th. Not to be completely outdone, Uranus ceases retrograde on Friday, July 29.

The best, however, is yet to come. August is the time for the
Perseid meteor shower, which appears to emanate from the constellation Perseus. While lasting over several nights, the meteor display (from the debris of Comet Swift-Tuttle, which last came by in 1992) peaks the morning of Friday the 12th with the Moon well out of the way. The shower is predicted to be better than the normal average of a meteor per minute in a dark sky.

It's a good time to look at the Dippers. In early evening the
Big Dipper of Ursa Major falls into the northwest while the Little Dipper of Ursa Minor stands on its handle, Kochab (Beta Ursae Minoris) above Polaris. Between the Dippers wags the tail of Draco the Dragon, while halfway between Kochab and the Big Dipper's end find Thuban, which (as a result of precession, the 26,000-year wobble of the Earth's axis) was the pole star during the time of ancient Egypt.

STAR OF THE WEEK: EPS DRA (Epsilon Draconis). Well, this one is another fine mess, which is a bit surprising given Epsilon Dra's status of nearly third magnitude (3.83, bright fourth) and its position in the main figure of
Draco (the Dragon) four degrees northeast of third magnitude Delta Dra. The name that might be applied to it, Al Tinnen (referring to a Serpent), is properly given to Delta but mistakenly as Altais, which refers to a goat. From Allen the name actually relates to the quartet of Delta, Pi, Rho, and Epsilon. It's best just to use the Greek letter. From the 19th century, Smyth and Chambers call Eps Dra "a fine double...A 5 1/2 light yellow, B 9 1/2 pale blue." While the subjective color of Epsilon Dra's brighter fourth magnitude component (4.01) class G (G7) component may not be far from the mark, that of seventh magnitude (6.71) Epsilon B's color is certainly well off it, the "blueness" a contrast effect, as may be the early estimated faintness. Both stars have been called giants, Eps Dra A a G7 giant, Eps B a K5 giant, such giant pairings somewhat unusual. Eps A's characteristics are pretty well defined. With a distance of 148 light years (give or take just 3) and a temperature of 5000 Kelvin (consistent with its class), which requires the addition of a bit of infrared radiation, "A" shines with the light of 60 Suns, which leads to a radius of 10 times solar. Theory shows that the star is most likely a helium-fusing (to carbon and oxygen) "clump" giant (as on as graph of luminosity vs. temperature such stars gang together) with a msss of about 2.5 times that of the Sun (though it could just be starting to brighten with a dead helium core; it's hard to tell) and an age of some 600 million years. But then there is Eps Dra B. Not only is the subjective color and early magnitude way off, but even modern observers are wildly divergent. While "B" was earlier ranked as a K5 giant, a later look suggested it to be an F5 giant, very different. If we adopt the former, then the temperature should be around 4100 Kelvin and the resulting luminosity ought to be 8 Suns, which cannot be matched by theory. If F5, the temperature should be 6400 K, which works, and which yields a luminosity of 3.4 Suns, a radius of 1.5 times solar and a mass of 1.3 Suns, and shows the star to be not a giant but instead a bit of an elderly dwarf. Eps Dra A is also classed as a metal-poor giant with weakened cyanogen (CN) in its spectrum, consistent with a lowered iron content relative to hydrogen, which runs around 45 percent solar. While we can make out some orbital motion, it's not enough to be able to fit an ellipse and to derive the orbital parameters. The first measurement of separation, 2.5 seconds of arc, was made by none other than William Herschel (the discoverer of orbiting doubles) in 1780. By 2012, the separation had increased to 3.2 seconds. The motion against the background in 129 years, however, is 16 seconds of arc, so the stars are surely true companions. They are now at least 145 Astronomical Units apart (the foreshortening not known) and from Kepler's third law and a summed mass of 3.8 times solar must take at least 900 years to orbit each other. They make a pretty pair, one worth a look. Maybe someday someone will actually make a study of them.


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