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

lightning

Photo of the Week. A powerful hidden electrical discharge lights the clouds of a thunderhead 60 miles away, See full resolution.


Astronomy news for the two weeks starting Friday, September 23, 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.


SSLive in town? Read FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. See the interview with Jim Kaler.


NEW! Read Dust to Dust in Stellar Stories.

COMING IN OCTOBER!, From the Sun to the Stars by Jim Kaler, World Scientific 2016, a new book based on From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.




The next skylights will appear October 7, 2016.

The Moon begins in its third quarter the morning of Friday, September 23, and after slimming as a waning crescent it passes new at the end of the month, then grows through its waxing crescent phase, which does not end until first quarter on the night of Saturday, October 8. You can see a marvelous pairing of the thin waning crescent with Mercury in eastern twilight the morning of Thursday the 29th, the planet a bit to the left. On the other side of the sky, the waxing crescent will make a similar, though not as close, a pair with Venus the evening of Monday, oxrober 3, the bright planet down and to the left of the Moon. The following evening finds the growing crescent between Venus and Saturn, and then on the evening of Wednesday the 5th the Moon will be directly to the right of the ringed planet, with the star Antares below. The Moon then heads towards Mars, which it will pass as our next fortnight begins. The Moon goes through apogee, where it is farthest from the Earth, on Thursday the 4th.

While Venus, Saturn, and Mars make a lovely progression up and to the left in the southwestern evening sky, after a long and beautiful run through southern Leo, Jupiter finally goes through conjunction with the Sun on Monday the 26th. We'll soon see it in the eastern morning sky. Mercury reaches its greatest western elongation on Wednesday the 28th, when it rises just as morning twilight begins, while at the far reaches of the Solar System, Pluto ceases retrograde motion against the background stars of Sagittarius and begins moving slowly eastward. Once Mars sets around 11 PM Daylight Time, the sky is free of ancient planets until dawn.

As the Big Dipper descends into the northwest, the Little Dipper stands high on its handle balanced on the North Star, Polaris. Between the two lies the tail of Draco, the Dragon, which, after winding around the Little Dipper, looks to the south with its fearsome head examining the region between Hercules and Vega in Lyra, rather oddly in the direction in which the Sun is moving through the swarm of local stars.

STAR OF THE WEEK: PSI DRA (Psi = Psi-1 = 31 Draconis), with an excursion to Psi-2 = 34 Draconis). In the nineteenth century, Smyth and Chambers wrote of fourth magnitude (4.23) Psi Dra in Draco: "A neat double star on the Dragon's back...easily identified, being on the same parallel as Gamma Ursae Minoris...A 5 1/2, and B 6, both pearly white." That is just the beginning of a remarkable tale. First, we need deal with the names. Psi Dra is commonly called Psi-1 in conjunction with otherwise unrelated fifth magnitude (5.43) Psi-2 Dra a degree to the east. Unfortunately, the two components of Psi-1 (Psi-1 A and B) have also been respectively labeled Psi-1 and Psi-2. To avoid confusion, "Psi-1" is thus often dropped and the binary called just "Psi Dra" or by its Flamsteed number 31 Dra, distant Psi-2 being 34 Draconis. We'll get to 34 Dra later. Hipparcos gives a distance of 74.9 light years (plus or minus 0.4) to Psi-1 = Psi, while more recent analysis yields 72.4 l-y. It does not make much difference in the final results. Psi A and B are clearly drifting through space together at a speed of about 30 kilometers per second relative to the Sun, and have maintained nearly constant separation, now 30.2 seconds of arc, over the past two centuries, so there is no doubt as to their binary nature. Farther out are 11th magnitude Psi-1 C (78 seconds), 13th magnitude D (101), and 15th magnitude E (69). "D" has been a constant companion for a century, so it might belong to the system. For now focus on Psi A and B. Psi A is a class F (F5) subgiant- dwarf with a temperature of 6340, a luminosity of about 6 Suns, and a mass of 1.43 times solar. It hosts a dim spectroscopic red dwarf companion of perhaps half a solar mass in an 18-year eccentric orbit at an average distance of 8.7 Astronomical Units (called Psi C, but not the same "Psi C" as above, so we need watch the context). Of much greater interest is Psi B, a rather sunlike class G (G0) star with a temperature of 6200 K, a luminosity double the Sun, and a mass of 1.19 Suns. Its velocity variations show the presence of not one, but TWO orbiting planets. The primary planet, Psi Bb, has a minimum mass of 1.53 times that of Jupiter and orbits with an average separation from Psi-B of 4.43 AU with a period of 8.53 years. More subtle variations in velocity point to another Jupiter-like planet averaging 6.8 AU out. In summary, Psi-1 Dra is a hierarchical triple, with a low mass star orbiting Psi-1 A and a pair of massive planets in a solar-system-like orbit going about the lesser star Psi-1 B, making it look rather like 61 Cygni. Separated by at least 700 Astronomical Units, Psi B with its planets and Psi A with its red dwarf take at least 10,000 years to orbit each other. Is there an earth going about Psi-B as well? If so, Psi-A would glow in its sky with a brightness at least six times that of our full Moon. What about old Psi-2 = 34 Dra? The fifth magnitude (5.43) class F2-3 bright giant stands out quite nicely by itself. At a distance of 1055 light years (give or take 68), it is quite unrelated to Psi-1. With a temperature of 6530 Kelvin, it radiates at a rate of 560 Suns and has a radius of 18.5 solar radii. Its projected equatorial rotation speed of 48 kilometers per second gives a rotation period under 20 days. Application of theory yields a substantial mass of 4.2 Suns, an age of about 20 million years, and shows the star to have a dead helium core in transition to becoming a true red giant, whereupon the core will burn to a mixture of carbon and oxygen. After sloughing off its outer layers, the C/O core will be revealed as a white dwarf with a mass about 80 percent that of the Sun. From its vantage point, Psi-1 would be an 11th magnitude dim bulb that would merit little consideration. (Most of the discussion of Psi-1 Dra is from M. Endl, E. J. Brugamyer, W. D. Cochran et al.in The Astrophysical Journal, 818:34, 2016 February 10, with thanks; thanks also to Jerry Diekmann for suggesting the star system.)


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