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

Floating Clouds

Photo of the Week. A flotilla of fair clouds floats across the sky.

Astronomy news for the two weeks starting Friday, September 11, 2015.

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. 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.


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.

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 Bearly North in Stellar Stories.

The next skylights will appear September 25, 2015.

The Moon begins the fortnight very late in its waning crescent phase and, as we open the show, is effectively invisible as it passes new on Saturday, September 13, when it eclipses the Sun. But don't get too excited about it, as the eclipse is partial and visible only from southern Africa, Antarctica, and oceanic points between. The Moon then emerges for us in western dusk as a narrow crescent the evening of Monday the 14th. The night of Friday the 18th finds the thickening crescent passing just three degrees north of Saturn, while the following night the Moon glides above Antares in Scorpius with Saturn now to the west. The crescent phase ends at first quarter the morning of Monday the 21st shortly after moonset in North America, after which it fattens in the waxing gibbous. As the Moon passes full the evening of Sunday the 27th (during our next session) we will witness a spectacular total lunar eclipse that's visible throughout the Americas with the Moon also near perigee, where it is closest to Earth and appears largest in the sky (though not by much). So prepare now for this most memorable Harvest Moon, the one nearest the autumnal equinox. The Moon goes through apogee, where it is farthest from Earth, on Monday the 14th.

Of equal interest, the Sun passes the autumnal equinox (where the ecliptic intersects the celestial equator, the Sun moving south) the morning of Wednesday the 23rd at 3:21 AM CDT shortly before dawn. On that day (excluding the effects of atmospheric refraction, solar diameter, and the exact timing of equinoctial passage) the Sun will rise due east, set due west, be up for 12 hours, and down for 12, hence the term "equinox." The Sun will also formally set at the north pole and rise at the south pole, all in all quite the event as the northern hemisphere moves from summer into fall.

Earth aside, the other planets are no less intriguing. Venus dominates the morning sky. Rising well before dawn, our "sister planet" (not much smaller than Earth) reaches greatest brilliancy on Monday the 21st just after the Moon passes third quarter. Rising not long before the start of morning twilight, Mars lags behind Venus and is not terribly bright, just second magnitude. The red planet passes less than a degree north of Regulus in Leo on Thursday the 24th. Following these two, Jupiter rises as the morning sky lightens. Back in the evening, Saturn, to the northwest of Antares goes down by 10 PM Daylight Time, so look early and let the crescent Moon be your guide (see above).

While the summer constellations will be with us for some time, the stars of autumn are on their way. Cygnus, the Swan, with bright Deneb (the Swan's head) at its northern end, rides high in early evening, the Milky Way running down the middle. It's followed to the east by Pegasus (Perseus's Flying Horse), which is recognizable by its Great Square. Off the northeastern corner run the star streams that in part represent Andromeda, who was rescued by the great hero.

STARS OF THE WEEK: KAPPA CYG, IOTA CYG, AND, WHY NOT, IOTA-1 CYG (Kappa, Iota, and Iota-1 Cygni, a three-for-one-special.) Separated by 2.7 degrees, Kappa and Iota Cygni lie in northwest Cygnus (the Swan) near the border with Draco, Kappa actually Flamsteed 1, Iota 10 Cygni. Look somewhat under 10 degrees north-northwest of Delta Cyg. They are a curious pair of almost identical fourth magnitude brightness (respectively 3.77 and 3.79). Just under a degree northwest of Iota is sixth magnitude (5.75) 7 Cygni, which also goes by the name Iota-1, plain Iota then being Iota-2. The name is rarely used, however, Iota-2 usually called just "Iota." Star names do get messy. Though Kappa and Iota are very different beasts -- Kappa is a class G (G9) giant, Iota a class A (A5) main-sequence dwarf -- they are even more curiously at almost identical distances: Kappa is 124.2 light years away (give or take 0.5), while Iota is 122.3 light years (with a formal error of just 0.3) from us. Factoring in a fair bit of infrared radiation from a surface with a well-defined temperature of 4990 Kelvin, Kappa Cyg shines with a luminosity of 47.4 times that of the Sun, which leads to a radius of 9.2 times solar, small for a giant. An interferometer measurement of angular diameter gives 8.5 solar radii, nine percent smaller (with no idea why except that giants can have fuzzy edges). Theory yields a mass 2.5 times that of the Sun and an age of around 600 million years. On the other hand, Iota's temperature is not well defined at all, two listings ranging from 7300 to 8260 Kelvin with a mean of 7880 Kelvin. Iota's luminosity of 32.4 Suns (nearly all of which is in the visual spectral domain) gives us a radius of 3.2 times solar and a somewhat lower mass of 2.2 Suns. Kappa is an ordinary core-helium-fusing giant, while Iota is at near the end of its hydrogen-fusing life of around a billion years and (the mass difference aside) will before long turn into a giant rather like Kappa, the two nicely illustrating how one kind of star transforms itself into another. As are so many stars in its class, Iota is a rapid spinner with a projected equatorial velocity of 229 kilometers per second, resulting in a rotation period of under 16.6 hours. Rapid rotation results in a distorted star with a notable equatorial bulge, which may be the cause of the temperature problem, as stars are cooler at their equators than they are at their poles (an effect called "gravity darkening"). Kappa and Iota Cyg even have rather similar motions. Each is coming toward us at respective speeds of 29 and 20 kilometers per second, and each is speeding across the line of sight in sort of the same direction at 25 and 23 km/s. Separated by 6.2 light years they are clearly not gravitationally bound, yet their similarity of motion suggests a common origin in the same, now long-gone, birth cloud.

Are there others? Certainly not Iota-1 (woops, 7) Cygni, as it's 351 light years away (with an uncertainty of 9), three times farther than Kappa and Iota. A class A1 (nearly "colorless") dwarf with a temperature of 8830 Kelvin, 7 Cyg radiates at a rate of 48 Suns, yielding a radius of 3.0 times solar and a mass of 2.4 Suns, the star in the latter half of its hydrogen-fusing lifetime of 630 million years. With a lower projected equatorial rotation speed of 52 km/s (which could of course just mean that the star's rotation axis is significantly tilted to the line of sight), Iota-1 could take as long as 2.9 days to make a full rotation. None of the three stars has any trace of a companion, all seemingly single. Whether there are planets remains unknown. Iota does not even seem to have an infrared-radiating debris ring, while Iota-1 Cyg has not been examined. If someone were on either Kappa or Iota, each would shine at magnitude -2.7 in the other's sky, three times the brightness of Sirius in ours and roughly similar to that of Jupiter.

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