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

Orange clouds

Photo of the Week.Orange sunset from above the clouds.

Astronomy news for the week starting Friday, September 19, 2014.

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.


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 The Harp in Stellar Stories.

The Moon fades away this week in its waning crescent phase, new Moon passed the morning of Wednesday, September 24. The crescent goes five degrees south of Jupiter the morning of Saturday the 20th, the two making a fine pair. Then look for the rising crescent below the planet and to the right of Regulus in Leo the following morning, that of Sunday the 21st. Your last look at the ultrathin crescent will be the morning thereafter, that of Monday, the 22nd, when it lurks well below Leo's luminary. You'll not see our companion again this week, not until the early twilit evening of Friday the 26th as the new Skylights begins. The Moon goes through apogee, where it is farthest from Earth on its more or less elliptical orbit, on Saturday the 20th.

Saturn, still in Libra just to the east of Zubenelgenubi, sets ever earlier, now just half an hour or so after the end of evening twilight. Mars, though only slowly falling behind Earth in orbit, is not a lot better, going down only a bit over half an hour after Saturn. East of Saturn, the red planet is slowly approaching its namesake Antares in Scorpius (the two of similar color) and will pass only three degrees north of the star on Saturday the 27th. West of Saturn, Mercury makes a particularly poor appearance in twilight even though passing its greatest eastern elongation on Sunday the 21st. In even less of a show, Pluto ceases retrograde motion against the Milky Way in Sagittarius and begins to creep once again to the east. On the other side of the sky, bright Jupiter is up by 3 AM, while Venus sinks ever farther into morning twilight, becoming very hard to see.

The biggest event involves our own planet, as at 9:29 PM Central Daylight Time (10:29 EDT, 8:29, MDT, 7:29 PDT) on Monday the 22nd the Sun crosses the Autumnal Equinox in Virgo, and autumn begins in the northern hemisphere, (spring in the southern). Aside from details like the extended disk of the Sun and atmospheric refraction, on that date days and nights are of equal length, the Sun rises and sets due east and west, rises at the south pole and sets at the north pole. It's an event that can't be missed whether you look for it or not.

After dark we now see the upswing of the ecliptic as it passes through dim Capricornus and then Aquarius, the latter lying north of the stellar harbinger of fall, Fomalhaut in Piscis Austrinus, the Southern Fish. North of Scorpius and Mars find the giant pentagon than makes much of Ophiuchus, the Serpent Bearer, the two parts of the snake, Serpens Caput and Cauda (the head and tail), lying to either side. Above Ophiuchus is Hercules, the great Hero of ancient times, while to the east enter the constellations of the Andromeda myth.

STAR OF THE WEEK: OMI SCO (Omicron Scorpii). Fifth magnitude Omicron Scorpii (at 4.55 just over the border from fourth), in northern Scorpius, is just barely the westernmost star of the "Scorpion's Pentagon" that (going clockwise) includes third magnitude Sigma Scorpii (one of the "Arteries"), first magnitude famed Antares, obscure 22 Scorpii, and the outlier double star Rho Ophiuchi. Three of them (Rho, actually both "Rho's," 22, and Sig Sco) are hot, massive blue-white class B2 stars, while Antares is a cool M1.5 supergiant and Omi Sco (the second faintest) is a white class A (A5) "bright giant," a rare class that falls somewhere in between. Adding to the lot, Antares has a hot class B (B2.5) companion. Evolving, Antares and Omi Sco were once hot dwarfs, making them even more part of the small family. The Pentagon is within one of the most complex and beautiful parts of the Milky Way, a vast region filled with bright diffuse and reflection nebulae and dark interstellar clouds. Especially well-known is the Rho Oph dark cloud, a star-forming region that stretches to the east of the eponymous duplicitous star. Though second-faintest of the five, Omi Sco stands out as both the most distant, 880 light years (give or take 130), and thus not surprisingly as the most reddened and obscured by interstellar dust. Interstellar dimming becomes stronger with shorter wavelengths, which makes stars look "redder" than they actually are. Interstellar reddening (which can be measured by comparing the observed technically-defined color with that expected from the spectral class) is closely tied to the visual obscuration, which can then be found. Omicron Sco is dimmed by 2.27 magnitudes. Were the line of sight clear, the star would shine at a prominent second magnitude (2.28) and might even have a proper name.

From distance and a temperature of 8390 (which places nearly all the radiation in the visual band of the spectrum), we find a luminosity of 7045 times that of the Sun, consistent with the star's classification as a bright giant. From luminosity and temperature, the radius comes out to be 40 times that of the Sun, about half the size of Mercury's orbit. The rotation period (from an equatorial rotation speed of at least 23 kilometers per second) is under 86 days. There seems to be some sort of infrared excess, which implies a surrounding disk of some sort. The mass of Omicron Scorpii is close to eight times that of the Sun, which places it near the limit above which stars explode as supernovae. Once it sloughs away its outer layers it will more likely die as a massive white dwarf not far below the white dwarf mass limit of 1.4 solar masses, perhaps one made of neon and oxygen rather than of oxygen and carbon. Now just over 28 million years old, Omi Sco was born as yet another class B2 dwarf, or at least close, fitting it right into the other stars of the Pentagon, except of course for more massive Antares, which was born as a class O star. Omicrom Sco may become unstable as it cools and spend some time as a Cepheid variable, and will briefly also grace the sky as a red supergiant, though one not as grand that which dominates the Pentagon right now.

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