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

Sky and mountains

Photo of the Week.Sky above all....

Astronomy news for the week starting Friday, September 5, 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. 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 begins our week late in its waxing gibbous phase, rocks by full phase on Monday the 8th (about the time of Moonrise in North America), then the rest of the week dims through the early part of its waning gibbous phase. While the coincidence of perigee (when and where the Moon is closest to Earth, the night of Monday the 7th) is not as good as it was at the last full Moon, this "supermoon" (on the average 12 percent larger than an apogean Moon) is almost as good. There are no planetary passages of note, unless we count one well north of Neptune on Monday the 8th and another past Uranus on Wednesday the 10th. The latter is a bit of a curiosity, as the waning gibbous Moon will occult, or pass in front of, the planet as would be seen from the far north (it's not worth a trip to Siberia). With the Sun approaching the autumnal equinox in Virgo, this full Moon will be riding the southern ecliptic not far from the vernal equinox in Pisces, the constellation's dim stars mostly hidden by lunar brightness.

In the evening, though both are near setting by 10 PM Daylight Time, Saturn and Mars are still quite visible as twilight draws to a close. They are widening the gap between them, faster-moving Mars only slowly falling behind the Earth as both orbit the Sun. By the end of the week, the red planet will be roughly halfway between Saturn and Antares in Scorpius, the star slightly the fainter but of similar color to Mars (hence "Ant-Ares" after the Greek god of war). In the morning sky, Jupiter and Venus are separating as well. Though still bright, Venus is a challenge as it rises half an hour after the break of dawn. Jupiter on the other hand is shifting in the other direction, rising earlier, now just after 3:30 AM within the confines of dim Cancer, the planet quite unmistakable as it climbs above the eastern horizon.

The Moon will wash out most of the stars, only the brighter ones shining through. In the northwest, though, the faithful Big Dipper is still seen, all but one of its stars second magnitude. The stellar brightness scale was invented around 130 BC by the Greek astronomer Hipparchus of Nicea, who divided the naked eye stars into six categories, first magnitude the brightest, sixth the faintest. The system, though modified and set on a modern mathematical scale, is still in use today. There are but 22 first magnitude stars, which actually run into magnitude zero and -1 (Venus going to -5!). Each full magnitude is about 2.5 times brighter than the next fainter one. Oddly, the brightest and faintest of the first magnitude set, Sirius and Adhara, are in the same constellation, Canis Major, Orion's larger hunting dog. Both stars are coming onto the scene in late morning skies to the southeast of the Hunter, giving us a preview of winter.

STAR OF THE WEEK: EPS GRU (Epsilon Gruis), the last of the stars of third magnitude and brighter to be covered by the Star of the Week and included in the STARS website! Northern autumn is marked best by the constellations of the Andromeda myth as well as by the appearance of the 18th brightest star in the sky, Fomalhaut, which represents the mouth of Piscis Austrinus, the Southern Fish. To the south-southwest of Fomalhaut lies a set of stars that were in older times linked to the fish's tail, but around 1600 were separated away as Grus, the Crane. The figure was cemented in astronomical lore by its inclusion in Bayer's Uranometria. The luminary, Al Nair, closes in on first magnitude, while Epsilon at magnitude 3.49, fourth brightest star in the constellation, just barely gets over the line into third magnitude. Lying south-southeast of the main constellation (which really does look like a giant stalking bird) where it is barely noticed, Epsilon Gru is a white class A (A3) dwarf with a temperature of about 8600 Kelvin (the average of a fairly sizable spread). From temperature (needed to account for a bit of ultraviolet radiation) and a distance of 129 light years (give or take 2.5), we find a luminosity of 50.6 Suns and a radius of 3.2 times solar. The theory of stellar structure and evolution then yields a mass 2.4 times that of the Sun. The age is uncertain, but it seems to be more than halfway through its hydrogen-fusing lifetime of 600 million years (pretty short as compared to the solar lifetime of 10 BILLION years, showing the stark inverse effect of mass on stellar duration), after which the helium core will collapse, turning the star as a whole into a much more luminous red giant. Epsilon Gru's singular feature seems to be its high equatorial rotation speed, the observed values averaging 247 kilometers per second, which gives the star a rotation period of under 0.65 days. It's high enough to stir the surface gases so as to prevent separation of the elements, some falling under the effect of gravity, others lofted upward by radiation. The high speed will also make the star somewhat oblate, which then gives problems with temperature determination (the poles hotter, the equator cooler). Since we do not know the axial tilt, the speed may be even higher. But it does not look as if there is anybody there to admire the star, as there is no evidence for the debris disk that often accompanies planets, and thus there is no chance of life, which in any case would probably be precluded by the star's short lifetime. Neither are there any known stellar companions, the star seemingly quite alone.

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