SKYLIGHTS

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

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Photo of the Week.Coastal trees honor the dawn.


Astronomy news for the week starting Friday, April 10, 2009.

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

Clear skies and thanks to Skylights' * reader.


Go to STARS for previous stars of the week. Last week's Skylights is still available. Access Skylights' Archive and photo gallery. Find out what happened in astronomy at Astronomy Updates.
The Constellations has a linked list with locations and brightest stars. Constellation Maps show the locations of the constellations. The 151 Brightest Stars lists through magnitude 2.90. For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way as seen from the northern hemisphere. Watch a total eclipse of the Moon and an annular eclipse of the Sun. Moon Light presents photos of the Moon. 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.
Go from Day Into Night, with 83 linked illustrations. See the The Aurora and the Midnight Sun. 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.

View Jim Kaler's "From Pluto to Planets: What Other Stars are Telling us" at the World AstroCast 2008 archive, brought to you by the Astronomy Section of the Northampton Natural History Society, England.

Having just passed its full phase on Thursday, April 9, our Moon now wanes in its gibbous phase during the entire week until it hits third quarter the morning of Friday the 17th, shortly after sunrise in North America. We thus have the opportunity to see the morning daylight Moon to the south and southwest near its perfect last quarter. The morning of Monday the 13th, the Moon will be just to the west of Antares in Scorpius, and will pass just to the south of the star after sunrise. Far enough to the west, in Hawaii, the Moon will actually occult the star. By the following morning, the Moon will have planted itself just between the classical outlines of Scorpius and Sagittarius. Just before third quarter, on Thursday the 16th, our neighbor passes apogee , where it is farthest from Earth.

Both evening and morning hold planetary treasures. By sundown, Saturn is well up in the southeast, as it holds its place in southeastern Leo to the east of Regulus, the planet in slow retrograde (westerly) motion against the stars. Nearly, but not quite, of magnitude zero, the ringed planet is about as bright as Orion's Betelgeuse, though of a different color. Look for it to the south around 11 PM as it crosses the celestial meridian.

Rising around 4 AM, about an hour before dawn, Jupiter is now eminently visible in the southeast as twilight lights the sky. But now we also have far brighter Venus making the morning scene. Ceasing retrograde on Wednesday the 15th, our sister planet rises prominently just after the beginning of twilight. In the coming months, indeed until late fall, it will blaze away for those who rise early enough to see it. In lesser planetary news, Mars rather invisibly passes conjunction with Uranus on Tuesday the 14th.

Look in early evening to see that great symbol of the sky, the Big Dipper, high in the northeast as it moves to cross the meridian nearly overhead around midnight. Look then just to the south of it to find the pair of stars that represent the modern constellation Canes Venatici (the Hunting Dogs) and then a bit more to see the delicate sprawl of stars of the Coma Berenices cluster. As the Dipper climbs for those in the northern hemisphere, so does the famed Southern Cross climb for those in the cognate latitudes of the southern hemisphere.

STAR OF THE WEEK: EPS CRU (Epsilon Crucis). Here is one of the most viewed of all stars, while at the same time being one of the most obscure. How can that be? At fourth magnitude (3.59, just fainter than third), Epsilon Crucis has a prominent place as the "fifth star" of one of the most famed of all constellations, Crux, the Southern Cross, lying almost on a line between brighter Delta Crucis (the Cross's most northerly star) and first magnitude Acrux (Alpha Crucis). The Southern Cross, an icon of the southern hemisphere, appears prominently on the flags of several nations, states, and territories, notably those of Australia, New Zealand, and Brazil (as well as on the banners of Papua New Guinea, Samoa, and some others). Epsilon takes its place among the brighter quartet on all of them except for the flag of New Zealand, where it is curiously absent. So anytime anyone in these five-star lands salutes or admires their national symbol, they at the same time must view our Eps Cru. With the exception of one flag, we see the star placed as it is viewed in the sky. On Brazil's flag, however, the Cross (with Epsilon) is reversed, following an ancient tradition where the stars appear as they do on a celestial globe, as in a God's-eye view from outside the sky, and as the stars also do on the ceiling of Grand Central Station in New York City. That said, nobody else seems to pay much attention to this otherwise conspicuous star. Certainly not the astronomers. It's hardly examined at all. Too bad, since while appearing as just one more class K (K3.5) giant, the star does have differences to it. At a distance of 228 light years, Epsilon Crucis shines with a luminosity of 330 Suns from a coolish 4150 Kelvin surface. A significant giant, the star has swelled to a radius 35 times that of the Sun, nearly half the size of the orbit of Mercury. Luminosity, temperature, and theory then tell of a star with a mass 1.7 times that of the Sun and reveal that the star is not just sitting there quietly fusing helium into carbon and oxygen, but is in a state of flux, most likely still brightening and swelling with a dead helium core, though it may have passed helium ignition and be fading -- it's impossible to tell. (One source suggests a lower mass of 1.4 Suns). With a mass of 1.7, the star began life some two billion years ago as a class A dwarf. The metal content is normal, about 80 percent solar. As a star in evolutionary transit, Eps Cru is also an irregular variable that changes erratically (or so we surmise, since no variation period is known) between magnitude 3.4 and 4.0. In spite of its variable nature, the star does, however, serve as a calibrator for other studies. It seems though that it will remain best known as it waves in the breezes of southern skies. (Thanks to Steven Raine, who suggested this star, and to Andre Bordeleau of the Montreal Planetarium, who researched the flags and wrote about them in the October 2008 issue of the "Planetarian," from which the above discussion was taken.)

For more on the sky, visit the Earth and Sky Skywatching and General Science pages.
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