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

Stormy Sky

Photo of the Week. Stormy skies will clear to reveal the stars.

Astronomy news for the two weeks starting Friday, May 8, 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 May 22, 2015.

We begin with a fat waning gibbous Moon that passes last quarter the morning of Monday, May 11, with the Moon high in the sky. The Moon thence runs through its waning crescent, which is terminated by new Moon on Sunday the 17th. Your last look at the ultra-thin crescent will be the morning of Saturday the 16th. As it fades, the Moon passes north of Neptune on Tuesday the 12th then as bit south of Uranus three days later. More significant, it goes through perigee, where it is 5.5 percent closer than average, the night of Thursday the 14th.

The waxing crescent will become readily visible in western twilight the evening of Tuesday the 19th, after which it will make some delightful visits. The evening of Wednesday the 20th, find it below brilliant Venus, which in turn will be below Castor and Pollux in Gemini. The following evening the crescent will be down and to the left of the planet, between it and the star Procyon in Canis Minor. One more evening takes the Moon well to the left of Venus and above Procyon, but now down and to the right of Jupiter, the planet lying above the Moon the night of Saturday the 23rd.

The bright light in the western evening sky, everyone's favorite UFO, is Venus. And it will keep on getting brighter until early July when it will also rendezvous with Jupiter, making an even scarier UFO as it did a few years back.

Not only is Venus getting brighter, it's setting about as late as it can, past 11:30 PM, almost two hours after the end of twilight. Well past the meridian as the sky darkens, Jupiter sets shortly after local midnight (1 AM Daylight Time), maintaining its position to the west of Leo and Regulus. About as Jupiter sets, Saturn transits low to the south just above the three-star head of Scorpius, Antares below. The ringed planet passes opposition to the Sun as our fortnight ends, on Friday the 22nd, when it will rise at sunset, cross to the south at local midnight, and set at sunrise.

Southwest of Jupiter find the roundish head of Hydra, the Water Serpent. The longest constellation in the sky, Hydra, plunging to the southeast, wraps a third of the way around the sky, its tail almost reaching to Libra. Hydra's modern cognate in the southern celestial hemisphere, Hydrus, the Water Snake (it does get confusing), is contained within 30 degrees of the South Celestial Pole south of bright Achernar at the end of Eridanus (the River), and peaks in the evening for those south of the tropics in December. In the early evening, northerners find the Big Dipper nearly overhead, orange Arcturus high to the southeast.

STAR OF THE WEEK: THETA HYA (Theta Hydrae), with a nod toward OMEGA HYA. Bright fourth magnitude (3.88), Theta Hydrae plays a prominent role as the first (and easternmost, though one might pick Omega) star in the long body of Hydra (the Water Serpent). Just seven or so degrees southeast of the fearsome animal's roundish head, 2.3 degrees north of the celestial equator, the star rather surprisingly carries no proper name. Its significance is not so much in the star itself, but in its history and companionship. A common hydrogen-fusing dwarf at the cool end of class B (B9.5), Theta Hydrae lies 113 light years away (give or take 6), which with a modest calculated addition of ultraviolet light from its 10,460 Kelvin surface yields a luminosity of only (for a class B star) 37 Suns. Luminosity and temperature then give a radius of 1.9 times solar. From a well-determined projected rotation speed of 89.5 kilometers per second, not particularly fast, Theta Hya must rotate in under a day. Theory gives a mass of 2.5 Suns, and shows the star to be quite young, just starting out along its 600 million year dwarf lifetime. At first Theta disappoints. It's listed in older literature as a rare Lambda Bootis star. Lambda Boo stars have weird metal-deficient chemical abundances. They are suspected of having accreted surrounding birth gases in which various chemical elements (e.g. calcium, titanium, nickel, and iron) have been depleted by attachment onto dust grains, the dust and the elements in question then wafted away by stellar winds. Alas, 'tis not so. Modern analyses don't reveal any particular Lambda Boo spectral signatures. But then there are the three companions, tenth magnitude Theta Hya B 20 seconds of arc away from A, thirteenth magnitude C at 101 seconds, and twelfth magnitude D at 82 seconds (all approximately as of this writing), making a veritable cluster. Alas alack, they are all moving much too fast relative to the primary star to be actual travelling companions, and thus they are all just line-of-sight coincidences.

So what's so special? There is a deficiency of close, hot white dwarfs attached to warmer, brighter stars. They are devilishly difficult to find. In the visual realm, the light of the primary overwhelms them. A number of class A stars have been found from space-based observations to be anomalously bright in the far ultraviolet, revealing invisible hot companions that could only be white dwarfs. Class B is more difficult, however, as these hotter stars have their own strong ultraviolet components. But the white dwarfs are slowly revealing themselves, Theta Hya among them, joining Regulus and 16 Draconis, in Theta's case the far ultraviolet suggesting a tiny white dwarf with a temperature between 25,000 and 31,000 Kelvin. Such pairings are important. Since higher mass stars die faster, the initial mass of the now-defunct white dwarf constrains the upper limit to white dwarf production and the lower limit to that of supernovae, not to mention additional information on the statistics of binary stars and their possible interactions among their companions. Maybe now Theta can have its proper name.

Oh yes, Omega. Omega Hydrae (we might say in the neck of the beast) is a fairly common, understudied class K2 giant-bright giant more recently classed a K2 supergiant. It stands out for its luminosity and mass. At a distance of 896 light years (give or take 76), it shines with the light of 982 Suns from a 4650 Kelvin surface, from which we calculate a radius 48.5 times that of the Sun, or nearly a quarter of an Astronomical Unit. It weighs in at 4.5 to 5 solar masses depending on whether it is cooking its core helium into carbon and oxygen or about to begin the process. Without sufficient mass to make a supernova, the star will die as a fairly heavy white dwarf.

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