SKYLIGHTS

Skylights featured on Astronomy Picture of the Day

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

Photo of the Week. Clearing clouds reveal the waxing crescent Moon and a lonely star.

Astronomy news for the week starting Friday, August 22, 2008.

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.

Support 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.

NEW! Heavens Above: Stars, Constellations, and the Sky from Recorded Books.

NEW FOR SUMMER! Take a tour of the Milky Way as seen from the northern hemisphere.

Hear and watch Jim Kaler, "From Pluto to Planets: What Other Stars are Telling us," on World AstroCast 2008, Saturday, August 30, 2 PM CDT (19h GMT) at http://www.ustream.tv/channel/world-astrocast-astronomy, brought to you by the Astronomy Section of the Northampton Natural History Society, England.

The Moon fades away this week. Beginning in its late waning gibbous phase, it passes third quarter the night of Saturday, August 23, then spends the remainder of our period as a waning crescent, new Moon to be passed on Saturday the 30th. With a clear eastern horizon, you might be able to admire the last glimpse of the narrow crescent the morning of Friday the 29th. Two days after third quarter, on Monday the 25th, the Moon passes perigee, where it is a bit over five percent closer to the Earth than average. As the crescent wanes, look for Earthlight on the nighttime side of the Moon.

The Moon passes no planets this week, in part because so many are ganged together over in western twilight, unfortunately pretty much out of sight. Saturn sets just after sunset, followed by Mercury (which is making a poor western appearance), then Venus and Mars, all going down in twilight. The only one with a hope of being seen is Venus, and only because of its great brilliance. Neptune is up in the east at the end of twilight, followed by Uranus as twilight ends, but these require a knowledge of exactly where to look as well as optical aid.

That leaves us once again with Jupiter, which now dominates the evening sky within the confines of northern Sagittarius. The giant of the Solar System (11 times the diameter of Earth) can now be seen crossing the meridian to the south around 9:30 PM Daylight. You can admire it for several more hours until it sets just after 2 AM, when Uranus transits the meridian and Betelgeuse in Orion rises. Jupiter is in a fine stellar setting a couple degrees due north of the bright star Nunki (Sigma Sagittarii) in the bowl of Sagittarius's Little Milk Dipper.

Several first (and zero) magnitude stars dot the evening sky, the set dominated by the Summer Triangle of Vega in Lyra (nearly overhead in early evening), Deneb in Cygnus (east of Vega), and Altair in Aquila (to the south). To the far northwest find orange Arcturus in Bootes (the brightest star of the northern hemisphere), while to the far south you can admire Antares in Scorpius.

STAR OF THE WEEK: XI BOO (Xi Bootis). Rare is the naked-eye star that has a luminosity and mass less than that of the Sun. Fifth magnitude (at 4.6 just barely) Xi Bootis offers not just one of them, but two, presenting itself to us as a fine, easily-viewed double star, the two components lying anywhere from 2.5 to 7 seconds of arc apart, depending on where they are on their 152-year mutual orbit.

Xi Bootis B goes around Xi Boo A (the brighter and more massive of the two, placed at the cross) in the clockwise direction with an orbital period of 151.6 years at an average separation of 33.5 Astronomical Units. In reality, both go around each other. North is down, and the scale around the edges is in seconds of arc. The dot-dash line is the orbit's major axis. The orbital plane is tilted to the line of sight by 41 degrees, so that Xi Boo A does not appear at the focus of the orbit (where it actually is). Binaries like this one are critical in the determination of stellar masses. (From W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, U. S, Naval Observatory.)

Though not within Bootes' classical outline, the star is easily found about nine degrees due east of Arcturus. Both members of the binary are hydrogen-fusing dwarfs. The brighter (Xi Boo A) is a fifth magnitude (4.76) class G8 star with a temperature of 5550 Kelvin, while the fainter (Xi B) is a seventh magnitude (6.78) cooler (4600 Kelvin) class K4 star. Contrast effects make them more colorful than they really are, Xi Boo A given as yellow, B as "reddish- violet." Smythe called them orange and purple. Were it not for their proximity to us, the pair would never have made naked-eye status. One of the closest star-systems to Earth, it lies a mere 22.1 light years away, and was only recently bumped from the closest-100 star-system list by discoveries of closer very faint red dwarfs. Xi Boo A and B take just over a century and a half to orbit about each other at a mean separation of 33.5 Astronomical Units, just a bit farther than Neptune is from the Sun. A rather high eccentricity takes them between 50.5 and 16.4 AU apart. They were last closest together in 1909, and will not be again until 2054. Application of Kepler's laws give a combined mass of just 1.6 times that of the Sun, the individual masses coming in at 0.9 and 0.7 solar. Distance and temperature give respective luminosities of 0.5 and 0.1 times that of the Sun, Xi Boo B being underluminous for its mass. Radii then come in at 0.75 and 0.5 solar. Both are, like the Sun, magnetically active, the effect dominated by Xi Boo A, which seems to have an activity period of two to four years with an outer corona estimated to be at a temperature of more than 10 million Kelvin. A longer cycle may take 25-30 years. Other variations give a rotation period of 6.4 days, a quarter that of the Sun. Too bad the activity is not being seen up close. A low metal content, three quarters that of the Sun, coupled with the eccentric orbit, would seem to preclude any planets (planet-holding stars tending to be metal-rich), and indeed, none has ever been found or even indicated. An old study suggested the gravitational effect of a third low-mass companion, but it has never been confirmed. Several other fainter stars hover nearby, but these are all just in the line of sight.

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

Copyright © James B. Kaler, all rights reserved. The written contents and (unless otherwise specified) the photograph are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes.