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. The near-full Moon watches over the rising shadow of the Earth at sunset.

Astronomy news for the week starting Friday, May 9, 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.	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.
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.

Attend the 120th annual meeting of the ASP, May 31 to June 3, 2008 in St. Louis, MO, cosponsored with, and part of, the Summer Meeting of the American Astronomical Society.

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

The Moon waxes from crescent through first quarter this week, the quarter passed on Sunday, May 11, with the Moon going down the western sky, after which it fattens through the waxing gibbous as it heads toward full phase next week. The evening of Friday the 9th finds the Moon just to the west of Mars, while by the following evening (Saturday the 10th), the Moon will have moved to the other side of the red planet. Now in Cancer, Mars stands well to the east of classical Gemini. A fine sight in the evening's west to the left of Castor and Pollux , Mars does not i set until just after local midnight (1 AM Daylight Time).

Then it's Saturn's turn. On the evening of Sunday the 11th, the Moon will lie rather well to the west of Saturn and Leo's Regulus (Saturn the brighter of the pair), then the following night (Monday the 12th) it will be just to the east of them, having passed just three degrees to the south of the planet shortly before Sunset. Saturn now transits the meridian just at the Sun sets, then stays up chasing Regulus until 2:30 AM Daylight.

The biggest and (excepting Pluto) smallest planets make a splash, as (anticipated last week) on Friday the 9th, Jupiter stops moving westerly against the stars of northeastern Sagittarius and begins to move retrograde as the Earth prepares to swing between it and the Sun. Look for the giant planet rising in the southeast around 12:30 AM Daylight.

Mercury then passes greatest eastern elongation (22 degrees to the east of the Sun) on Tuesday the 13th. Given that the evening spring ecliptic stands well up against the horizon, this closest planet to the Sun is giving its very best evening showing of the year. Look for a bright "star" low in western evening twilight, the planet not setting until formal twilight is over.

Look high in late evening to find Ursa Major's Big Dipper. Then, as told to generations of those learning the constellations, follow the curve of the handle to the south through Bootes' Arcturus (the brightest star of the northern hemisphere) and then to Virgo's Spica. Down and to the left of Spica is one of the dimmer constellations of the Zodiac, the distorted box that makes Libra, the Scales, which 3000 years ago held the Autumnal Equinox, hence its name, signifying the balance of days. The constellation is perhaps best known by its two beloved stars that represent the outstretched claws of Scorpius, Zubenelgenubi (the Southern Claw) and Zubeneschamali (the Northern).

STAR OF THE WEEK: IOTA LIB (Iota Librae). South of the famed third magnitude Claws of the Scorpion, Zubeneschamali and Zubenelgenubi, lies the dimmest of Libra's classical stars, fifth magnitude (though at 4.54 just barely) Iota-1 Librae. Immediately to the east lies sixth magnitude Iota-2, making the two look like a naked-eye double in the mold of Mizar and Alcor, or perhaps Epsilon Lyrae. Sadly, no, as Iota-2, a class A (A3) dwarf lies 240 light years away, much closer than Iota-1, which stands at a distance of 377 light years, the two just a line-of-sight coincidence. Iota-1, however, does not need Iota- 2, as it is already a quadruple star on its own.

Iota Librae Ab goes around Iota Aa (the brighter and more massive of the two, placed at the cross) every 23.5 years at an average separation of 14.9 Astronomical Units. The points are individual observations to which the best orbit is fitted. The dot-dash line is the orbit's major axis. Since the orbit is tilted by 26 degrees to the plane of the sky, it is somewhat distorted from the way it would look if seen face-on. The arrow at lower right shows the direction of motion. Though Aa is placed at the focus of the Ab's orbit, in truth, both stars go about each other, orbiting around a common center of mass, which is not determined for this pair. North is down, as would be seen in a telescope. The scales around the edges are in seconds of arc. Far off the page is another pair (Iota B and C) that orbits this one. (From an article by B. D. Mason, G. G. Douglass, and W. I. Hartkopf in the Astronomical Journal, vol. 117, p. 1023, 1999, image courtesy of W. I. Hartkopf.)

Iota-1 was initially identified as a class A-peculiar star (with high silicon content), and then as a B9 subgiant-dwarf pair with magnitudes of 5.1 and 5.6 (but see below). Iota-1 Aa and Ab (as they are called) orbit each other with a period of 23.469 years at an average separation of 14.9 Astronomical Units (half again as far as Saturn is from the Sun), which at Iota-1's distance translates into a very difficult-to-separate 0.13 seconds of arc and that requires sophisticated instrumentation to split. A significant eccentricity takes them between 18.6 and 11.3 AU apart, "periastron" (the closest separation) last taking place in 1994, the next in 2018. Orbital period and separation then lead to a combined mass of 6.05 times that of the Sun. An estimated temperature of 11,000 Kelvin for both stars coupled with distance gives respective luminosities for Aa and Ab of 149 and 94 Suns, radii of 3.4 and 2.7 times solar, and masses of 3.1 and 2.9 Suns, which sum to almost exactly that determined through the orbit, showing that the various parameters are very accurate. Theory shows that both stars are also very clearly dwarfs about 60 percent of the way through their hydrogen-fusing lifetimes. Completing the whole set, not quite a minute of arc away lies another pair, Iota-1 B and C, respectively 10th and 11th more or less solar type class G (G4 and G8) dwarfs that are keeping pace with their larger class B mates (Aa and Ab), showing them to be real companions. The separation of Iota-1 B and C of two seconds of arc leads to a physical separation of at least 230 AU. Given subsolar luminosities and combined masses of 1.7 solar, the orbital period must be at least 2700 years. The G-dwarf pair is separated from the bigger B dwarfs (Aa and Ab) by at least 6600 AU, giving a period of at least 195,000 years. Given these various numbers to be accurate, from each of the pairs, the other would appear beautiful if not spectacular. From Aa or Ab, B and C would each roughly shine with the light of 10 times that of Venus separated by up to 2 degrees. A resident of B or C (and there is no evidence for any) would see the two B dwarfs each radiating the light of a full Moon up to 8 minutes of arc (0.13 degree) apart, the orbital movement quite wonderful to behold.

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.