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. Sagittarius (left) and Scorpius (right) sprawl across the picture, honoring Jupiter, the bright light at far left to the east of the Little Milk Dipper (as it appeared in March of 2008). The Milky Way runs down from left to right, while Antares shines toward upper right.

Astronomy news for the week starting Friday, May 16, 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.
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.

This is the week of the full Moon, the Planting Moon, Milk Moon, Mother's Moon, Flower Moon, which takes place just after mid-month, the night of Monday, May 19, about the time of Moonrise in North America, the full Moon thus rising at Sunset. That the full Moon rising in twilight looks so large is but an optical illusion whose origin is still argued. Illusion or no, it still makes for a beautiful sight. Only 12 hours after full, the Moon goes through apogee, where it is farthest from Earth, thus reducing the so-called " spring tides" that take place when the solar and lunar tides add together (at full and new phases). Earlier in the week we see a fat waxing gibbous, while the remainder of the week is spent in the waning gibbous. Just a day past full (the night of Tuesday the 20th), the Moon makes a close pass to Antares in Scorpius, appearing just to the east of the star by the time they both rise in the southeast.

The early evening sky presents a trio of planets. Though not as good as last week, Mercury is still on display in the west-northwest during twilight. Higher up, find Mars to the left of, and up from, Castor and Pollux in Gemini, the planet itself in Cancer, the next constellation of the Zodiac to the east. Higher still is Saturn, which makes a fine coupling with Regulus in Leo, the planet just a bit to the east of the star, the two slowly pulling apart. Mars then makes something of a transition by setting at local midnight (1 AM Daylight), followed a little over an hour later by Saturn. By that time, however, Jupiter is nicely up in the southeast, the planet rising in Sagittarius at midnight Daylight time to join the evening crowd, a late guest if you will.

This is Big Dipper season, this most famed of asterisms a part of Ursa Major, the Great Bear. Look for the seven-star pattern nearly overhead in early evening. Then look to the second star in from the end of the handle, Mizar, to try to find its dim naked-eye companion Alcor just to the northeast of it. South of the curve of the handle and parallel to it is a pair of stars that make most of the modern constellation Canes Venatici, the Hunting Dogs, while to the south of these lies Coma Berenices, a lacy cluster that needs a dark sky with little Moonlight, which this week does not provide.

STAR OF THE WEEK: 78 UMa (78 Ursae Majoris). With nothing but Flamsteed and catalogue numbers, 78 Ursae Majoris may look anonymous. But if you've looked carefully at Ursa Major's Big Dipper, it may have caught your attention. Look first at the second star in from the handle, Mizar, which has Alcor as an outlier. Then to the next star in, fifth magnitude (4.93) Alioth (Epsilon UMa), which is remarkably similar, except that its "companion," 78 UMa, is angularly just farther away. Along with many others, 78 and Alioth are both part of the Ursa Major cluster, the nearest such cluster to the Earth, our star 78 UMa at 81 light years, the average at 80. However, while Mizar and Alcor are probably true orbiting companions, Alioth and 78 are most likely not: see below. No matter, as 78 UMa is a fine double in its own right, one component of which is a lesser version of the Sun.

78 Ursae Majoris B goes around 78 UMa A (the brighter and more massive of the two, placed at the cross) with a period of 106 years at an average separation of 31 1/4 Astronomical Units. Observations (the colored points) are missing where the stars were too close together. Tilted through a 50 degree angle to the plane of the sky, the orbit is quite noticeably foreshortened, as can be seen by the off-center displacement of the true ellipse's major axis (the dot-dash line). The arrow at lower right shows the direction of motion. Double star observers measure the position of the fainter star relative to the brighter rather than the actual motions of both stars around the common center of mass that lies between them. North is down, as would be seen in a telescope. The scales around the edges are in seconds of arc. (From an article by M. Scardia et al. in the Monthly Notices of the Royal Astronomical Society, vol. 357, p. 1255, 2005, image courtesy of W. I. Hartkopf.)

The main star, 78 UMa A, is a class F (F2) dwarf at 7000 Kelvin coupled with a G-type (G6) 5500-Kelvin dwarf that have respective luminosities of 4.6 and 0.87 times that of the Sun, radii of 1.5 and 0.9 solar, and (from theory) masses of 1.5 and just under 1.0 solar (perhaps as low as 0.8 solar). Rotating quickly, more than 92 kilometers per second at the equator, 78 UMa A spins in less than 0.8 days. The orbital period of the pair of 106.4 years coupled with an average separation of 31.25 Astronomical Units (1.25 seconds of arc on the sky) yields a combined mass of 2.7 times that of the Sun, somewhat above that found from evolutionary theory. A significant eccentricity of 0.41 takes the stars between 49 and 18 AU apart. They were last closest in 1921 and will be again in 2027. The stars are both quite young, consistent with the 250 million-year age of the UMa cluster. Can 78 and Alioth be a real pair? Given their masses and a separation of at least 1.4 light years, the orbital period would be more than 10 million years. The two are so far apart, however, that the influence of the UMa cluster would prevent any stable orbit. Mizar and Alcor alone, at a distance of five light years, would have roughly a third of the gravitational influence of Alioth. But 78's sky would be glorious. Alioth would glow with the light 7.5 times that of our Venus at its brightest. Even Mizar would rival the planet. From Alioth, 78 UMa might at times in its orbit appear as a close near-naked-eye double, the two a bit over a minute of arc apart, the brighter almost another Venus, the fainter about equal to our Sirius. (Thanks to Jerry Diekmann, who suggested this star.)

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.