SKYLIGHTS

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Astronomy news for the two weeks starting Friday, August 12, 2016.

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.

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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, now in Chinese translation.

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.


SSLive in town? Read FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. See the interview with Jim Kaler.


Coming in October, From the Sun to the Stars by Jim Kaler, World Scientific 2016, a new book based on From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.


NEW! Read Dust to Dust in Stellar Stories.



The next skylights will appear August 26.

And a remarkable fortnight it is. As we open, we are just coming off the Perseid meteor shower, and we should still get a few the morning of Saturday, August 13. And it still won't be much bothered by the Moon, which is only in its early waxing gibbous phase and sets well before dawn begins to light the sky. The Moon finally passes full ("the green corn moon, the "grain moon") the morning of Thursday the 18th, bright and high in the western sky. The Moon then quickly transitions to its waning gibbous phase, which ends at last quarter the night of Wednesday the 24th, followed by a bit of the waning crescent.

The morning of Thursday the 25th, the Moon will rise just barely west of bright Aldebaran. For the eastern and southern US, the Moon will occult or cover the star, though (except in Texas, where it will be dark) in twilight or, as in the northeast, after sunrise. In addition the full Moon will undergo a brief eclipse. But don't get too excited about it as the Moon only barely clips the Earth's penumbral (partial) shadow, the "event" quite undetectable.

It's far better to watch Saturn, which are beautifully visible in the southwest in and after evening twilight. Saturn, north of Antares (the luminary of Scorpius) and to the east of Mars, ends its retrograde (easterly) motion against the stars on Saturday the 13th, and is so far away that it hardly seems to move at all, while Mars is so close to us that its orbital motion to the east can be witnessed near night-to-night as it rapidly overtakes the ringed planet. On Tuesday the 23rd, Mars will pass just 1.8 degrees north of Antares and then on Thursday the 25th four degrees south of Saturn. We then see a remarkable stack of celestial bodies with Saturn on top, Mars in the middle, and Antares on the bottom. It's a good time to compare colors. Saturn is a yellow-white, while Mars and Antares are reddish, the star's very name, meaning "rival of Mars," Ares the Greek version of the god of war. While their colors are similar, Antares, a huge supergiant, is clearly the redder of the two. Look early, as they all set around midnight daylight time. `

Stretching our fortnight a bit, a day after Mars passes Saturn, on Friday the 26th, Mercury (practically invisible) goes 5 degrees south of Venus, and then on the next day, Venus goes just 0.07 degrees north of Jupiter, the events very difficult to see because of bright western twilight. Want more? Mercury passes greatest eastern elongation on Tuesday the 16th, Neptune is occulted by the Moon on Friday the 19th (though in daylight and only in the far north), and the Moon is at perigee, closest to Earth, on Sunday the 21st.

For a bit of relief from all this activity we turn to the stars. In early evening, white Vega, the second brightest star in the northern celestial hemisphere, lies nearly overhead, while orange Arcturus falls to the southwest, sandwiched between faint Coma Berenices with its sprawling cluster and the head of Serpens, the Serpent, which wraps itself around Ophiuchus. Well south of Arcturus, blue-white Spica sparkles in Virgo.

STAR OF THE WEEK: XI SCO (Xi Scorpii). In the 1930s modern boundaries were drawn around the 88 accepted constellations such that they included as many of the Bayer Greek-lettered and Flamsteed-numbered stars as possible. As a result, some of the formal outlines look mighty peculiar. Scorpius, for example, has a long thin panhandle only five degrees wide sandwiched between Libra and Ophiuchus that rises from the south near the ecliptic to nearly 12 degrees south of the celestial equator. Within it we find three of Bayer's stars, Phi, Chi, and the brightest, fourth magnitude (4.19) Xi Sco. The star is a bit of a bouncing ball. Ptolemy put in Libra, while Bayer moved it to Scorpius as Xi Sco. Flamsteed then put it back into Libra as Xi Librae. Because it was number 51 on his list in that constellation, it also became known as 51 Librae. But as the star was in more modern times flipped back into Scorpius, "51 Lib" is never used. It's at least triple, and may be sextuple, or something in between. Writing in the 19th century, Smyth and Chambers tell us it's "a fine triple star...A 4 1/2, bright white; B 5, pale yellow, C 7 1/2. grey," the first measures of separation made by William Herschel in 1780-1782. The multiplicity disallowed a Hipparcos parallax, so the distance of 91 light years is only roughly known. However, 4.6 minutes of arc away is ANOTHER triple that seems to be co-moving with Xi, which does have a Hipparcos parallax that gives a supportive 82 light years. It seems fairly safe to stick with 90 l-y. Xi Sco A and B, magnitudes 5.16 and 4.87 ("B" oddly the brighter), are mid- class F stars (a somewhat uncertain F4 and F5, both hydrogen-fusing dwarfs) a mere second or so of arc apart. With estimated temperatures of 6600 and 6530 Kelvin, we find respective luminosities of 5.2 and 6.9 times that of the Sun, which gives similar masses of 1.5 Suns. Both again seem to be evolutionary dwarfs. Seventh magnitude (7.3) Xi C may be on the warm side of class G (G1 to G4), and with a luminosity near 80 percent solar carries roughly a solar mass.

Lambda Lup Xi Scorpii A is seen to go about Xi Sco B (or B around A: it's not clear) with a period just short of 46 years in a rather highly elliptical orbit in which they average 18 Astronomical Units apart, about the distance Uranus is from the Sun. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

We can check the numbers by analyzing the stellar orbits. Xi A and B go around each other with a period of 45.9 years at an average separation of 18.2 Astronomical Units, a rather large eccentricity carrying them between 32 and 5 AU apart. They were last closest in 1997. Kepler's third law then gives a combined mass of 2.9 times that of the Sun, very close to the evolutionary values. Xi Sco C, now 7.0 seconds of arc from the AB pair, has gone far enough around to have had an orbit fitted to the data. It takes the outer star 1500 years to go around the inner double in a more circular path at an average separation of 215 AU. Now Kepler's laws give a combined ABC mass of 4.3 solar. Subtracting the AB mass from the ABC mass then allows a mass for "C" of 1.4 Suns, too high but not bad given the uncertainties of the big orbit.

Then next door, too faint to have a Greek-lettered name, is the other apparent triple. It's rather similar, with seventh and eighth magnitude A and B (G8 and K3) 12 seconds of arc apart, eleventh magnitude AC separated by 83 seconds. From its motion "C" is probably a line of sight coincidence, which is hardly surprising given the density of the stars in Scorpius. Stranger still, Xi Sco is also listed as the "D" companion to the fainter triple. On the whole it looks like we may have a quintuple, with the fainter set at least 7400 AU away from Xi, taking at least a quarter of a million years to make the journey all the way around. It's rather hard to see how they can stick together. (The discussion of the star's name is from Morton Wagman's "Lost Stars," McDonald and Woodward, 2003.)


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