SKYLIGHTS

Skylights featured on Astronomy Picture of the Day

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

Photo of the Week. Mars (to the upper right) and Saturn (lower left) bracket the three-star head of Scorpius on June 5, 2016, with Antares down and to the right of Saturn, the trio making a striking sight. See a labelled image and full resolution.

Astronomy news for the two weeks starting Friday, August 26, 2016.

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.	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.

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.

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.

To 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.

Live 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 September 9.

We begin with the Moon in its waning crescent phase as it heads towards new Moon on Thursday, September 1, when it will eclipse the Sun. But don't make any travel plans. Since the new Moon is fairly close to its apogee, which takes place on Tuesday the 6th, it will be too far away from us to cover the Sun completely, resulting in an annular eclipse, seen as a bright ring of sunlight around the lunar disk. Besides, the eclipse path goes over south central Africa and the Pacific Ocean, none of it, not even the partial stages, visible in North America. Your last view of the slim crescent will be in eastern twilight the morning of Tuesday, August 30.

The Moon then pops up as a waxing crescent the evening of Friday the 2nd. It quickly passes three planets, first Mercury then Jupiter on the 2nd, the next day Venus. The Moon will actually occult Venus and Jupiter, but not as seen from North America. Besides, the three planets fall in western twilight and, except for perhaps Venus, are hard to see. And that's too bad because the night of Saturday the 27th, Venus and Jupiter will appear only half a degree from each other. Formal conjunction takes place during the day in North America, when the two will pass a mere 0.07 degrees apart. But only in alignment, as Jupiter is some four times farther away from us than Venus.

Viewing difficulty is hardly the case for Mars and Saturn, both of which are nicely visible in the southwest in early evening, Mars now to the southeast of Saturn, the ringed planet above Antares in Scorpius, the trio making a fine sight. The Moon adds to it. The night of Wednesday the 7th, the Moon will be northwest of Saturn, while the night of Thursday the 8th it will fall to the northeast of the planet and northwest of Mars. The Moon finally reaches first quarter on Friday the 9th.

As a more minor event, Neptune, five times fainter than the eye can see alone and 4.5 times farther away from us than Jupiter, goes through opposition to the Sun near the Pisces-Aquarius border on Sunday the 2nd.

As Scorpius moves off to the west, its place in early evening to the south is taken by Sagittarius, which holds the Winter Solstice and the center of the Galaxy, which, vastly farther than the stars that make the constellation, is dominated by a supermassive black hole of some four million solar masses. To the west, orange Arcturus descends the sky.

STARS OF THE WEEK: CHI AND PSI SCO (Chi and Psi Scoprpii), a two-for-one special. We could even have a three-for-one, all in northern Scorpius, if we include Phi Sco, giving us a consecutive run of Phi, Chi, and Psi. Unfortunately, Phi Sco is one of Bayer's "lost stars." It does not exist. It's been confused with both 48 Librae and 49 Librae, which are just over the border of Scorpius with Libra, but modern boundaries place the latter two firmly in Libra, not in Scorpius. We are then left with fifth magnitude Chi (5.22) and Psi (4.94), which with Xi Sco (the brightest of the trio) occupy the most northerly part of the narrow panhandle of Scorpius that juts northward between Libra and Ophiuchus. The only Flamsteed stars farther north are faint-fifth magnitude (5.43) 16 Sco (a class A3 dwarf 252 light years away) and a solar clone, 18 Scorpii. Other than position, Chi and Psi two have nothing to do with each other. Chi is a relatively ordinary class K (K3) giant 378 light years away (give or take 15), while Psi is class A (A3) subgiant at a distance of 156 light years (plus or minus 10). Chi Sco is a common helium-fusing giant with a temperature of 5270 Kelvin. Adding in some infrared radiation, the star is seen to shine with the light of 249 Suns, which leads to a radius of 28.9 solar. Chi Sco is a calibrator for the interferometric measures of stellar angular diameters, from which we derive a very supportive 27.1 solar radii, showing that the parameters are all pretty close to the mark. Theory then gives a mass around 1.6 times that of the Sun (such stars with a range of masses having similar properties, so precision is difficult). The iron content is around 3/4 that of the Sun.

Psi Sco, with a temperature of 8590 Kelvin, radiates at a rate of 19.2 times that of the Sun, which in turn gives as radius of 2.0 solar radii. Other values of mass from the literature average 2.05 Suns. Theory shows it to be a hydrogen-fusing dwarf rather than a subgiant. A rather slow equatorial rotation velocity of at least 36 kilometers per second yields a rotation period of less than 2.0 days. Yet there seems to be no serious separation of elements of the kind we see in so many class A stars, so the rotation speed is probably quite a lot higher (rotation mixing the elements) and the rotation pole tipped more or less towards us. Psi Sco is roughly halfway through its hydrogen-fusing dwarf lifetime of a billion years, while Chi is just over two billion years old. As seen from Chi, Psi would be a bit fainter, sixth magnitude, Chi from Psi a bit brighter, fourth magnitude. From each, the Sun would shrink to invisibility within the distant stars of Taurus, tenth magnitude from Chi, eighth from Psi.

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.