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 href="http://epod.usra.edu/blog/2006/04/sundog-and-parhelic-circle .html">6 7 8 9

Photo of the Week.Peaceful twilight.

Astronomy news for the week starting Friday, August 15, 2014.

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.

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.

Read From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

NEWEST! FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. Read the interview with Jim Kaler.

NEW! Read The Harp in Stellar Stories.

Skylights will next appear on Friday, August 29.

We start our fortnight with the Moon in its waning gibbous phase, which is terminated at third quarter the morning of Sunday, August 17, after Moonrise in North America. It then fades away as a waning crescent as it heads toward new Moon on Monday the 25th. The morning of Friday the 22nd, the slimming crescent will fall between the stars Procyon (down and to the right) and the Pollux and Castor pair (up and to the left). The following morning, that of Saturday the 23rd, the Moon will make a glorious dawn triangle with Jupiter and Venus (Venus the lower and brighter of the two). Then turn around and watch the Moon pop up as a waxing crescent in the early evening sky. Our first glimpse of it will be in western twilight the evening of Monday the 26th, though it will be a lot more obvious a day later. The Moon passes apogee, where it is farthest from the Earth, on Sunday the 24th.

We are witness to the rare delight of two pairs of close planets, one in the evening sky the other in the morning. Moving rapidly eastward against the background stars, Mars crossed the border from Virgo into Libra last Sunday the 10th, and finally passes conjunction with much slower moving Saturn on Wednesday the 27th, the red planet four degrees to the south. Look into the southwest as twilight draws to a close. By 10:30 PM Daylight Time they are close to setting. But if you miss them, all you need do is wait until morning for much brighter Venus and Jupiter. The two planets pass conjunction with each other the night of Sunday the 17th. When they rise, just after the beginning of twilight, they will be less than half a degree apart, Jupiter to the right. The last time they performed like this was in 2012, when they blazed forth together in the evening sky, some taking them for UFOs and invading space aliens. While Venus sinks out of sight into twilight, Jupiter climbs in the other direction, the planets thus quickly separating.

As the Moon moves out of the way, we see the glory of the Milky Way, which is made of the countless stars of the disk of our Galaxy. In mid to late evening the Milky Way plunges down from Cygnus (which turned upside down is the Northern Cross with Deneb at its top), through Aquila and Scutum, and into Scorpius and Sagittarius, finally disappearing below the southern horizon. It's split into two streams by the Great Rift, a lumpy layer of dark, cold, dusty interstellar clouds in which stars are being born. To the northwest, the Big Dipper in Ursa Major winds around the celestial pole, which is closely marked by Polaris, while in the northeast, the rising "W" of Cassiopeia brings a sign of the coming fall.

STAR OF THE WEEK: HD 140283 (HD 140283 Librae). And now (once again) for something completely different. Excluding its nuclear-burning core, the Sun is composed of 92 percent hydrogen atoms and 8 percent helium atoms. That's 100 percent. All the other elements (loosely called "metals") sneak in at a mere 0.15 or so percent, so they don't show up in rounded-off H and He abundances. Oxygen leads the pack, followed by carbon, neon, nitrogen, and so on down to uranium, which must be there but is too sparse to be detected. The hydrogen and most of the helium atoms come from the Big Bang, the H primordial, the He from nuclear reactions that took place within the first three minutes of the life of the Universe. All the other elements (and a bit of helium) were formed in the nuclear cauldrons of massive stars and supernovae that, through their winds and explosions, blew them into interstellar space. The metal content of the stellar birth clouds therefore increases with time, so that young stars have higher metal abundances than old stars. The very FIRST stars, however, those formed right after the Big Bang, should have had no metals at all. We can't find them. Presumably, the first stars were all massive and therefore exploded, so there are none left to see. How far back in metal content, and thus toward the Galaxy's birth, indeed toward that of the Universe, can we go? Not visible to the naked eye, but a clear binocular object, seventh magnitude (7.21) HD 140283 (in the Henry Draper spectral catalogue) is certainly a stop along the way. Only 190 (give or take under 2) light years away in Libra, HD 140283 is the closest of the really low-metal stars. With a temperature of 5580 Kelvin, the iron abundance relative to hydrogen is 0.0040 that found in the Sun. The oxygen value is higher, but still only 2.1 percent solar. The star is classified as "sdF3," the "sd" standing for "subdwarf." Subdwarfs were once thought to be dwarfs that are sub-luminous for their classes. Because of their low metals, however, they are actually hotter and smaller for their luminosities, the low metal abundances giving them classes that are anomalously warm. (All are of class F or cooler. Don't confuse them with similarly named class O and B subdwarfs, which are highly evolved and are on their way to becoming white dwarfs.) Because they are ancient, subdwarfs such as these all have low masses, below about 80 or 90 percent that of the Sun; those of higher initial mass have long since burned away. HD 140283, however, once a real subdwarf, is actually a subgiant that is turning into a giant with a dead helium core. From its temperature, luminosity (3.8 times that of the Sun), chemical composition, and theory, HD 140283 seems almost as old as the Universe itself, which dates at 13.8 billion years. Whatever the final word, the star is certainly among the oldest there are. While close to us now, HD 140283 is a visitor from the ancient Galactic halo, and is just passing through the orderly disk at a speed of 361 kilometers per second relative to the Sun, just a bit more than the true subdwarf Groombridge 1830. Whatever the details, HD 140283 is among the oldest things you can see other than the Universe itself that surrounds you. (Thanks to H. E. Bond et al. in the "Astrophysical Journal Letters," 765:L12, 2013.)

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.