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. Shadowed evening clouds.

Astronomy news for the week starting Friday, May 10, 2013.

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.	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.

NEW! From the Sun to the Stars: the OLLI Lectures 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 Twins in Stellar Stories.

NEW! The Tides.

The Moon spends the week waxing through its crescent phase. For those with clear northwestern horizons it will be just barely visible the evening of Friday, May 10, in bright twilight with Venus just up and to the right of it: a difficult sight at best. By the evening of Saturday the 11th, the growing but still thin crescent will be much more visible and will make a fine display as it sits a few degrees below bright Jupiter. By the following night, the Moon will have moved to appear up and to the left of the planet. On the night of Tuesday the 14th, look for it passing south of Castor and Pollux in Gemini. Continuing to fatten, the crescent phase finally ends at first quarter the night of Friday the 17th about the time of Moonset in North America. Look for Regulus to the northeast of it. On Monday the 13th, our companion passes apogee, where it is farthest from the Earth on its monthly round.

Even though Venus, as noted above, is coming into visibility, it has not yet arrived in any obvious way. And with Jupiter setting just after the end of twilight, the planetary sky is diminishing. The only planet we have to admire in any serious way is Saturn. Which is perhaps enough for now. Well up in the southeast in early evening about 15 degrees to the east of Spica (the two making a distant pair), the ringed planet crosses the meridian to the south around midnight Daylight Time. Practically on the Libra-Virgo border, Saturn stays with us the entire night, not setting in the southwest until dawn lights the eastern sky. Among the ancient planets, that leaves Mercury, which passes superior conjunction with the Sun (on the other side of it) on Saturday the 11th as it prepares for a decent evening showing in early June.

Leo, with its sickle-shaped foreparts, lies high to the south in early evening, the Big Dipper well to the north of it and nearly overhead. Between the Lion's head and the Dipper's bowl lie the dim stars of modern Leo Minor, the smaller Lion. Below the Sickle are the equally faint stars of modern Sextans, the Sextant. High to the southeast in early evening find bright orange Arcturus, which lies at the southern end of kite-shaped Bootes, the Herdsman. It begins a progression of constellations that extends eastward, the first of which is semi-circular Corona Borealis, the Northern Crown. Its counterpart, Corona Australis, will be out of sight below summer's Sagittarius.

STAR OF THE WEEK: PI LEO (Pi Leonis). Tucked into western Leo just under five degrees south southwest of Regulus and just north of the border with Sextans lies fifth magnitude (4.70) Pi Leonis, a giant star of a somewhat different breed. Most, of class K (usually of the K0 or K1 subvarieties), are quietly fusing helium into carbon and oxygen in their deep cores. But they have to get there first. Class M (M2) Pi Leo may be a good example of a red giant in transition. At a distance of 406 light years (give or take 15), the star is all by itself, with no known companions, so we can concentrate on it alone. With a temperature of 3710 Kelvin, just right for the class, Pi Leo radiates much of its energy in the infrared part of the spectrum. Taking that and distance into account, we find the star to shine with the light of 775 Suns, which with temperature gives it a radius of 68 times that of the Sun, or 0.31 Astronomical Units, about 80 percent the size of Mercury's orbit. Pi Leo is just close enough to the ecliptic to be occasionally occulted by the Moon. The time it takes for the star to disappear behind the lunar advancing lunar limb gives a radius of 70 solar radii, while interferometry gives 66, the average of which is right on the mark. Given the luminosity and temperature (plus the theory of stellar structure and evolution), Pi Leo's mass should be close to 1.5 times that of the Sun (though with a fairly large uncertainty). Pi's claim to any sort of fame is that it is most likely close to the high-luminosity tip of the "red giant branch" of evolving stars. When the hydrogen in the core of a star is all converted to helium, the core contracts, hydrogen fusion expands outward into a shell, and the star itself swells and brightens. For stars with masses below about double solar, as the core squeezes down and its density climbs, the electrons in the ionized gas begin to fall into a state of "degeneracy" (like that in a white dwarf) in which they can get no closer. At some point, the internal temperature gets high enough that the helium nuclei can fuse to carbon and oxygen. The degeneracy is then relieved, but explosively in a tremendous "helium flash" whose energy is absorbed by the star and is largely invisible at the stellar surface. Assuming the stellar parameters and our premise to be correct, after a hydrogen fusing lifetime of 2.7 billion years, Pi Leo has been ageing and swelling for the last 220 million. It has perhaps another couple million years left before the helium flash takes place and it dims and shrinks some to become one of the ordinary class K orange giants that dot the skies and are so much a part of our familiar constellations. Of course the star might be in that dimming state as well, or even brightening for the second time with a dead carbon core to become a more advanced giant, a possibility supported by a tenth of a magnitude variation. We can only wait and see.

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.