SKYLIGHTS

PLEASE NOTE: SKYLIGHTS has moved to http://stars.astro.illinois.edu/skylights.html.

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 -- Full List Restored!

Photo of the Week. Polaris, the bright star toward the bottom, and the faint stars of northern Camelopardalis (the Giraffe), float above the dome of the four-meter telescope on Kitt Peak, the 2.3-meter telescope of the Steward Observatory seen in silhouette against the glorious sky. See full resolution.

Astronomy news for the week starting Friday, November 13, 2009.

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.	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.
Tour the Milky Way as seen from the northern hemisphere.	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.

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.

The Earth Science Pictures of the Day (see above) have been restored.

Read Hot Crab Sandwich in Stellar Stories.

NEW. "Heaven's Touch: From Killer Stars to Seeds of Life, How We Are Connected to the Universe," Princeton University Press.

Ever have one of those weeks where nothing much seems to happen? With one quite nice exception (see below), here's one now. It can of course be a blessing, as you can then concentrate on other things, such as the serene beauty of the nighttime sky. The Moon starts us off in its morning waning crescent phase, then passes new on Monday the 16th, whereupon it waxes as an evening crescent. The morning of Saturday the 14th finds the Moon just down and to the right of Spica in Virgo. It's a good chance to admire box-like Corvus, the Crow, which will lie well to the right of the star. Your last look at the morning crescent will be on the morning of Sunday the 15th, whereas your first glimpse of the evening crescent will be on the night of Tuesday the 17th. The view gets much better after that with the Moon higher each night, earthlight on the Moon's nighttime side on fine display.

Enjoy Jupiter now, to the south in mid-twilight (and still in northeastern Capricornus), the planet bright enough to be seen in dusk. It's also among the brightest sources of radio radiation in the sky, compliments of a huge magnetosphere that traps particles from the solar wind. Then within an hour of one another, between 10 and 11 PM, Jupiter sets and both Mars and the star Sirius rise, Mars well to the north of the star to the east of Cancer's Beehive cluster. After a goodly wait, Saturn then comes up around 2:30 AM, the rings tightly closed, which dims the planet some. Venus, however, is leaving us. Not rising until after 5:30 AM, in twilight, the planet is getting increasingly difficult to see.

The one big event is the Leonid meteor shower, which takes place in and around the morning of Tuesday the 17th and seems to emanate from the constellation Leo. Peaking every 33 years, the shower was a big hit a decade ago. The leavings of Comet Tempel-Tuttle, the shower is more or less back to its normal small action. There is, however, a possibility of some bursts of activity, so it may be worth a look in a dark sky.

New Moon of course is special, as it allows you to see -- depending on where you live -- a dark sky. With the summer constellations -- Lyra, Cygnus, Aquila and the rest of the gang -- leaving for the western sky, look high to the south to find the Great Square of Pegasus, the main portion of the classic Flying Horse. To the southeast of it, perhaps you can find the small box of stars that makes Equuleus, the Little Horse, also of ancient origins.

STAR OF THE WEEK: RR LYR (RR Lyrae). When we concentrate on naked- eye stars, we obviously miss many that are faint but great, prime examples being the ninth magnitude black-hole binary Cygnus X-1 and fast-moving 10th magnitude Barnard's Star, which holds the record for motion across the sky. RR Lyrae, a seventh-eighth magnitude star in northeastern Lyra 10 degrees to the northeast of Vega, falls well into this family. It's not just a famed variable star, but the prototype of an enormous and deeply important set of them, the "RR Lyrae" stars, which abound within a good fraction of globular clusters, hence the alternative term of "cluster variables." RR Lyrae and the rest of the gang are short-period, low-metal versions of the giant/supergiant Cepheid variables epitomized by Delta Cephei, Zeta Geminorum, and Eta Aquilae. RR itself, a helium-fusing "giant," varies between magnitudes 7.06 at maximum brightness to 8.12 and back over a period of a mere 13.604 hours. The pulsations are driven by regions of partially ionized gas below the stellar surface that valve outflowing heat.

RR Lyrae is the prototype of the RR Lyrae stars, which pulsate in radius and brightness over very short periods. RR itself changes by about a magnitude over a mere 13.6 hours, the brightness rising quickly to maximum that is followed by a slow decline. The star is brightest during maximum expansion velocity. The maxima and minima do not quite match those stated in the main text because of additional, and not-understood, variations. (From Burnham's Celestial Handbook, R. Burnham, Jr., Dover Publications, NY, 1978.)

Most such giants fall into class K or the cool end of class G. A very low iron (really, metal) content of only 7 percent that of the Sun makes RR smaller and hotter than an ordinary giant, rendering it a nominal mid-class-F (F5) star. But as a pulsating variable, the surface temperature varies as well, so the class goes from A8 to F7 (typical of the full range of RR Lyrae stars). The temperature itself is not well known but seems to average somewhere between 6200 and 7000 Kelvin. The Hipparcos parallax satellite gives a distance of 940 light years, but a better measure of 850 light years comes from a Hubble parallax. Using that one, and factoring in 5 percent dimming by interstellar dust, the luminosity comes in between 30 and 85 times that of the Sun, the radius -- as noted above, small for a giant -- somewhere between 4 and 8 times solar depending on the temperature used.

RR Lyrae and its ilk are denizens not of the Galaxy's thin disk (which hosts the Sun and most of the stars around us), but like the globular clusters are members of the vast surrounding Galactic "Population II" halo (the stars of the disk "Population I"). As such they are the evolved progeny of lower mass solar (really subsolar) type stars that have lost a good fraction of their masses. The disk stars like the Sun have somewhat circular orbits about the center of the Galaxy and have low speeds relative to one another. Halo orbits, though, are highly elliptical, the stars and clusters screaming through the disk at high speeds. RR Lyrae is no exception. It's speeding past us at 285 kilometers per second, close to 20 times what is normal for the surrounding stars of the disk. Though RR Lyrae stars are reliable variables, their periods and amplitudes (ranges of brightness) can change over weeks and years. RR Lyrae is particularly vulnerable to it, its variations having superimposed further variations on them of 41 days and 4 years, whose origins (after nearly a century of study) remain mysterious.

No matter what their temperatures or pulsation periods, which range between 0.2 to 0.8 days, RR Lyrae stars all have roughly the same average absolute visual brightnesses (absolute visual magnitudes of -0.7), which makes them excellent distance indicators when seen in clusters or in other galaxies (the absolute magnitude the apparent magnitudes they would have if 32.6 light years away).

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.