SKYLIGHTS

Skylights featured on Astronomy Picture of the Day

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

Photo of the Week. A fine circumhorizontal arc, caused by sunlight refracting through ice crystals, courses across the sky south of the Sun.

Astronomy news for the week starting Friday, August 15, 2008.

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.

NEW! Heavens Above: Stars, Constellations, and the Sky from Recorded Books.

NEW FOR SUMMER! Take a tour of the Milky Way as seen from the northern hemisphere.

We begin the week with the Moon near the end of its waxing gibbous phase as it approaches full on Saturday the 16th. With the Sun descending the ecliptic from the Summer Solstice, our full Moons are now riding upward from the Winter Solstice, this one -- the "Green Corn Moon," the "Grain Moon" - - lies between classical Capricornus and Aquarius just to the northeast of Neptune, which passes opposition to the Sun as our week begins, on Friday the 15th. Having just had an eclipse of the Sun on August 1, this full Moon will be eclipsed, though just partially and seen in Europe and Asia, the event not visible in North America. The remainder of the week then sees our companion in the waning gibbous phase. On Monday the 18th, it passes a few degrees south of Uranus.

Some nice planetary action takes place nearly invisibly in western twilight, as Saturn, Venus, and Mercury gather together. On Friday the 22nd, Mercury and Venus pass conjunction just 1.2 degrees apart, the event unfortunately not visible without special equipment. At least we know it is happening.

So once again, for planetary viewing we turn to lonely Jupiter, which now dominates the southern sky at the end of twilight, the giant planet now crossing the meridian due south (for those of us in the northern hemisphere) around 10 PM. Setting ever earlier, Jupiter now goes down around 3 AM, leaving us planet-less but for Uranus and Neptune, Uranus transiting just as Jupiter sets.

Mid-August in mid-evening finds the Scorpius-Sagittarius pair (Sagittarius to the east, Scorpius to the west) crossing the meridian to the far south. Due north of Sagittarius, white Vega shines nearly overhead in Lyra, while Arcturus sparkles orange-ly to the west. Sagittarius is now easily located by looking down and a bit to the right of Jupiter, though a bright Moon will blot much of it out. Atop Scorpius, once the Moon is out of the way, look upward to Ophiuchus (the Serpent Bearer), then to Hercules, then to the head of Draco, the Dragon.

STAR OF THE WEEK: X HER (X Herculis). While obscure -- hardly anyone studies the star -- and just over the line into seventh magnitude (6.58 on a sort-of average), X Herculis still has a couple stories to tell. First, if one does not pay attention, it is easy to mix it up with brighter, rather sun-like Chi Herculis, the Greek "Chi" written much like Roman "X." The confusion is not helped by X and Chi being close together in far northwestern Hercules. They are close only in direction, though, as Chi Her is a mere 52 light years away, while X Her, at 450 light years, is almost nine times farther. Curiously, they share high speed relative to the Sun, Chi moving at 80 kilometers per second, X Her at a very substantial 110, seven times average, both stars visitors from different parts of the Galaxy. More importantly, the "X" tells us not that the star is necessarily mysterious, but is a variable star, the seventh one found in Hercules that already did not have a name (the discovery-progression of variables in a constellation starting with "R"). Called a "semi-regular" variable (in the trade, an "SRb" star), this deep red class M giant (the class probably variable as well, given as M7 or M8) varies with an average 95 day period between magnitudes 6 and 7 with a longer two-year additional variation superimposed on it.

The visual light curve of the semi-regular variable X Herculis is plotted against Julian Date. JD 2453750 is January 14, 2006, JD 2454500 February 3, 2008. The black points are visual estimates, the green ones more-accurate photoelectric measures. The star varies between magnitudes 6 and 7 with a rough 95 day period. Note the superimposed longer-period variation. Courtesy of the American Association of Variable Star Observers (AAVSO).

Typical of its class, it is one of the cooler stars in the sky, its temperature hovering at a measured 3300 Kelvin. Such extreme red giants are entering the last stages of their lives with dead carbon-oxygen cores, and are -- like the red giant variable Mira -- preparing to slough off their outer envelopes in the process of becoming white dwarfs. Distance and temperature (to account for a LOT of infrared radiation) give a luminosity of 680 times that of the Sun and a radius of 80 Suns, 0.38 Astronomical Units, about the size of Mercury's orbit. Oddly, the radius from direct measure of angular diameter gives a wildly different number, 180 solar, 0.84 AU. When compared to theory, the luminosity is too low for the temperature. An M7 giant should have a temperature closer to 3000 Kelvin, which would give a luminosity of 3200 Suns and a radius of 200 solar (nearly the size of Earth's orbit), closer to the mark. The birth mass is then around one to 1.5 times that of the Sun or so. The problem may also be compounded by a surrounding disk-like structure observed in radio radiation that is caused by windy mass loss. A low metal content implied by high velocity might also confuse theory. Clearly, this much neglected star (which shows us something of what will happen to the Sun) has some problems and needs additional work to sort out its parameters, in this sense "X" really denoting some mystery. Best appreciate it now, as it is dying fast, at least on an astronomical time scale.

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.