Sierra NightSky for the period starting Friday, July 25, 2008 by Jim Kaler

Let your soul stand cool and composed before a million universes" Walt Whitman

Current Moon PhaseThe Moon's current phase, courtesy of USNO.
Sierra NightSky for the period starting Friday, July 25, 2008 by Jim Kaler

Passing last quarter during daylight (in North America) on Friday, July 25, the Moon begins to fade away as it enters its waning crescent phase and heads towards new on Friday, August 1, at which time it will totally eclipse the Sun and present a fine sight for people in Europe (Russia) and Asia (China), but not in North America. A partial solar eclipse will be visible in extreme northern North America, and will cover nearly all of Europe and Asia except for Spain, Portugal, and Japan. (Paired with this eclipse will be a partial lunar eclipse on August 16, which will be invisible in North America as well.) Back to home, the last glimpse of the thin crescent will be the morning of Thursday, July 31, in eastern twilight. Look next week for the waxing crescent in western evening twilight. On Tuesday the 29th, the waning crescent goes through perigee, bringing higher-than-usual tides to the coasts.

Of the ancient planets, the only one now readily visible is Jupiter, which is seen rising gloriously in the southeast in early evening. Bright and beautiful, hard to miss, Jupiter crosses the meridian to the south around 11:30 PM Daylight. Its retrograde, westerly, motion against the background stars has returned it to a lovely position just to the northeast of the Little Milk Dipper in Sagittarius, the locally bright stars making the planet's motion from week-to-week quite obvious. Of the "missing" bright planets, only Mercury makes some sort of news, as it passes superior conjunction with the Sun (on the other side of the Sun) on Tuesday, the 29th. Saturn and Mars are quite lost in evening twilight.

With the Moon nicely out of the way, we have a good chance to witness one of the better meteor showers of the year, the Southern Delta Aquarids, named after a southern star in Aquarius that marks the region of the sky from which the meteors seem to come (which involves the direction of the meteoroid stream about the Sun combined with the orbital motion of the Earth). The shower is visible from late July into early August with a broad peak around Monday, July 28, when under a dark sky it will produce perhaps 20 meteors a minute. The best time to look will be around 3 or 4 AM, to the south. The parent comet, from which meteor showers derive (as the comets slough off dust and rock), is unknown.

Summer is a fine time to catch the two celestial crowns. Look for a semi-circle of stars, Corona Borealis, the Northern Crown, to the northeast of bright Arcturus. Then if you have a good southern horizon and don't live too far north, look directly beneath the Little Milk Dipper of Sagittarius (now well-marked by Jupiter) to find the more ragged semi-circle of Corona Australis, the Southern Crown.

STAR OF THE WEEK: GAMMA CRB (Gamma Coronae Borealis)

To the northeast of Arcturus in Bootes lies one of the most graceful of celestial figures, the semi-circle that makes Corona Borealis, the Northern Crown, in mythology the Crown of Ariadne. Set into the center of the curve, like a genuine jewel, is the luminary, third magnitude Alphecca. Immediately to the east of it lies fourth magnitude (3.81) Gamma Coronae Borealis (of no proper name). It is not one star, but two, a double with components in tight orbit about each other. The brighter (Gamma CrB A) is a fourth magnitude class B (on the cool side, B9) subgiant (suggesting the cessation of core hydrogen fusion, but see below); the fainter (Gamma B) is a sixth magnitude (5.60) class A3 hydrogen-fusing dwarf. The two are always less than a second of arc apart and quite difficult to separate through the telescope. The discovery of duplicity in 1826 by F. G. W. Struve is thus quite remarkable. At a distance of 145 light years, they orbit every 92.94 years at an average separation of 32.7 Astronomical Units, roughly the distance between Neptune and the Sun, a relatively high eccentricity of 0.48 taking the physical separation between 49 and 17 AU. The next close approach will take place in 2024.

The orbital plane is tilted almost into the line of sight, just 5 degrees off, so that the stars are not that far from eclipsing one another. Analysis of the orbit via Kepler's Laws yield a combined mass of 4.1 times that of the Sun. The individual masses are not available from the orbital data, so we use the theory of stellar structure. From the spectral classes, the respective temperatures for Gamma A and B are 11,000 and 8800 Kelvin. Distance and apparent brightness (with small corrections for ultraviolet radiation) give respective luminosities of 48 and 9 times solar, radii of 1.9 and 1.3 solar, and masses 2.6 and 1.85 times that of the Sun. The combined "evolutionary mass" is then 4.45 time that of the Sun, not far off that derived from the binary orbit, showing that all the parameters are in pretty good shape. The hotter one is sometimes classed as an A0 or A1 star, but that lowers the mass by only a tenth of a Sun or so. Theory shows that the stars are both quite young, and that Gamma CrB A is not a subgiant at all but a dwarf, although a greater one than its mate. Another point of contention is that Gamma A is listed as a Delta Scuti- type variable with a brightness range of about six percent over a period of 0.7 hours, but it seems too hot for that category. Eventually, in a few hundred million years, Gamma A's internal fire will indeed go out, and Gamma B will watch it turn into a distended giant, but one far enough away that Gamma B should remain unaffected, the pair eventually dying as a double white dwarf.

Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler.

Sierra NightSky thanks to Jim Kaler.
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Astronomy Picture of the Week

total lunar eclipse
Total Lunar Eclipse, Feb. 20, 2008
Credit: Stephen "AstroGeek" LaFlamme

Explanation: This image of the total lunar eclipse was captured with a Meade LPI imager connected to an AstroTech 66mm refractor. About 50 two-second exposures were aligned and stacked with Registax followed by further processing with PhotoShop. Image taken in my backyard observatory located in Bridgewater, Massachusetts. Feb. 20, 2008.