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Astronomy Picture of the Day

Thunderhead

Photo of the Week.. An anviled thunderhead floats beneath a clear blue sky, while shafts of setting sunlight shine through holes in the clouds.

Astronomy news for the week starting Friday, May 17, 2002.

Phone: (217) 333-8789
Prepared by Jim Kaler.

If you like stars, go to STARS: Portraits of Stars and their Constellations, compiled from previous stars of the week. Enjoy photographs of the January 20, 2000 total eclipse of the Moon. Watch planets move against the background stars. See sunsets, rainbows, the Moon and planets, and other sky phenomena in Sunlight.
As busy as last week was, is as quiet the week to come, as if the heavens are taking a bit of a rest. The Moon, approaching its first quarter at the start of the week, reaches that phase, when it is 90 degrees to the east of the Sun, on Sunday the 19th amidst the stars of central Leo, just up and to the left of the first magnitude star Regulus. Regulus, like Spica in Virgo, is practically on the ecliptic, the apparent path of the Sun, and both stars can be occulted, or covered, by the Moon. The Moon's orbit is tilted by 5 degrees to the ecliptic path, however, and this month the Moon passes well north of the two stars, making the tilt quite visible. It is because of the orbital tilt that we do not have eclipses every month, as the Moon usually passes either above or below the Earth's shadow, missing a lunar eclipse, or passes above or below the Sun, missing a solar eclipse. About 4 days after the quarter, on Thursday the 23rd, the Moon passes perigee, when it is closest to the Earth.

The great planetary gathering of 2002 is slowly breaking up, though there are treats yet to see. Mercury and Saturn have sunk out of sight, while Mars is finally beginning to remove itself from the western evening sky, and by the end of the week will be setting just about the time twilight ends, leaving the stage to the two brightest planets of the Solar System, Venus and Jupiter. Venus, the brighter, is still climbing higher, away from the Sun, while Jupiter is sinking lower. As a result, they are approaching each other as they prepare to swap places on June 3. Of course they approach only in angle. Venus is to the side of the Sun 1.4 Astronomical Units (the distance between the Earth and Sun) away, while Jupiter is far to the opposite side of the Sun at a distance of nearly 6 Astronomical Units. That Jupiter is so bright is testimony to its great size and reflecting power, the planet over 10 times the diameter of Earth, Venus slightly smaller.

With the winter stars now gone, the spring stars dominate the early evening, and the stars of summer begin to make a serious impact in the later night. With bright orange Arcturus crossing the meridian to the south (at least from north of the Tropics) before midnight, find its white counterpart Vega climbing up in the northeast, the star the luminary of exquisite Lyra, the Harp. Between Arcturus (in Bootes) and Vega lie the graceful curve of Corona Borealis, the Northern Crown, and the figure of Hercules, which honors the powerful hero of ancient times. Corona Borealis's southern counterpart, Corona Australis, lies below Sagittarius, and will be with us later in the true summer.

STAR OF THE WEEK. DELTA BOO (Delta Bootis). In spite of its prominent position within Bootes, the Herdsman, Delta Boo carries no proper name at all, probably because it shines down at us at only faint third -- and nearly fourth -- magnitude (3.47). Bayer seemed to use position in the constellation as much as brightness in naming by Greek letters, as Delta not only ranks fifth, but is outstripped by brighter Eta Bootis (Muphrid). Delta Boo is a class G (G8) giant located 117 light years away. From its yellow-white 4840 Kelvin surface, it shines with the light of 59 Suns. It is a rather typical "clump" star, one of a breed of similar temperature and luminosity that is fusing helium into carbon in its deep core, one that carries a mass 2.5 times that of the Sun. Not all that large for a star called a "giant," its diameter deduced from temperature and luminosity is 11.2 solar. It is large and close enough, however, to have had its angular size determined, from which we come up with a similar diameter 10.4 times solar (from which we then find a somewhat warmer temperature of 4990 Kelvin). The star stands out in two ways. Its metal content is rather low, only 40 percent that of the Sun. As a result it is spectroscopically classified as a "CN weak" star (from the strength of absorptions of cyanogen, CN). Delta Boo also has a well-known 8th magnitude (7.8) class G (G0) dwarf companion that is quite similar to the Sun, just a bit warmer (5900 Kelvin), 80 percent less luminous, and 87 percent the radius. The companion is a bit warmer than expected for its luminosity, and might be a marginal "subdwarf," in keeping with the low metal content of the main star (the two go together). Since the companion is nearly two minutes of arc away from its brighter neighbor, Delta Boo B shows no orbital motion, but since the two stars are moving in lockstep through space, there is no question about their connection. At least 3800 Astronomical Units (Earth- Sun distances) apart, the two take at least 120,000 years to orbit each other. From Delta Boo A, the smaller star would shine 30 or so times brighter than Venus does in our sky, while from Delta Boo B, the giant would glow with the light of 2.5 full moons.

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.
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