Skylights featured on Astronomy Picture of the Day

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Skylights featured four times on Earth Science Picture of the Day: 1 , 2 , 3 , 4 .


Photo of the Week.. The Sun, as seen with low magnifying power on Thursday, October 30, 2003, displayed not one, but two, gigantic sunspot groups on either side of the solar equator. Note that the edge (the limb) of the Sun is dimmer than the center (a phenomenon called limb darkening), a consequence of the Sun being spherical and gaseous. The result is that we look slightly deeper into the Sun at the center than at the edge, and therefore to hotter and brighter gases. .

Astronomy news for the week starting Friday, November 7, 2003.

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

Clear skies and thanks to Skylights' * reader

If you like stars, go to STARS: Portraits of Stars and their Constellations, compiled from previous stars of the week. Want to know what happened? Last week's Skylights is still available. Access Skylights' Archive and photo gallery, now available from December 21, 2001, to the present. SG
Enjoy photographs of the January 20, 2000 total eclipse of the Moon. Watch planets move against the background stars. See the Moon move and pass just below the star Nu Virginis, almost occulting it. See sunsets, rainbows, the Moon and planets, and other sky phenomena in Sunlight.

The Stargazer may be playing in a planetarium near you.

Go back and see what happened with a growing Skylights' Archive and Photo Gallery.

For more about stars and constellations, visit the new Essay Pages.

This week we celebrate our two constant companions, the Moon and the Sun. The Moon begins the week not quite full, passing the phase on Saturday, November 8, when it will plunge through the shadow of the Earth to be eclipsed, the second total eclipse of the year to be seen in North America. This one is beautifully timed for the east, not quite so for the west. The partial phase, wherein the Moon enters the full shadow of the Earth, begins at 5:32 PM Central Time (add one hour for EST, subtract an hour for MST, two hours for PST). For the eastern part of the US and Canada, the eclipse will begin after sundown (and, since the Moon is full, Moonrise). For the west, the Moon will rise already partially eclipsed, actually producing quite the fascinating sight. The Moon fully enters total shadow at 7:06 PM (CST), leaves it at 7:31 PM, and then finishes the partial stage at 9:04. Totality does not last very long, as the Moon is a bit south of the ecliptic, and really just clips full shadow. The shadow is not fully dark, but is lit by sunlight scattered by and refracted through the Earth's atmosphere, its brightness depending on atmospheric opacity, mostly that caused by volcanic eruptions. The result at central eclipse at 7:18 (CST) is that the south part of the Moon will be notably brighter than the northern part, which will be closer to the central part of the shadow.

The Sun (which with the Earth is the cause of the eclipse in the first place) got additionally into the act through the great coronal mass ejection of last week, the event caused by the collapse of magnetic fields associated with the huge sunspot groups that paraded across the apparent solar disk. (The "corona" that surrounds the Sun and is seen during a solar eclipse is heated to two million degrees and supported by solar magnetism.) Though the Sun is now on the downside of its 11-year cycle, it is still capable of producing powerful magnetic activity. The ejections from the corona of the Sun hit the Earth and disturbed the magnetic field, producing northern and southern lights, and disturbing communications. More events are always possible (but impossible to predict until we see the Sun actually eject the matter), so keep your eyes on the nightly skies (though not on the Sun itself, which is dangerous to look at).

Beyond the Moon and Sun, Venus, slowly climbing the southwestern skies in evening twilight, passes north of Antares in Scorpius the night of Sunday, November 9, the planet still difficult to see. The Moon will then be seen to the northwest of Saturn the night of Wednesday, the 12th, then to the northeast of it on Thursday, the 13th. Mars, of course, still glowers to the south in the evening hours, while bright Jupiter dominates the early morning skies to the east, below the brave figure of Leo.

This is Cassiopeia season, the famed "W" appearing upside down as it crosses the meridian to the north of overhead around 10 PM, the constellation that represents the ancient Queen set within a remarkable part of the Milky Way, which can be admired once the Moon moves out of the way.

STAR OF THE WEEK: 6 CAS (6 Cassiopeiae). The Star of the Week celebrates its 300th star with a magnificent white class A (A3) supergiant, one of the most luminous stars of the Galaxy. Don't let its apparent fifth magnitude (at 5.43, near sixth) dimness fool you. At a distance of roughly 8100 light years, it is hugely far away, and partly hidden within Cassiopeia by the Milky Way's interstellar dust that dims it by 1.9 magnitudes (a factor of 5.75). Were the dust not there, the star would shine at an easily-visible near-third magnitude (3.57). The star is too dim, however, to have obtained a Greek letter name (much less a proper name), but was bright enough for John Flamsteed to have recorded it as star 6 in the constellation of the Queen. 6 Cas is much too far away for parallax measure, so we have to use indirect means. It is a member of the huge loose "Cas OB5" association of hot O and B stars (which were born at roughly the same time) that includes Rho Cas. From estimates of the distances of the members based on their spectra and thus assumed luminosities, we find the association's distance, which for any specific star becomes rather problematic. Nevertheless, it is all we have. Assuming 8100 light years for our "6," and correcting for interstellar dimming and a small bit of ultraviolet radiation from the roughly 9000 Kelvin surface, we find a magnificent luminosity of 195,000 Suns!, which yields a radius 170 times that of the Sun (0.8 Astronomical Units, bigger than the orbit of Venus), and from the theory of stellar evolution a mass of 25 times solar, all these statistics placing 6 Cas in league with such stars as first magnitude Deneb. Rotating with a minimum speed of 50 kilometers per second, the star may take as long as 170 days to make a full spin. Like most such immense stars, 6 Cas is slightly variable (by 0.09 magnitudes), and is classed as an "irregular," with no apparent period. It is also possessed of a variable strong wind that ejects matter in blobs at speeds of up to 180 kilometers per second. Such huge stars evolve fast, 6 Cas a mere 6.4 million years old. Its only fate is to develop an iron core and then to collapse and explode as a superb supernova, creating a spinning neutron star in the process, one of immense density and the size of a small town. If that is not enough, 6 Cas seems to have a companion 1.4 seconds of arc away, an eighth magnitude class A2 star, and if really a companion and not a line-of-sight coincidence, also a supergiant, but of lesser quality, one that shines with a brilliance of "only" 19,000 Suns, rendering it 10 solar masses, and on the border between producing a supernova or a rare neon/magnesium white dwarf. (Most white dwarfs are made of carbon and oxygen; the massive end of the white dwarfs have fused their stuff to heavier elements). If the companionship is real, the two are separated by at least 3500 Astronomical Units and take at least 35,000 years to orbit. Given this distance apart, from 6 Cas proper the companion would appear with the brightness of 1600 full Moons, while from the companion, 6 would shine 1/100 as bright as the Sun in our Earthly sky.

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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