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Re: [ISTATALK-L] Skylights
On Sat, 23 Oct 2004 17:13:22 -0500
Jim Kaler <kaler@xxxxxxxxxxxxxx> wrote:
>Skylights, University of Illinois Department of Astronomy.
>Astronomy News for short week starting Saturday, October 23, 2004.
>Phone (217) 333-8789.
>Prepared by Jim Kaler.
>Find Skylights on the Web at
> http://www.astro.uiuc.edu/~kaler/skylights.html,
>and Stars (Stars of the Week) with constellation photographs at
> http://www.astro.uiuc.edu/~kaler/sow/sow.html.
>
>See "The StarGazer" at a planetarium near you: visit
> http://www.astro.uiuc.edu/~kaler/sg.html
>
>Interested in Astronomy and Astro Education? Join the Astronomical
>Society of the Pacific (an international organization) to get the
>outstanding astronomy magazine Mercury and a variety of other
>benefits. Call 1-415-337-1100, then press 1.
>
>"Vault of the Heavens: Exploring the Solar System's Place in the
>Universe," an accessible astronomy course on audio CD with 100 page
>study guide narrated and written by Jim Kaler, is now available at
>Barnes and Noble in stores and on line.
>
>The Moon starts its week in the waxing gibbous phase as it heads
>toward full, the phase reached the night of Wednesday, October 27,
>when there will be a magnificent lunar eclipse that is beautifully
>timed for continental North America.
>
>On that night, the Moon will pass just to the north of the central
>part of the Earth's shadow. The "penumbral phase" (when the Moon
>falls into partial shadow of the Earth) is only barely visible and
>is here ignored. The Moon begins to enter total shadow (called the
>"partial eclipse phase" because only part of the Moon is obscured)
>at 8:14 PM CDT. (Add an hour for EDT, subtract an hour for MDT, 2
>hours for PDT; subtract another hour if you are on standard time).
>The eclipse then becomes total at 9:23 PM CDT. Mid-eclipse, when
>the Moon is as dark as possible, falls at 10:04 PM CDT. The events
>then reverse, with the Moon beginning to leave the total shadow at
>10:45 PM CDT, the show over at 11:54 PM CDT. While Alaskans will
>see the eclipse, Hawaiians will see only the last parts of it.
>
>Since the Earth's atmosphere scatters sunlight into the realm of
>total shadow, the Moon does not become completely dark during
>totality. The brightness of the eclipsed Moon depends on the state
>of the terrestrial atmosphere, particularly on the degree of
>volcanic activity prior to the eclipse, as dust and aerosols dim
>the amount of light getting through. The colors on the Moon can be
>quite lovely, especially if the eclipse is viewed with binoculars
>or a telescope.
>
>As anticlimax, Neptune, deep in Capricornus, ends retrograde motion
>on Sunday the 24th. The morning sky near dawn does much better
>with the planets, where brilliant Venus clearly rules. However,
>look below Venus to find bright Jupiter, which is now clearing the
>horizon in modest twilight. In the other direction, to the west,
>find Saturn, which is now rising just after 11 PM Daylight Time.
>Though still in Gemini, Saturn is very close to the border with
>Cancer, and is nicely pointed to by Gemini's Pollux and Castor.
>
>As the Moon dims into totality, watch the stars come back out.
>Since the Sun is now well past the autumnal equinox, the fully
>eclipsed Moon will be the same degree to the east of the Vernal
>Equinox, and south of the classic figure of Aries. To the
>southwest lies Neptune's Capricornus, one of the dimmer Zodiacal
>constellations, while the Summer Triangle of Deneb, Vega, and
>Altair moves off to the northwest.
>
>STAR OF THE WEEK. ZETA CAP (Zeta Capricorni). Dipping into the
>eastern lower half of mostly faint Capricornus, find the modest
>fourth magnitude (3.74) "yellow" class G (G4) supergiant ("yellow"
>referring not so much to actual color but to the part of the
>spectrum where much of its light emerges) Zeta Capricorni. The
>"supergiant" tag implies that the star must be fairly distant to
>shine so faintly, and sure enough, it measures 122 light years
>away. The temperature of 5050 Kelvin (not that much cooler than
>our Sun) allows for a small correction for some infrared light,
>which with distance yields a luminosity 490 times that of the Sun,
>and a resulting radius 29 times solar. These are distinctly
>smallish values for a star called "supergiant," and more like those
>of a "bright giant." Since the term comes from simple examination
>of the spectrum, however, it remains in use, showing that some
>stars are not quite what they are at first made out to be.
>Combination of temperature and luminosity suggest a star of about
>four solar masses, which makes it under 200 million years old, one
>that is now most likely fusing helium into carbon in its deep core
>and will "soon" (on an astronomical timescale) begin to brighten
>once again to close to real supergiant status. The interest in
>Zeta Cap lies not in these characteristics, however, but in its
>chemical composition and in its classification as a "mild barium
>star," one rich in that heavy element, and rich in other elements
>as well that are created under high temperature conditions through
>capture of neutrons (Zeta Cap enriched in such elements as
>zirconium and the "rare earths" by a factor of 10). In some stars,
>chemical enrichment comes from interior nuclear reactions, the
>newly-formed elements raised upward by convection. In others,
>however, they come from contamination from a companion that has
>since died and left a legacy of its own nuclear reactions brought
>to the current giant star through mass exchange. Zeta Cap and all
>other barium stars (epitomized in the naked-eye sky by Alphard)
>fall into the latter category, the earlier (once more massive and
>brighter) star now reduced to a dead cinder, a white dwarf. While
>the white dwarf is unseen, measures of Zeta Cap's spectrum (through
>the Doppler effect) reveal a companion in a 6.5 year period at a
>separation of around 6 Astronomical Units. Another "companion," a
>12th magnitude star at a separation of some 20 seconds of arc, is
>also a white dwarf, but the alignment is merely a line-of-sight
>coincidence, again showing that not all things are what they at
>first seem to be.
>
>
>
>****************************************************************
>Jim Kaler
>Professor Emeritus of Astronomy Phone: (217) 333-9382
>University of Illinois Fax: (217) 244-7638
>Department of Astronomy email: kaler@xxxxxxxxxxxxxx
>103 Astronomy Bldg. web: http://www.astro.uiuc.edu/~kaler/
>1002 West Green St.
>Urbana, IL 61801
>USA
>
>Visit: http://www.astro.uiuc.edu/~kaler/ for links to:
> Skylights (Weekly Sky News updated each Friday)
> Stars (Portraits of Stars and the Constellations)
> The StarGazer (a new planetarium show)
>*****************************************************************
>
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