Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 22, 2000
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.
The Moon wanes through its crescent phase early in the week,
passing new on Monday, the 25th, Christmas day. By the night of
Tuesday, the 26th, Boxing Day in Canada in the UK, it will be just
visible as a waxing crescent in the southwest shortly after
sundown. Two days later it passes its apogee, its farthest point
from Earth.
Venus, in the far southwest, grows ever more lustrous, while
Jupiter, on the other side of the sky, to the east, climbs yet
higher in early evening, Saturn up and to the right. Venus will
pass 1.3 degrees north of Uranus the night of Saturday, the 23rd.
The event will be visible in binoculars and will provide a
convenient way of finding the dim planet, the seventh out from the
Sun. Mercury makes less-visible news as it passes superior
conjunction with the Sun (on the other side of the Sun) also on
Christmas Day, just two hours after new Moon.
Overwhelming all, however, is a partial eclipse of the Sun that
will be visible on Christmas Day, Monday the 25th, throughout North
America. Usually, the new Moon passes above or below the Sun; this
time new Moon occurs while the Moon is near crossing the ecliptic
(the solar path), so it will pass across the solar disk. The event
is actually a polar partial eclipse, but with the Earth's northern
axis now turned away from the Sun, it centers over northern Canada.
Nowhere will it be total. Time of occurrence depends mostly on
time zone and longitude. In the central US, it will begin around
9:45 AM (CST), be at maximum around 11:15, and end about 12:50 PM.
In the New York City area, the times are 11:10 AM (EST), 12:45 and
2:20 PM, while near San Francisco they are 7:35 AM (PST), 8:20 AM,
and 9:15 AM. The coverage of the Sun by the Moon depends more on
latitude: the farther north of the central US, the better the
event. Chicago and New York see 55 percent. The west coast does
not fare so well, however, with only 19 percent coverage in San
Francisco. Please do NOT try to see the eclipse directly, as any
exposure of the eye to brilliant sunlight can cause retinal burns
and permanent damage. Instead, simply make a pinhole in a piece of
cardboard and PROJECT the image onto a piece of paper. (Do NOT
look through the pinhole!) Such pinhole projection produces a
beautiful -- and safe -- optical image. You can also merely stand
under a leafed tree (admittedly hard to find in winter), and see
the eclipse projected on the ground through the pinhole spaces
between the leaves (or pine needles).
The constellations of the zodiac this time of year brighten from
southwest to northeast, from dim Capricornus (through which Venus
is now sweeping), Aquarius, and Pisces, to the brighter flat
triangle that makes Aries, and into Taurus (which now holds Jupiter
and Saturn) and finally Gemini, the most northerly of them and the
container of the Summer Solstice. Take heart, the Sun will be
there in less than six months. A good holiday season to all.
STAR OF THE WEEK. SHERATAN (Beta Arietis). The most prominent
part of Aries, the Ram, historically the "first" constellation of
the Zodiac (as it held the Vernal Equinox in ancient times), is a
thin flat triangle of stars that Bayer lettered (from east to west)
Alpha (Hamal), Beta (our Sheratan), and Gamma (Mesarthim). The
first two are also in order of brightness, just-barely-third
magnitude (2.64) Sheratan ranking second behind Hamal. The name
originally referred to both Sheratan and Mesarthim, and evoked "two
things" that have been lost to time, the name now applied to Beta
Arietis alone. At first, the star looks like a very ordinary,
white, mid-class A (A5) main sequence dwarf, one in the normal
process of fusing hydrogen to helium in its core. At a distance of
60 light years, and with a temperature of 8200 Kelvin, it is
pumping 22 solar luminosities into space. However, it keeps a
secret from the eye, a companion that has been known for a century
and that is visible only by means Doppler motions in the spectrum
(that detect line of sight movement). While such discoveries are
not at all unusual, Sheratan stands out as a result of the
extremely high eccentricity of the orbit (0.88), the companion
trapped in a record-holding elongated path. Moreover, the star is
an observational treasure. The two stars are so close together
that they cannot be separated directly through the telescope; all
we ever actually see is one star (again common, as to allow
detection via the spectrum requires the stars to be close and
moving quickly). However, sophisticated observation of Sheratan
with an interferometer, a device that makes use of the interfering
properties of light to resolve ultra-fine detail, allow (as for the
brighter component of Mizar) the pair to be resolved. The masses
of the stars (through gravitational theory) can then be measured
with high accuracy. Averaging 0.64 Astronomical Units apart (89
percent Venus's distance from the Sun), a star with the mass of the
Sun (1.02 solar) orbits a double-solar-mass (2.00) star every 107
days. Since luminosity is very sensitive to mass, 95 percent of
the light of the system is produced by the heavier star. The huge
eccentricity adds the spice. As they wheel around each other, the
smaller one (undoubtedly a class G star like the Sun) approaches as
close as 0.08 AU (only 20 percent Mercury's distance from the Sun),
and then half an orbit later loops around at 1.2 AU, 16 times
farther away and 20 percent farther than Earth from the Sun. No
close planets could survive the gravitational onslaught. Such
stars, in which the doubling is "visible" by two techniques (only
about 40 are known, Sheratan one of the brighter), allows accurate
assessment of the theoretical relation between stellar mass and
luminosity, and provides powerful evidence that the theory is
correct. The higher mass star will die first. In a couple billion
years, the lower mass G star will be the king of the pair, while
the current luminary will be a shrunken dim white dwarf.
****************************************************************
Jim Kaler
Professor of Astronomy Phone: (217) 333-9382
University of Illinois Fax: (217) 244-7638
Department of Astronomy email: [EMAIL PROTECTED]
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)
Astronomy! A Brief Edition (links and updates)
*****************************************************************
--
This is the CPS Science Teacher List.
To unsubscribe, send a message to
<[EMAIL PROTECTED]>
For more information:
<http://home.sprintmail.com/~mikelach/subscribe.html>.
To search the archives:
<http://www.mail-archive.com/science%40lists.csi.cps.k12.il.us/>
