Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 30, 2001.
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.
We begin the week with the Moon two days day shy of first quarter,
the phase reached on Sunday, April 1st (no fooling!), the lunar
disk to the east of Jupiter. Four days later, in its waxing
gibbous phase, the Moon passes perigee, when it is closest to the
Earth and angularly largest. Many are the lunar cycles. The
period of the phases (the synodic period, that relative to the Sun)
is 29.531 days, from which comes our calendrical month. The period
relative to the stars (the sidereal period), the true orbital
period, however, is 27.322 days: the time the Moon takes to go from
one constellation, around the sky, and back. The elliptical lunar
orbit rotates, which moves the point of perigee forward (it goes
all the way around in 8.85 years), so the period from one perigee
to the next (the "anomalistic month") is a bit longer, 27.555 days.
The lunar orbit is also tilted relative to the Earth's orbit.
Twice a month the Moon crosses the ecliptic (the apparent solar
path) at the "nodes." The lunar orbit wobbles, causing the nodes
to regress and to move around the ecliptic oppositely from the
motion of perigee over a period of 18.61 years. The lunar period
from one node and back to that node (the "draconic month" of 27.212
days) is therefore somewhat shorter than the sidereal period.
(Since eclipses can take place only when the Moon is near a node,
the draconic month is important in their prediction).
We also begin the week with Venus just barely past inferior
conjunction with the Sun, leaving something of a hole in the
western sky, the familiar glow of the planet now gone. Venus is,
however, already becoming visible in the eastern morning sky, where
it will reach full brilliance in about a month. For the moment, we
are left with Jupiter and Saturn, which still shine brightly to the
west in early evening, Saturn setting around 10 PM, bright Jupiter
an hour later, leaving us with no planets to see at all. But do
not despair, as Mars now begins to encroach on the evening sky, the
red planet, in deep southern Ophiuchus to the east of Antares in
Scorpius, rises about midnight.
The richness of the winter sky is escaping to the west. Just to
the east of Orion runs the Milky Way, the disk of our Galaxy. As
the spring constellations rise, we look perpendicular to the
Galactic plane and find far fewer stars. As Ursa Major,
represented by the Big Dipper, passes nearly overhead (in mid-
northern latitudes) around midnight, look about 30 degrees to the
south to see the lacy sprawl of stars that makes the Coma Berenices
star cluster, the constellation that holds the Galaxy's north pole.
Though the stars are fewer toward the Galactic pole, the lack of
obscuring interstellar dust allows a view of countless distant
galaxies.
STAR OF THE WEEK. HAEDUS I (Zeta Aurigae). Southwest of Capella
(the "she-goat) in Auriga lies a neat triangle of stars, the
"Kids." In older tradition, the top star (Almaaz) is excluded, the
term belonging to the bottom two, in Latin known as the "Haedi,"
the eastern (and faintest of the set) one now called Haedus I, the
western Haedus II. Starting at the north and going clockwise,
Bayer named the three in alphabetic order Epsilon, Zeta, and Eta
Aurigae. At bright fourth magnitude (3.75), Haedus I is the
fainter of the pair (indeed of the three), though only because it
is -- at a distance of 850 light years -- the farther. (Haedus II
is 220 light years away, Almaaz 2000 light years, the three Kids
only a line-of-sight coincidence.) Quite the brilliant star,
Haedus I is visually 1700 times more luminous than the Sun. Its
spectrum, however, shows that it is not one star but two in mutual
orbit, an orange class K (K4) supergiant (or bright giant) and a
hot blue class B (B5) star circling each other every 972.183 days
(2.66 years). Our delight in the system lies not in the duplicity,
but in the orientation of the orbit, which lies within 3 degrees of
the line of sight. As a result, the stars of Zeta Aurigae eclipse
each other. Every 2.66 years, the smaller but still-bright B star
hides completely behind the larger, cooler K star, and the combined
visual light drops by 0.15 magnitudes (about 15 percent), not much,
but noticeable to the practiced eye. (Coincidences abound in
astronomy: though the Kids have nothing to do with one another, two
of them are eclipsing binaries, the other Almaaz.) Detailed
analysis of the eclipse and of the velocities of the stars tell
their stories. Averaging 4.2 Astronomical Units apart, the two go
around each other in an elliptical orbit that takes them from 5.9
AU to 2.5 AU. The K star's mass is 5.8 times that of the Sun, the
radius 148 solar (about the size of Venus's orbit), the temperature
3950 Kelvin, and the luminosity 4800 solar. The B star's
parameters are mass 4.8 solar; radius 4.5 solar; temperature 15,300
Kelvin; luminosity 1000 solar. The total luminosity of the system
of 5800 solar is greater than the visual luminosity because the B
star produces much of its radiation in the ultraviolet, while the
K star radiates significantly in the infrared. From comparison
with theory, the pair was born 80 million years ago. Each will
ultimately be converted to a massive white dwarf, the K star now
well on its way.
****************************************************************
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)
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