Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 16, 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.
The Moon is now on something of a Friday cycle, the third quarter
reached today, Friday, March 16. The cycle is soon to be broken,
however, as the phase period of 29.5 days slightly exceeds the 4-
week period of 28 days, causing the next new Moon to take place on
Saturday, March 24th. As the lunar disk approaches new, its
crescent wanes toward the eastern dawn sky.
The morning sky is filled with dim and barely accessible planets.
On the morning of Sunday the 18th, the Moon will pass in front of
(that is, occult) the largest asteroid Ceres, the event visible
across much of the Atlantic Ocean, Africa, and the middle east (but
not, unfortunately, in the Americas). It then passes Neptune the
morning of Tuesday the 20th and Uranus the following day, finally
passing south of Mercury (now being lost in morning twilight) on
Thursday, the 22nd. Not to be left out, distant Pluto, to the west
of Uranus and Neptune, begins its retrograde motion on Sunday, the
18th. The evening sky remains glorious, however. Though the Sun
is now rapidly catching up with Venus, the brilliant planet -- to
the west at sundown -- still does not set until after the end of
evening twilight. Higher in the sky, Saturn and bright Jupiter
march easterly through the bright stars of Taurus.
But it is the Earth and Sun that really make the news, as the Sun
passes the vernal equinox in Pisces, crossing the celestial equator
at 7:31 AM Central Standard Time (8:31 Eastern, 5:31 Pacific Time)
on Tuesday, the 20th, near the time of sunrise. That day the Sun
will rise due east and set due west (very noticeable for those with
east-west roads), the Earth's rotation axis will be perpendicular
to the line to the Sun, and the days and nights will be
approximately equal at 12 hours each. The Sun also technically
rises at the north pole and sets at the south, beginning 6 months
of north polar daylight, 6 months of south polar night. These
times and events are, however, slightly modified by the facts that
the Sun is an extended disk and that refraction in the Earth's
atmosphere raises the Sun a bit, rendering the equinox day slightly
longer than 12 hours and causing the polar Sun to have already
risen.
With the advent of spring we also begin to say farewell to the
winter constellations (Orion, Canis Major and their cohort), hello
to the spring, Leo and the Big Dipper now climbing the sky in mid-
evening, orange Arcturus glowing brightly in the east. Just down
and to the right of Regulus in Leo, between Regulus and Procyon,
see if you can find the circlet of dim stars that makes the head of
Hydra, the Water Serpent (part of the Jason myth), the longest
constellation of the sky, which stretches below both Leo and Virgo,
finally ending near Libra.
STAR OF THE WEEK. TUREIS (Rho Puppis). Within the great ship
Argo, in which Jason sailed to find the golden fleece, is a group
of stars that represents a "little shield." The term was
erroneously applied in Greek to the star Aspidiske (Iota Carinae)
and then in Arabic to the star we now know as Rho Puppis (Carina
the hull of the ship, Puppis the stern). Lying rather prominently
to the west of Wezen in Canis Major, Tureis shines at an easily-
visible third magnitude (2.81), and is one of the most northerly of
the brighter stars of the constellation. From its distance of only
63 light years, this yellow-white class F (F6) giant star radiates
6 times more energy than the Sun from its 6540 Kelvin surface. As
a giant with a mass about 1.5 times that of the Sun, it has
recently (or will soon) shut down its internal hydrogen fusion.
Tureis, otherwise quite ordinary, makes its mark as one of the
sky's brightest "Delta Scuti" variable stars. The Delta Scuti
stars represent the low-luminosity tail of the bright giant and
supergiant Cepheid variables that in turn are represented so well
by Mekbuda in Gemini. Having lower masses, luminosities, and radii
than classical Cepheids (Tureis only twice the solar size), they
pulsate subtly and quickly. Tureis changes by only about 10% over
a precisely known period of 0.14088143 days (3 hours 22 minutes 52
seconds). Far lesser variations make the star pulsate at 0.13 and
0.16 days as well. The variations (which influence the spectrum)
once led us to believe that the star had a close companion, but
that is no longer believed. Though Tureis is only 6 times brighter
than the Sun, it is anomalously classed as a far grander "bright
giant," which for unknown reasons is typical of Delta Scuti stars.
Among the Delta Scuti crowd, Tureis closely sets the low-
temperature limit. While having no close companion, it does seem
to have a distant encircling 14th magnitude neighbor about which
little is known. Lying at least 570 astronomical units away (14
times Pluto's average distance from the Sun), this (probable) class
M5 red dwarf (similar to Proxima Centauri) takes a minimum of
10,000 years to orbit, if in fact the two are connected at all.
****************************************************************
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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