Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 29, 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.
Our Moon, Earth's ancient companion, ends the year, the century,
and the millennium (technically anyway) as a growing crescent. On
the second day of 2001 it will pass its first quarter amidst the
dim stars of Pisces just to southeast of the Great Square of
Pegasus, thereafter waxing toward full. As it moves against the
starry background, it beautifully encounters the sky's brightest
planet. Tonight, Friday the 29th, the earthlit crescent Moon will
make a close pass to (falling just below) Venus, the modest star
Deneb Algedi (Delta Capricorni) roughly between the two. The Moon
will pass beneath Saturn next Friday evening, the ringed planet now
shining to the east at sundown just above and to the right of
brilliant Jupiter. The morning hours host only one planet now,
Mars, which continues to brighten among the stars of eastern Virgo
as the Earth slowly catches up with it. The red planet will not
become an evening object until the beginning of April, when it
finally rises before midnight.
Once again the Earth is on stage, as it passes perihelion with the
Sun, when it is closest to the Sun on its elliptical orbit, on
Thursday, January 4 at a distance of 147,097,500 kilometers
(91,402,150 miles). Rather obviously, since the northern
hemisphere is now in the dead of winter, the variation in distance
from the Sun -- which is not great, only about 3.5 percent -- is
not the cause of the seasons (which are produced solely by the tilt
of the Earth's axis relative to the vertical to the orbital plane).
Aphelion, when the Earth is farthest from the Sun, will take place
this year on the United States' national holiday, the Fourth of
July, when we will be 152 million kilometers (94.5 million miles)
away. All things being equal, the variation in distance to the Sun
should cause the Earth's southern hemisphere to have greater
seasonal extremes than the northern (Argentina's summer coinciding
with perihelion, winter with aphelion), but the effect is quite
lost in the inequality of land masses on Earth, most of the
moderating oceans being in the south.
The autumn stars, Pegasus, Aries, and the rest, begin their annual
flight to the west as the winter stars take over. Orion is now up
as the sky darkens. He makes his great transit across the southern
meridian around 11 PM, and serves as the centerpiece for a host of
bright stars and constellations that encircle him. The stars of
Orion, together with those of Canis Major, Taurus, Perseus, and
others, are part of a celestial ring of bright, relatively nearby
stars called "Gould's Belt" (after B. A. Gould, a prominent
nineteenth century astronomer) that tilts slightly relative to the
Milky Way, the band of light created by the disk of our Galaxy (the
portion of the Milky Way running to the east of Orion faint and
difficult to see). Happy New Year to all; may it bring health,
peace, and prosperity.
STAR OF THE WEEK. MESARTHIM (Gamma Arietis). Shining third among
the stars of the flat triangle that make the classical figure of
Aries, the Ram (for that reason gaining the Gamma designation),
Mesarthim actually takes fourth place in the constellation after
non-named 41 Arietis, which glows softly off to the northeast. The
name (derived from Arabic) originally came from the same root as
that for the Beta star, Sheratan (meaning "the two"), but was
corrupted by mistranslations into its current form. Famed from
history, Mesarthim is also called "the first star of Aries," as
during the ancient times when the stars were being systematically
organized, it was the closest of the Ram's stars to the vernal
equinox. (Precession, the 26,000 year wobble of the Earth's axis,
has since shifted the equinox westward to Pisces.) Shining at us
from a distance of 204 light years, Mesarthim is one of the classic
double stars of the sky, its two components, of nearly equal
brightness, an easily separable 8 seconds of arc apart and known
since 1664. Both actually white, the (just slightly) fainter one
(Gamma-1, since it is the more westerly) has been called "pale
grey," which is a visual contrast effect. Both fifth magnitude
(Gamma-1 4.83, Gamma-2 4.75), they combine to make a mid fourth
magnitude (3.9) star. Gamma-1, a class B (B9) dwarf, is the
hotter, its temperature 11,000 Kelvin. Gamma-2, a bit
controversial, has been classed both as an A (A1) (probably) dwarf
and as a B (B9.5) subgiant (meaning that it may be starting to
evolve), the temperature between 9200 and 9800 Kelvin. Though
Gamma-1 is a bit dimmer to the eye, it is actually the more
luminous, as the higher temperature causes more of its light to
shine in the invisible ultraviolet. Gamma 1, around 2.8 solar
masses, radiates 56 or so solar luminosities into space, whereas
Gamma-1 (around 2.5 solar masses) releases somewhere between 43 and
52 times the power of the Sun. While Gamma-1 is relatively
ordinary, Gamma-2 is an "Ap" star,the "p" standing for (spectrally)
"peculiar." It is now known that such stars are actually highly
magnetized, Mesarthim-2's magnetic field roughly 1000 times the
strength of Earth's. Like the prototype, Cor Caroli-2, the
magnetism is concentrated into zones in which it aids in the
separation of chemical elements (Gamma-2 notably high in silicon).
As the star rotates, these concentrations swing in and out of view,
allowing the rotation period to be found (in this case 1.609 days)
and causing subtle visual variations. Sophisticated spectroscopic
examination and measures of Doppler shifts allow astronomers to
create "pictures" of the surfaces of such stars, Gamma-2 having the
distinction of being the first to be so treated. The stars of the
pair are separated by at least 500 astronomical units (over a dozen
times Pluto's distance from the Sun), and take at least 5000 years
to orbit each other.
****************************************************************
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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