Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, May 4, 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 approaches full the early part of the week, passing that
brilliant phase on Monday May 7th among the dim stars of Libra,
thereafter waning through gibbous, rising after sunset. Sending so
much sunlight back to Earth, the full (or near-full) Moon is so
bright that the surrounding stars are all but invisible.
In eastern dawn, Venus also passes through its greatest possible
brightness, on Friday the 4th, the planet shining brightly amidst
the equally dim stars of Pisces and very close to the vernal
equinox, the point on the celestial equator passed by the Sun on
the first day of spring. A view through the telescope shows Venus
as a large crescent. Though we see only a small section of Venus's
daylight side, its proximity to us still throws a maximum amount of
sunlight in our direction. The observations of Venus's phases was
one of the key proofs to Galileo that Copernicus was right, that
the Earth really does go about the Sun.
Western twilight holds the disappearing treasure of bright Jupiter,
Saturn, an hour to the west and in a bright sky, difficult to view.
If you can find it, perhaps you can also find Mercury down and to
the right early in the week. Jupiter will soon follow into near-
invisibility. Brightening Mars, however, makes up the difference,
as it now rises in Sagittarius around 11:30 PM Daylight Time.
Mimicking Venus as a celestial marker, Mars now lies only a couple
degrees above the center of the Galaxy, the center of the Milky
Way.
The range of celestial brightness is astonishing. As bright as
Venus may appear to us, the full Moon is 1000 times brighter, and
the Sun a million times brighter yet (so bright as to be able to
damage the eyes). Venus, however, is at maximum 15 times brighter
than the brightest star, Sirius, and a remarkable 25,000 times
brighter than the faintest star we can see without optical aid.
With telescopes the range is even more astonishing: the Hubble
Space Telescope can go another 2.5 billion times fainter!
Two other items of note include Neptune, which begins retrograde
motion in Capricornus on Thursday the 10th. The most distant large
planet, Neptune is now 90,000 times fainter than Venus, and
requires a telescope to see. The bright Moon will unfortunately
wash out one of the year's better meteor showers, the "Eta
Aquarids," which will peak the morning of Sunday, May 5. The
debris of Halley's Comet, the shower typically produces some 30
meteors per hour in a dark sky.
In spite of the Moon's brightness, look to the south in late
twilight to find the great figure of northern spring, Leo the Lion,
the bright star Regulus at the end of the "Sickle" that makes the
great beast's head.
STAR OF THE WEEK. PRAECIPUA (46 Leonis Minoris). While Leo is
easy to admire, its modern-constellation counterpart, faint Leo
Minor, which rides the back of the Zodiac's King of the Beasts, is
not. Leo Minor, the "Lesser Lion," is so dim that few bother with
him, and is so unimportant a constellation that no one would think
of making Leo into "Leo Major," thus raising Leo Minor's rank.
Because the modern constellations were invented long after proper
names were assigned, most well after Bayer assigned Greek letters
(and to some degree were carved from other constellations), the
names of their stars can be a bit of a mess. Only one star in Leo
Minor carries a Greek letter name, and that is "Beta;" there is no
Alpha, and Beta Leonis Minoris is not even brightest, coming in
second. The little constellation does have its charm however,
mostly in a flat quadrilateral with the constellation's luminary,
bright fourth magnitude (3.83) 46 Leonis at the eastern end (the
"46" a Flamsteed number, the only Flamsteed number whose star ranks
"number 1" in a constellation). Leo Minor is also one of the few
modern constellation whose brightest star carries a proper name, 46
Leo Minoris also called "Praecipua," or "Chief," a "modern" term
from Latin telling that "46" is the brightest star. Praecipua is
otherwise ordinary, an orange class K (K0) giant-subgiant with a
temperature of 4690 Kelvin. At a distance of 98 light years, it is
not quite up to average giant brightness, radiating 32 solar
luminosities into space, from which we derive a modest diameter
(for a giant) 8.5 times that of the Sun. A star of around 1.5
solar masses, once a hydrogen-fusing cool class A star, it is now
evolved, and is quietly fusing helium to carbon in its core. The
star is known to be somewhat metal poor compared with the Sun, its
iron content down by about a third. Of most interest perhaps is
how well we know it. Recent accurate measures of angular diameter
by the Navy Interferometer show it to be 0.00254 seconds of arc
across (the separation of car headlights seen from a distance of
80,000 kilometers, 20 percent of the way to the Moon), which gives
it a physical diameter 8.2 times that of the Sun, the agreement
with the previously calculated diameter showing that we know the
size, temperature, luminosity, and distance very well.
****************************************************************
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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