TEA WITH THE SCORPION
Four o'clock in the afternoon. Ready for a break, you glide down
the ecliptic to Sagittarius's Teapot. And who
do you find for company in the next chair to keep you company? A
giant scorpion. But fear not. Since ancient times, since the gods
put him in the sky after doing in Orion,
he has harmed no one. Instead, Scorpius
and Sagittarius (to the east) bring spectacular glory to northern
summer, and even more if you can travel to southern climes.
Bright and beautiful, the two figures
are abundant in informal asterisms. Not only does the Archer pour
his own tea from the fabled Teapot, he provides the milk from his
five-star, upside-down, Little Milk
Dipper. Perhaps Scorpius is placid because Sagittarius's Bow
and Arrow point right at him, ready to repel if need be. The
Scorpion provides his own sub-patterns with the Arteries (Tau and Sigma
Sco) that surround his heart (Antares) and the Stinger at the
end of the graceful curve of his body (made of Shaula and Lesath), with which he might threaten
Between the classic constellation figures
lies the Winter Solstice, which the Sun passes at its most southerly reach to mark
the beginning of northern winter and perhaps to take some hot tea
himself. Though the modern constellation boundaries give the
Solstice to Sagittarius, Scorpius actually extends farther to the
south (the ecliptic passing rather well to the north of both
classic patterns). The farther you travel north of about 45
degrees north latitude, the more you will miss of the Scorpion's
Scorpius is best known for its luminary, first magnitude Antares, whose reddish color can confuse
the beginner into thinking that it's Mars (hence the name, "Ant-
Ares," Ares the Greek god of war). This magnificent red class M supergiant is nominally the
15th brightest star in the sky, though instability caused by huge
size (three-fifths that of the orbit of Jupiter) and luminosity
(some 60,000 Suns) can change its rank. Weighing in at 15 or more
solar masses, Antares began life only 10 million years ago as a
hot, blue class O hydrogen-fusing main sequence dwarf. Now dying,
most likely with a helium-fusing core (though the burning process
could be farther along), it does not have much time left before it
explodes as a supernova.
If it were to go tonight, the destroyed star would shine in the sky
with the light of a gibbous Moon even
though 600 light years away.
The rest of the constellation is loaded with bright blue class B
stars, many of which are related to each other through the
Scorpius-Centaurus Association, which
includes Antares and consists of at least three loose, expanding
subgroups. Take the B stars away, and the Scorpion would vanish.
Among the more fascinating of them is Dschubba (Delta Scorpii). Normally
shining at magnitude 2.3, in 2001-2002 it developed a surrounding
disk in part as a result of rapid rotation, and turned itself into
a "B-emission" star (the disk radiating bright emissions at colors
characteristic of hydrogen). By 2004, the star had reached nearly
first magnitude, which rather changed the appearance of the
constellation. After a couple years, Dschubba settled down, though
it has yet to return to normal, and may not for a long time.
Over on the other side of the tea table, Sagittarius puts the lie
to the standard notion that the "Alpha" star is always the
brightest in the constellation. Alpha Sgr (Rukbat), far to the south of the classic
figure, is a miserable fourth magnitude, while the luminary is
bright second magnitude Epsilon (Kaus
Australis, the southern star of the Bow), followed by, of all
things, Sigma (Nunki, in the Dipper).
Even the two Beta stars (Beta-1 and
2) are only fourth magnitude.
The true glory of the two constellations, however, is in their
setting within the broadest and brightest part of the Milky Way, the combined light of the billions of
stars that inhabit the disk of our Galaxy. Sadly, for those living
in moderate northern latitudes, the Earth's thick atmosphere dims
this part of the Milky Circle. The view from the southern
hemisphere, with the Archer overhead, is one of the most
spectacular sights nature has to offer, with sheets of stars
seeming to cascade downward toward the horizon like celestial
waterfalls. By amazing coincidence, the Milky Way's center-line,
the Galactic Equator, runs almost exactly through both the winter
and summer solstices. There is nothing physical in the crossings;
they are just products of our times and the 26,000-year
../celsph.html#equ">precession of the Earth's axis, which has
brought us a temporary alignment.
Set within the Milky Stream are hordes of clusters and both bright and
dark nebulae, many of which are glorious in small telescopes or
even binoculars. Just to the west of Antares, and thus easy to
find, is the pretty globular
cluster Messier 4. Containing maybe 200,000 or so stars, it
shines at roughly sixth magnitude even though 5600 light years
away. A few degrees north lies the grander globular Messier 80.
One of the densest of all such ancient collections (which go back
nearly to the time of the Galaxy's birth), the light from its
nearly half-million stars is dimmed by a distance of 27,000 light
On the other side of the Scorpion, to the northeast of the Stinger,
lies a fabulous splash of stars over a degree across. Easily seen
by eye alone, the sprawling open cluster Messier 7, 975 light years distant, is perfect for
binoculars. To the northwest of M 7 is bright, young Messier 6. Swinging eastward, Sagittarius contains one
of the brightest of globular clusters, Messier 22, visible to the
naked eye to the northeast of the Milk Dipper's handle and a
startling sight when happened upon with binoculars. "Only" 10,000
light years away, M 22 closes in on a million stars. In the
general neighborhood of Sagittarius and Scorpius, and extending
into southeastern Ophiuchus, is an
immense concentration of fainter globulars.
Sagittarius also tops out in bright gaseous nebulae (made mostly of
glowing hydrogen lit up by nearby hot stars). Foremost is the huge
Lagoon (Messier 8), an easy binocular
object roughly 5000 light years away and centered on yet another
young open cluster. Just to the northwest, and at about the same
distance, lies the Trifid Nebula, Messier 20, which is cut by dark,
dusty lanes and that, like the Lagoon, is set within dark clouds
that are actively birthing
Vastly larger dark clouds thread their way through the Milky Way's
background, the most prominent of which is the southern extension
of the Great Rift, which to the
south of central Cygnus splits the Milky
Way in two. Some of the clouds are so dark and obvious, especially
as seen from southern latitudes, that the Incas of Peru named them
as "dark constellations." The Rift and associated features are
made of myriad dusty clouds so opaque that they block out all light
from the millions of stars in back of them, in a sense throwing
their shadows toward the Earth.
The dust keeps out heating starlight, and the darkest clouds are
thus naturally cold, near absolute zero, which allows the formation
of molecules and ultimately the collapse of the gas to form stars.
Well over 100 such interstellar molecules have been discovered,
including molecular hydrogen (which dominates), water, ammonia,
methanol, ethanol, and formaldehyde, as well as several not found
on Earth. The current record is one that contains 13 atoms,
cyanodecapentayne (HC10CN). Among the most interesting are those,
such as acetic acid, that may lead to an understanding of the
formation of life.
The ancients eerily seem to be calling out to us, guiding our eye.
Follow the Archer's arrow about four degrees (a bit less than the
separation between the Big Dipper's
bowl stars) southwest and you pass just two degrees south of the
road that leads to the center of the
Galaxy, which lies just on the Sagittarius side of the border with
Scorpius. The Galactic Center, at a distance of 26,000 light
years, is vastly farther than the stars of either constellation,
and more comparable to the distances of the globular clusters.
There is so much dust in the line of sight that optical radiation
is dimmed by 30 magnitudes: one ray of light in a trillion gets
The Center was instead discovered by radio astronomers, radio waves
easily penetrating the dust. In the early days of radio astronomy,
no one knew how many discrete radio sources there were, so they
were at first named after their constellations of residence. The
first source found in Sagittarius -- in the direction of the
Galactic Center -- was thus called "Sagittarius A." Within this
extended radio patch is a point source, "Sagittarius A*," which is
without much question the true center of the Galaxy itself.
Measurements reveal it to be comparable to the size of the Solar
System, while the orbits of the stars that surround it (observed in
the transparent infrared) give a mass of more than three million
Suns. Such conditions firmly indicate the center of the Galaxy to
be a "supermassive black hole." While no light can escape from
within the beast itself, it glows around the outside with radiation
from a circulating gaseous disk.
Just a few hundred light years from the Galaxy's center lies
Sagittarius B, which divides in two as Sgr B1 and B2. The northern
part of Sgr B2 is the most productive molecule factory known in the
Galaxy. Among a huge number of other molecules, it contains
glycolaldehyde, an eight-atom molecule touted as a form of "sugar."
So when teatime comes around again, Sagittarius may not only serve
milk with your tea, but as you chat with the Scorpion, can sweeten
it for you as well (one hopes with the real thing), much as these
constellations sweeten the sky of northern summer.
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the August/December 2008 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.