By Jim Kaler

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 Sagittarius.

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 curved tail.

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 years.

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 stars.

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 through.

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.