By Jim Kaler

Outside of the deep southern hemisphere, in the realm of the sky where the ancient Greeks and their predecessors held forth, the modern constellations (invented between roughly 1600 and 1800) are drab and hard to find. Try pointing out Lynx, Camelopardalis, or Vulpecula to the neighbors. A notable exception (which comes with a caveat) is the celestial Shield, northern summer's Scutum. One of the few nationalistic creations to survive (one thinks of the unmourned demise of Charles II's "Robur Carolinium"), Scutum -- originally "Scutum Sobiescianum" -- honors John Sobieski, the Polish king who held off the Turks in their attempt to take Vienna in 1683. The constellation itself, invented a year later by Hevelius, is unremarkable. Even the luminary, Alpha Scuti (a common class K3 giant 199 light years away), is but fourth magnitude. What Scutum DOES have is its setting. To get the figure's full glory, we have to look to the Milky Way and (here's the caveat) include a bit of the southwestern corner of Aquila.

Scanning to the south from Altair toward Sagittarius, we encounter a striking curl of stars, a half-oval Tiara (behold a new asterism!) with third- magnitude Lambda Aquilae (a B9 dwarf 125 light years distant) as its diadem and whose southwestern end is marked by fourth magnitude Beta Scuti, a magnificent G5 bright giant and spectroscopic double 920 l-y off that shines with the radiance of 3100 Suns. To the south of Beta, more or less framed by Beta and Alpha, is a prominent patch of the Milky Way, a shield of sorts on its own that immediately draws the eye and that seems to guard the Tiara. The intimate relation between Aquila and Scutum is re-enforced by Scutum's five northern Greek-lettered stars, all of which carry Flamsteed numbers that belong to Aquila (Alpha Sct being 1 Aql, Beta 6 Aql), as England's famed astronomer never recognized Hevelius's creation. The connection goes even deeper by noting that Lambda Aql -- the Tiara's jewel -- is also the southwestern anchor of the now-defunct constellation Antinoüs, companion to the Roman emperor Hadrian, who is seen in Bayer's Uranometria as being held aloft by the Eagle.

The region's real treasure, though, lies in "The Box," a 6° X 8° (east-west) rectangle that is defined by the Tiara in its northeastern corner. One of the more heavily-populated of heavenly realms, in addition to Lambda Aquilae, Alpha and Beta Scuti, and the Shield's bright Milky Way, the Box contains six NGC-named open clusters (two of them Messier objects) with a seventh just a bit to the south, four fine variable stars (two visible to the naked eye), two planetary nebulae (one quite well-known), and a globular cluster (but no partridge in a pear tree).

Look first to R Scuti, which helps define the Tiara's southwestern extension. This strange class K supergiant, which because of its orange-red color stands out from its environment, varies from fifth to eighth magnitude (whence it temporarily leaves the Tiara) over a regular 142-day period, with shallow minima that take place in the middles of the maxima. It seems to be related to the class of "RV Tauri" stars, which are luminous versions of Cepheid-like pulsators that belong to the Galactic halo. Shining from a distance of 870 light years with an uncertain luminosity of 1500-2000 Suns and with a radius of 60 solar, at minimum R Sct turns itself into red class M star. It's entertaining to watch it come and go.

Far better known is Delta Scuti, which defines a whole class of "Delta Scuti stars." They are lower-mass, mid-temperature, somewhat-evolved versions of classic Cepheids (formed within classes A and F where the so-called "instability strip" of the HR diagram nears the main sequence). Unlike Cepheids, they typically vary by a tenth of a magnitude or so over periods of much less than a day. REALLY unlike Cepheids, they pulsate with multiple periods. Delta Sct itself varies by up to 0.2 magnitudes with a main period of 4.48 hours along with sub- periods of 2.3. 2.8, 2.9, and 20 hours all going on at the same time, making for messy disentanglement.

Largely unsung is V Aquilae, a semi-regular variable that, while lying on the Tiara about a degree east-southeast of Lambda, requires binoculars as it varies between magnitudes 6.7 - 7.7 with a 353-day period. You won't need to strain to find it, as V Aql is a relatively rare deep red "carbon star" 1200 light years away, the color the result of a very high abundance of carbon, whose molecules take out most of the blue portion of the spectrum (V's blue "photographic magnitude" 11 to 12). Such stars are in the final throes in their existence as advanced giants with dead carbon cores that are about to eject their outer envelopes to create planetary nebulae and turn themselves into white dwarfs. V Aql, a quarter again the size of Mars's orbit, lights the sky with a luminosity of perhaps as much as 20,000 Suns, its heavy carbon content the result of the internal fusion of helium to carbon, the latter dredged up from below by convection. Then, back over in Scutum is S Scuti, yet ANOTHER similar semi-regular variable carbon star at the southeastern edge of the patch of bright Milky Way, this one varying between about magnitude 7.0 and 8.0 over 148 days.

The open clusters -- all in Scutum -- are led by the brightest of the lot, Messier 11 (NGC 6705, the oddly named "Wild Duck Cluster"). Lying along the south side of the Tiara 6200 light years away, compact (just 20 light years wide), and filled with colorful giants, M 11 is a spectacular sight in a small telescope. With an age of 200 million years, it is also the oldest of the gang. M 26, at the Box's southern edge and not as bright, lies 1600 light years from us. With an estimated age of 73 million years, it's the second oldest. Of the remainder (NGC 6644, 6683, and 6704, and 6882, plus the outlier NGC 6649), NGC 6704 is at 9700 light years the most distant, NGC 6644 the closest (3800 l-y), NGC 6683 at 10 Megayears the youngest. NGC 6649 has the honor of being the most heavily dimmed by interstellar dust (which gives such character to the Milky Way), appearing nearly four magnitudes fainter than it would were there nothing in the way.

Of them all, NGC 6882 has perhaps the most singular distinction. Rather like NGC 2129 in Gemini and a few others, it does not actually exist! Which brings the cluster-count back to six. It takes more than a lump of stars to make a cluster. Its members must also have similar distances and motions. Surely the most infamous of non- clusters is "The Coathanger," Brocchi's Cluster, a tight group of nine stars in Vulpecula that have nothing to do with one another except to fool beginning observers.

Given the Sun's age of 4.6 billion years, where are all the OLD clusters? Fragile, easily ripped apart by tidal interactions with massive molecular clouds, for the most part they do not live very long as they scatter away their stars. The only place we find the elderly is more toward the Anticenter of the Galaxy, where the stars and interstellar gases thin out and rare clusters can survive for up to 10 billion years, whereupon they yield the age of the Galactic disk.

Older still is the Box's lone globular cluster, NGC 6712. While no Messier 13 (or M3 or M5 for that matter), it is not without interest. Vastly farther than any of the open clusters, 21,000 light years away, it contains between 100,000 and 200,000 stars, enough so that even with some interstellar dust- dimming, it makes it to 8th magnitude (though its spread-out nature makes it appear fainter). Members of an extended Galactic halo, formed in the early Galaxy shortly after the Big Bang, globular clusters are among the oldest objects known, with ages of some 12 billion years. Since they were born before the winds and explosions of evolving stars could build up much in the way of heavy elements to seed the star-forming gases of interstellar space, globulars have low metal contents, typically a few hundredths that of the Sun (with even lower extremes). NGC 6712 is, for globulars, metal-rich, having a tenth the solar metal-count, making it perhaps somewhat younger than the rest.

That leaves us with the two planetary nebulae. Planetaries, discovered in the late eighteenth century by William Herschel, are bright ionized shells and rings of gases that surround hot central stars. The nebulae are the compressed ejecta of dying advanced giant stars, while their central stars, nascent white dwarfs, are the progenitors' ex-nuclear-burning cores. The fainter and larger of the pair is IC 1295, in Scutum immediately to the east of NGC 6712. Appearing as a faint ring about minute of arc across, little is known about it. A very uncertain distance of 4000 light years would give it a diameter of about a light year. Much more prominent, enough so to have made it a Hubble target, is NGC 6751 in Aquila, which lies a bit over a degree to the south of Lambda Aql. Perhaps twice as far as IC 1295 (perhaps not, as distances to planetaries are notoriously poor), the diameter could be almost as large as IC 1295's. The Hubble image shows a bright partial ring whose interior is filled with spectacular radial filaments, the whole affair surrounded by a ragged envelope. The illuminating central star, with a temperature of 76,000 Kelvin, far hotter than any main sequence dwarf, appears to be in a heating phase, the old stellar core having been revealed not all that long ago, our Box -- tied with a ribbon of Milky Way and armored by a warrior's Shield - - filled with the treasures of evolving stars.

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 2011 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.