THE ARMORED BOX
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.