The Caldwell Objects

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about 11° — roughly the width of a fist held at
arm's length. NGC 6934 is about as far south of
Epsilon (ε) Delphini, the Dolphin's tail, as NGC
7006 is east of Gamma (γ) Delphini, the Dolphin's
nose (roughly 3¾°). The clusters have similar
integrated spectral types (F7 and F6, respectively)
and both display highly concentrated cores. NGC
6934 measures 118 light-years in diameter, while
NGC 7006 is some 17 percent larger. Yet NGC
6934 is the brighter of the two clusters by a factor
of 5. The reason for this apparent paradox is
simple; at a distance of 57,000 light-years, NGC
6934 is a little less than half as distant.
NGC 6934 remained anonymous for billions
of years, until William Herschel turned his
telescope to it in 1785 and became the first human
(on Earth, anyway) to perceive its primitive light.
Herschel called NGC 6934 a "beautiful object"
and found that it could be easily resolved at the
eyepiece. But here is the historical rub. Resolving
an object into stars had a different meaning and
significance in Herschel's day than it does now.
The degree to which a system could be resolved
was not as important to Herschel as the mere fact
that it could. Herschel made no distinction
between open and globular clusters. In fact, he
placed NGC 6934 into his Class I (bright
nebulae), which included objects such as NGC
7331 (Caldwell 30), a spiral galaxy in Pegasus,
and NGC 4697 (Caldwell 52), an elliptical galaxy
in Virgo. Of course, Herschel did not know the
true nature of any of these objects. He believed
that all nebulae were collections of individual
stars, and that resolving those stars was simply a
matter of telescope size and power. Herschel's
son, John, promulgated this theory, as Adm.
William Henry Smyth notes in his Cycle of
Celestial Objects: "[NGC 6934] is a mass of very
small stars, and is therefore installed among Sir
John Herschel's test objects for trying the
188
space-penetrating powers of telescopes."
John Herschel was more prophetic than he could
have imagined. Even today NGC 6934 remains a
proving ground for the world's most powerful
telescopes. The stars in NGC 6934's core are so
tightly packed that for years even the thin
atmosphere above Mauna Kea blurred them to
the point of overlapping. Only when the 8.1meter
Gemini North telescope demonstrated its
optical prowess in June 1999 did matters change.
The latest of Mauna Kea's many superlative
instruments, Gemini North "snapped" a few
images of NGC 6934 with an adaptive-optics
camera helping to compensate for atmospheric
instability. The images were astoundingly sharp,
showing individual stars at the cluster's very
heart. As it turns out, Gemini North produced
star images only 0.08" across, and that's about as
good as an 8-meter-class telescope ever can do at
the near-infrared wavelengths that were used.
The feat was comparable to a telescope in New
York City resolving the headlights of an
automobile in Mexico City, 2,000 miles distant!
Of course, the Hubble Space Telescope had
previously flexed its visual muscles at NGC 6934.
In 1999 HST revealed a population of blue
stragglers in NGC 6934's core. Blue
Deep-Sky Companions: The Caldwell Objects