The Caldwell Objects

stragglers are stars that appear to be bluer and
more luminous — and hence more massive —
than the other stars on a globular cluster's
hydrogen-burning main sequence. If they were
really the same age as the cluster's other stars,
blue stragglers should already have moved
beyond the main-sequence stage to become
swollen giants. But, at first glance, they are
younger than their siblings. And they really are,
in a sense. Astronomers believe that a blue
straggler forms when two stars merge, as often
happens in the crowded confines of a globular
cluster's core; such a merger produces a single,
young, massive star that hasn't evolved as much
as its similarly sized siblings. Blue stragglers
stand out on a globular cluster's Hertzsprung-
Russell (or H-R) diagram, which plots the
apparent magnitude of each star versus that
star's color.
Because of their advanced ages, globular
clusters generally lack bright, blue stars; such
stars have already swelled toward red-gianthood
(blue stragglers excepted). By measuring
just how bright a globular's bluest mainsequence
stars are, astronomers can estimate that
globular's age. The HST data place NGC 6934's
age somewhere between 13 and 16 billion years.
Along with many other globular-cluster age
estimates, this places a firm lower limit on the
universe's age.
Finding NGC 6934 is easy. As noted above, it
lies about 3¾° due south of Epsilon Delphini. If
your telescope sports a Telrad sighting device,
place the northern edge of the Telrad's outermost
circle on Epsilon Delphini and the globular will
be just inside the southern edge of the same
circle. Through 7x35 binoculars the cluster seems
impressively obvious, lying just north of a
sideways hook of binocular stars. But NGC
6934's apparent brightness in binoculars is an
illusion, since a similarly bright (magnitude-9.5)
star lies only 2' due
The Caldwell Objects
47
west of the cluster; light from the star melds with
that from the cluster to intensify the latter's
appearance. (The star is visible just to the right of
the cluster in the photograph on page 187.) At
23x in the 4-inch the two are barely separated; the
cluster looks like the head of a tiny comet
"kissing" the star to the west. A close pair of
roughly l0th-magnitude stars lies about 7' farther
west of the magnitude-9.5 star. Sweeping the
area with a 3° field of view reveals several long,
faint, dark streaks running more or less northsouth;
the most prominent of these lanes lie to
the east of the aforementioned "hook."
At 72x the cluster displays a dim core, which
suddenly appears dappled with dense clumps of
hazy starlight. The core is surrounded by a large,
symmetrical glow. A prolonged look will
enhance the core's apparent patchiness,
especially with averted vision — and an
occasional strong inhalation of air to replenish
189