The Caldwell Objects

104 & 106
conformists are formed either when the stars in a
double-star system slowly merge or when two
unrelated stars collide. Data from HST's Faint
Object Spectrograph established the temperature,
size, and rotation rate of one massive blue
straggler (BSS-19) in 47 Tucanae. And for BSS-19,
at least, astronomers favor the slow-merger
scenario. BSS-19 has a mass of 1.7 Suns; it is the
first globular-cluster blue straggler whose mass
has been measured directly. The cluster's
crowded center also contains rapidly spinning
pulsars discovered recently by radio
astronomers.
Finally, here's a little oddity. How many of
us have heard of, or given much thought to, NGC
121, a pathetic little globular cluster about 30'
north-northeast of 47 Tucanae's core and just 10'
from the edge of its outer halo? NGC
416
121 measures a mere 1.5' and shines with the
light of a magnitude-11.2 star. It is not plotted in
the first edition of Wil Tirion's Sky Atlas 2000.0,
though it is in the second edition. The cluster also
is not included in the Catalog of Parameters for
Milky Way Globular Clusters compiled by expert
William E. Harris (McMaster University). Why?
Because NGC 121 is an old globular cluster in the
Small Magellanic Cloud. By studying Hubble
Space Telescope images of NGC 121, astronomers
detected the first blue stragglers known in an
extragalactic globular cluster. The globular's true
size is 75 light-years — only one-third as large as
47 Tucanae. Still, seeing the two together offers
observers incredible depth perception! How
many observers have overlooked NGC 121 for
the glory of 47 Tucanae?
Deep-Sky Companions: The Caldwell Objects