The Caldwell Objects

evealed a significant blue-straggler population
in NGC 6101. It was found that these stars are
more centrally concentrated than the cluster's
similarly bright subgiants. This indicates that
NGC 6101's blue stragglers have larger masses
than their ostensible counterparts, supporting the
theory that they either are binary systems or have
been formed by the merging of stars. Fifteen
variable stars have been discovered in the region
of NGC 6101. Eleven of these appear to be RR
Lyrae stars, one of the types of variables used to
size up our galaxy.
To find NGC 6101 start with magnitude-1.9
Atria, Alpha (α) Trianguli Australis. The cluster
lies 3½° to the southwest of it. If that's too far a
jump, first locate 5th-magnitude Zeta (ζ)
Trianguli Australis 2° to the southwest of Atria;
the globular is another 2° south of Zeta Trianguli
Australis. NGC 6101 can be seen in binoculars.
Look for a tight triangle of 7th- to 8th-magnitude
stars whose westernmost, 7th-magnitude
member appears double. The cluster should be
just south of the triangle and will
107
appear as a dim glow with a slight central condensation.
It's best to have some magnification
handy if you want to confirm your sighting.
With an 8-inch telescope sited in Wellington,
New Zealand, I saw the cluster as a moderately
bright sphere surrounded by a dim gray halo.
The roughly 3'-wide core is fractured into three
parts. The innermost section is an uneven wedge
of suns. A dark lane separates this mass from a
tiny cap of stars to the northwest (Dunlop's "very
small" nebula?), while a chevron of starlight to
the south appears to have calved off the core.
Through Auckland Observatory's 20-inch Zeiss
reflector, the core of NGC 6101 looks herniated. A
splash of 13th- to 15th-mag-nitude suns crosses
the cluster's entire face, as if some disgruntled
artist shook a wet paint brush at it. The drawing
here is a composite of all these views. Through
his 12-inch telescope, Ernst Hartung discerned a
"rather faint but very rich globular cluster...
irregularly round, rising broadly to the centre,
about 3' across with rays of faint stars emerging."
The Caldwell Objects 425