52 AUSTRALIAN SKY & TELESCOPE July 2019
TARGETS by Sue FrenchDO 27: MARTIN GERMANO; SKETCH: SUE FRENCHNo NGC
Clusters
Look beyond the pages of the
classic catalogue to find these
star clusters.M
ost of the star clusters we
amateur astronomers observe
are included in the famed
New General Catalogue of Nebulae
and Clusters of Stars (NGC), but that
compilation does not include any
objects discovered after 1887. Since
then many more clusters have been
found that are well within the reach
of backyard scopes, so let’s visit some
of these oddities in the world of open
clusters and learn who found them.
Our featured deep sky wonders
dwell within the realm of the all-sky
chart at the centre of this magazine. I’ll
introduce our quarry from west to east
so that each succeeding cluster climbs
higher as you eyeball the one before.
In case you prefer to stalk clusters by
constellation, that info is given in the
table on page 54.
In 1961 Madona Dolidze published
lists of possible clusters noted during
her spectral studies at the Abastumani
Astrophysical Observatory. These groups
seemed to hold either stars with a
cluster-like distribution of spectral type
versus brightness or else hot young stars
with similar magnitudes.
Dolidze 27 in Ophiuchus is a sparse
cluster, but I’m fond of it anyway. It
brings to mind a simplified version
of the Gemini stick figure drawn on
many charts. My 105-mm refractor
at 17× shows a shallow curve of three
stars that’s easy to spot 1.6° north of
Zeta (ζ) Ophiuchi in the same fieldof view. At 76×
the northernmost
star in the curve
glows orange and
lies just outside the
northwestern border
of the group’s nominal
25 ′ span. The other two
stars belong to Do 27 and
mark the heads of the mini-Gemini
twins, the western one yellow and
the eastern one orange. The sketch
illustrates the figure as seen in my
mirror-reversed view.
Robert Trumpler published a study
of open clusters in 1930, including
37 anonymous groups that were later
named for him. The following year
Per Collinder produced a catalogue of
open clusters. The vast majority were
previously known, but all now bear
Collinder designations.
Our next two objects come from
these catalogues. The overlapping
clusters Collinder 316 and Trumpler
24 are fascinating to skygazers mostly
because they make up the curved tail
of the False Comet. This remarkable
object is nicely visible to the unaided
eye, but various observers interpret it
in different ways. For some the bright
and tight cluster NGC 6231 forms the
pseudo-comet’s head, while others see
it as Zeta^1 and Zeta^2 Scorpii. No matter
which strikes your fancy, you’ll find the
tail fanning out to the northeast. With
binoculars I’ve been able to enjoy theview of the ‘comet’.
The origin of the term False Comet is
something of a mystery. Some propose
that it was so dubbed by John Herschel,
largely to commemorate his ship’s
landing in False Bay, South Africa, near
which he conducted observations of
the southern sky. Others claim a more
recent origin by amateur astronomers
based solely on appearance. There’s
some evidence that Alan Whitman
inspired the name by pointing out
the “comet-like Milky Way patch in
southern Scorpius” in 1983. Others
think the term may have been coined
by observers here in the Southern
Hemisphere.
Resting 27′ northeast of the
4th-magnitude star 45 Ophiuchi,
Trumpler 26 is a fine cluster through
my 130-mm refractor. At 23× four
brighter suns, laid out like the hub and
tips of a three-bladed propeller, overlay
a grainy backdrop of threshold stars. At
164 × the hub star is a nice double, and
I can pick out 20 stars in a 7′ gathering
with indefinite borders.
Much farther north in Ophiuchus,tpThe open cluster
Dolidze 27 seems almost
bereft of stars, but its
brighter members form a
Gemini-like asterism. The
sketch illustrates the figure
as shown in the mirror-
reversed view through the
author’s 105-mm refractor.NNEE