48 AUSTRALIAN SKY & TELESCOPE April 2018
DOUBLE STARS by Ross GouldWandering west in Centaurus
Some tough and some easy targets for double star observers.
C
entaurus contains hundreds of
doubles accessible to moderate
telescopes. As always, I’ll try
to offer some easy and attractive pairs
as well as some more testing objects, a
few of them requiring medium-sized
telescopes (20 to 30 cm in aperture).
First up is R 165, a 7th-magnitude
pair nearly 3 ̋ apart, not difficult for
80 mm aperture. The yellow stars show
nicely close at 100× in a field that has
some middling-bright stars. It hasn’t
changed much since H.C. Russell
discovered it from Sydney Observatory
in 1880. It’s about 5 degrees east-
northeast from the nearest guide star,
2.7-magnitude Mu Velorum.
From R165, 1.25 degrees north-
northeast is HJ 4423, another
7th-magnitude pair with similar
separation and angle, slightly unequal,
and paler yellow, and again accessible
to 80-mm scopes. More difficult,
some 4.5 degrees north from R165 and
just west, is HJ 4409, with a brighter
5th-magnitude primary; the stars are
quite unequal (2.5 mags different) and
only 1.3 ̋ apart. I’d suggest at least 10
cm aperture for this one; with 18 cm at
180×, the companion looked small and
close. The primary is pale yellow.
Four degrees east from HJ 4409 isBSO 6, a wide, easy pair of 5th- and
7th-magnitude stars, yellow and duller
yellow. Rather nice through small scopes,
it has changed little since the 1830s.
Just over 2 degrees north-northeast
of BSO 6 is I 78, discovered in 1896 by
Robert Innes with a 17.5-cm refractor,
one of his first discoveries after moving
from Sydney to Cape Town. Solon Bailey
at the Harvard Southern Station had
found it earlier, in 1894, but failed to
publish until 1900. I 78 is nearly 400
light-years from us, and is a changing
binary that hasn’t yet had an orbit
calculated. It has become closer over
the years, without much change in
angle. In the early 1960s, Hartung —
in (Astronomical Objects for Southern
Telescopes) had noted “the stars are
equal and 15cm will just separate
them,” when the separation was 1.0 ̋. By
the 1990s it was around 0.75 ̋. With 18
cm in the late 1990s, I found the pair
barely separated with 180×.
Moving southwards, we come across
4th-magnitude Pi Centauri, or I 879,
included here for those who like severe
challenges. Pi was discovered by Robert
Innes in 1910 with a 9-inch refractor, as
a somewhat unequal and very close pair.
It has a fairly short period, 39 years, and
is best attempted when near maximumseparation, as it was when discovered.
At present it’s widening from being
closest in 2011 (about 0.05 ̋) and the
orbit ephemeris suggests 0.31 ̋ for mid-
2018 — 25 cm might show elongation
at high power this year. It will continue
to widen until about 2030. When last
in this wide section of its orbit, in the
1990s, I observed it when the separation
was 0.38 ̋ — 18 cm at high power gave a
hint of elongation; a 35-cm SCT showed
some image elongation at 430×, and just
about split at 800×.
Continuing southwards, BSO 5 is
an easy pair about 1.4 degrees west of
the attractive open cluster NGC 3766.
A very-long-period binary, the orange
stars are placed in a good starry field.
It’s nearby, about 45 light-years from us.
Southeast from BSO 5, and 0.75
degrees northeast from 3rd-magnitude
Lambda Centauri, CPO 11 is a pair of
blue-white stars, neatly separated at
100× with 18 cm, in a very fine field of
mixed bright and faint stars. This is a
beautiful and varied Milky Way region,
including a little clustering of magnitude
10 to 11 stars 10 ́ northeast, and the
small cluster Stock 14 some 20 ́ east.
Recently the primary star of CPO 11 has
been found to be a very close Wolf-Rayet
pair, discovered with an interferometer
at the European Southern Observatory.
The separation was measured at
0.00256 ̋; compare that with the
resolution limit of a 4-metre ’scope using
speckle interferometry, about 0.03 ̋.
Finally, moving northeast, 1.5 degrees
from CPO 11 is HDO 212. A fairly
tight pair at 1.0 ̋ and yellow-orange in
colour, it showed nicely at 180× with 18
cm aperture. Expect it to be elongated
rather than split with 10 cm.■ ROSS GOULD observes the sky from
the nation’s capital. He can be reached
at [email protected]Double stars of western Centaurus
Star Name R. A. Dec. Mag. Sep. Position angle measureDate of Spectrum
HJ 4409 (46 Cen) 11 h 07.3m -42° 38 ́ 5.2, 7.7 1.3 ̋ 252° 1995 ApSrCrEu
R 165 11 h 13.1m -47° 03 ́ 7. 5 , 7. 5 2. 8 ̋ 253° 1999 F8/G0IV
HJ 4423 11 h 16.5m -45° 53 ́ 7. 0 , 7. 3 2. 6 ̋ 278° 2000 F3V
Pi Cen (I 879) 11 h 21.0m -54° 29 ́ 4.1, 5.6 0.30 ̋ 110° 2 018 B 5 V
BSO 5 11 h 24.7m -61° 39 ́ 7.7, 8. 8 7. 5 ̋ 248° 2014 K5V+K7V
BSO 6 11 h 28.6m -42° 40 ́ 5.1, 7.4 13.9 ̋ 164° 2009 B9V
I 78 11 h 33.6m -40° 35 ́ 6.1, 6.2 0.7 ̋ 098° 1998 A2IV-V
CPO 11 11 h 40.6m -62° 34 ́ 6.9, 7.4 2.6 ̋ 219° 2012 O9.5
HDO 212 11 h 41.2m -61° 08 ́ 7. 5 , 7. 9 1. 0 ̋ 322° 2006 K1III
Data from the Washington Double Star Catalog