48 AUSTRALIAN SKY & TELESCOPE May | June 2017
DOUBLE STARS by Ross Gould
The sea serpent’s tail
Track down these nine stellar pairings for scopes of all sizes.
T
his column visited the central part
of Hydra three years ago; this time
I’ll describe double stars from
a more easterly end of this very long
constellation. First up is Beta Hydrae,
HJ 4478, the end-point of the previous
column. Discovered by John Herschel in
1834, this long-period binary has now
become much closer than the easy 2 ̋
separation of the 1830s.
In 1961 separation was a whisker
under 1.0 ̋ when Ernst Hartung
reported that it could just be separated
with 15 cm aperture. By the late 1990s
it was at 0.7 ̋, and with 18cm aperture
showed an uneven figure-8 shape.
Returning to Beta in 2015 with my
current favourite double star scope, a
210-mm refractor, I found the image
similar to the one I’d seen with 18 cm
years earlier, because Beta Hya had
continued to close. At 300× it was egg-
shaped; at 480× notched in the better
moments of seeing.
The stars are pale yellowish, slightly
unequal, and should just separate
with 25 cm aperture. There’s no orbit
calculated yet so we’ll have to see if it
continues closing. In recent decades the
position angle has changed faster as the
stars become closer. A revised parallax
puts Beta Hya at about 310 light-yearsfrom us, so the 2015 separation (0.58 ̋)
is still some 55 a.u. in projection.
Nearly two degrees north and a little
east from Beta Hya is DUN 116, one
of James Dunlop’s 1820s discoveries
from his back garden in Parramatta.
A wide easy pair of yellow magnitude 8
stars, with an 11th-magnitude star 30 ̋
north, even 60-mm scopes will show
it well. The main stars have similar
proper motions, so may form a very
wide gravitational pair; the third star is
merely an optical companion.
Nearly three degrees east and just
north from Beta Hya is HJ 4495, an
unequal pair with a yellow primary. It’s
uncertain whether this is an optical
pairing or a true binary; the proper
motions are large and similar, but change
thus far has been linear; which can,
however, appear in parts of an extended
elliptical orbit. At nearly 6 ̋ separation this
is another pair that 60 mm will show.
Medium apertures show a magnitude-13
field star nearby north-east.
Our next binary, JC 17, is
3.5-degrees east and just south from
Beta Hya. Again unequal brightness,
magnitudes 6 and 8, this off-white 3 ̋
pair can be seen with 60-mm scopes.
Another of the discoveries of Captain
Stephen Jacob from India, in 1848, ithas shown only a slight change in angle
since that time. A very nice pair.
Four degrees east from Beta Hya
is HWE 72, a tighter, unequal white
double. Again, it’s a long-period binary
that’s shown very little change since
discovery in 1876. With 18 cm aperture,
the companion showed at 180×, though
it was better seen at 330×. It’s an
attractive object, although apertures
under 15 cm may find it too difficult.
We now head 14 degrees north-east
from Beta Hya to find HJ 4556. An
easy, unequal, yellow pair in a widely
scattered star field, the moderately
bright stars make an attractive easy
object, accessible though faint with 60
mm, better seen with 100 mm or more.
Little has changed since John Herschel
found it in 1835.
Well eastwards again is H N 69, a
William Herschel discovery, a bright
and easy white pair of 6th-magnitude
stars, 10 ̋ apart, with two very wide
11th-magnitude field companions. A
nice object, it’s just over 3 degrees north
of the bright galaxy M83, and well
worth a visit when you’re in this area.
To finish, another John Herschel
pair, HJ 4606, is five degrees east from
3rd-magnitude Gamma Hya (with
R Hya between them). HJ 4606 is a
bright very pale yellow star with an
easy 10th-magnitude, wide companion.
In 1959 W.S. Finsen at the Union
Observatory in Johannesburg found that
the primary star is a slightly unequal
ultra-close pair (FIN 352), at 0.1 ̋
separation! In recent times it’s been a
little wider; the last measure, in 2006,
being 0.24 ̋. A 30-cm or larger scope
might show slight elongation of this pair;
very high magnification will be needed.■ ROSS GOULD observes the sky from
the nation’s capital. He can be reached
at [email protected]Double stars along Hydra’s tailStar Name R. A. Dec. Magnitudes Separation Position Angle MeasureDate of Spectrum
Beta Hya (HJ 4478) 11 h 52.9m –^33 ° 54 ́ 4.7, 5.5 0.6 ̋ 052 2015 B9 IIIpSi
DUN 116 11 h 56.7m –32° 16 ́ 7.7, 7. 8 18. 5 ̋ 088 2010 G8V+G8V
HJ 4495 12 h 06.1m –32° 58 ́ 6.7, 8.8 5.8 ̋ 319 1999 G0V
JC 17 12 h 10.0m –^34 ° 42 ́ 6.4, 8.0 3.2 ̋ 017 2000 A0V
HWE 72 12 h 13.6m –33° 48 ́ 6.5, 8.6 1.3 ̋ 164 1991 A0V
HJ 4556 12 h 54.3m –27° 58 ́ 7.7, 8. 8 4. 9 ̋ 084 2010 F9V
H N 69 13 h 36.8m –26° 30 ́ AB 5.7, 6.6 10.1 ̋ 189 2009 A7III
HJ 4606 13 h 41.5m –23° 27 ́ AB,C 6.6, 10.2 30.5 ̋ 354 2007 A1V
FIN 352 " " AB 6.9, 8.1 0.24 ̋ 313 2006 "
Data from the Washington Double Star Catalog