48 AUSTRALIAN SKY & TELESCOPE January 2018
DOUBLE STARS by Ross GouldSoaring through Volans
Pioneers of southern astronomy found many doubles within the flying fish.
V
olans, the flying fish, is one of
the far-southern constellations
invented by Bayer for his
Uranometria of 1603. Starting in
the west of the constellation, we find
the show-piece stellar pair Gamma
Volantis, or DUN 42. One of James
Dunlop’s discoveries, the bright,
unequal and well-separated stars are a
beautiful combination, and a fine object
through any telescope. Hartung called
the colours golden and pale yellow.
Zeta Vol (DUN 57) to the south-east
is a wide, easy pair, another of Dunlop’s
doubles found during his 1820s survey
of the southern sky from his backyard at
Parramatta. DUN 57 is a bright orange
star with a fainter, well-separated
companion, magnitudes 4 and 9. The
field has some fairly bright stars. A
nice object, it is accessible to 80 mm
aperture, but looks better with 140 mm.
About 1.7 degrees south of Zeta Vol
is the pair HJ 3997, a John Herschel
discovery, found in 1835 during a sweep
with his 18-inch reflector. Herschel
was surveying the southern sky with a
much better and larger telescope than
Dunlop, a decade earlier. Herschel
measured this pair the following year
with a micrometer on his 5-inch
refractor, and found the separation to
be 1.75 ̋. Early- to mid-20th century
measures suggest a slight widening, to
around 2.1 ̋; measures in 1995–96 were
slightly closer at 1.9 ̋. A small telescope
will show this fine pair, of equal
brightness and both pale yellow.
East and somewhat south, about
2.5 degrees from Zeta Vol is I 9. Its
7th-magnitude stars are only 0.8 ̋
apart, so a clean split will need 15 cm
aperture or more. Innes found I 9 using
a borrowed 6.25-inch (16-cm) refractor,
at the beginning of his career in double
star astronomy while he was in Sydney.
The only change since discovery is aslow decrease in angle. It appears to be
another binary of long period.
Epsilon Vol (RMK 7) was discovered
in the 1820s by Carl Rumker of the
Parramatta Observatory. It is more than
500 light-years from us, so the easy
separation of 6.0 ̋ equates to somewhat
over 1,000 a.u.; a huge orbit of many
centuries period. It is an easy and bright,
though unequal, pair; the primary white
with a yellowish companion. I thought it
a beautiful pair with my 18-cm refractor
at 100×, set in a fairly starry field. It will
be easy with 80 mm.
Nearly 1 degree south and just
east from Epsilon is I 192. With 18
cm at 100× it was a cream-coloured
star with a minute companion close
south, somewhat better seen at 180×.
I 192 was discovered by Innes at Cape
Observatory, South Africa, in his early
years there. He used the old 7-inch
refractor that had been obtained
decades earlier to follow up on John
Herschel’s 1830s Cape observations of
doubles. It had not been much used for
doubles, and by 1897 the drive clock was
erratic, so micrometer measures were
not possible. With it, Innes discovered
nearly 350 new doubles.
Kappa Vol is eastwards from Zeta
and Eta and midway in declination,making a triangle, being about 3
degrees from each of them. It is double
through an 8×50 finder, which shows
the 5th-magnitude stars as a neat little
pair. With an 18-cm refractor at 65× it
shows as a broad triple. The extra star is
of 8th magnitude, making a little arc of
three stars in an otherwise sparse field.
The last of our doubles this time is
HRG 19, discovered in 1882 by Lawrence
Hargrave — later an aviation pioneer —
during his five years as an assistant at
Sydney Observatory, where he used the
Merz 7.25-inch (18-cm) refractor for
discovering and measuring double stars.
HRG 19 is not a bright pair, though
within reach of 10 cm, with the 7th-
and 10th-magnitude stars an easy 4.4 ̋
apart. It appears to be a distant (500-
plus light-years) binary with a large, slow
orbit. With the 18-cm at 100×, it was a
delicate and fine, very unequal pair, at
an easy but fairly close separation. The
primary star is orange. Six arcminutes
north-east is a moderately bright star
with two fainter companions, and 15 ́
north-north-west is a wide couple... but
these are optical pairings.■ ROSS GOULD observes the sky from
the nation’s capital. He can be reached
at [email protected]DoublestarsofVolansStar Name R. A. Dec. Magnitudes Separation Position Angle MeasureDate of Spectrum
Gamma Vol (DUN 42) 07 h 08.7m -70° 30 ́ 3.9, 5.4 14.4 ̋ 296 2002 K0III+F2V
HJ 3997 07 h 35.4m -74° 17 ́ 7. 0 , 7.1 1. 9 ̋ 306 1996 B9IV+B9IV
Zeta Vol (DUN 57) 07 h 41.8m -72° 36 ́ 4.1, 9.3 15.6 ̋ 123 2011 K0III
Epsilon (RMK 7) 08 h 07.9m -68° 37 ́ 4.4, 7.3 6.0 ̋ 023 2010 B6IV
I 192 08 h 08.9m -68° 59 ́ 7. 3 , 9. 6 1. 8 ̋ 172 1991 A0
I 9 08 h 14.7m -73° 48 ́ 7. 3 , 7. 5 0. 8 ̋ 103 1996 A8III
Kappa (BSO 17) 08 h 19.8m -71° 31 ́ AB 5.3, 5.6 63.8 ̋ 060 2011 B9III-IV
" ""BC 5.6, 7.737.5 ̋ 030 2010 A0IV
HRG 19 08 h 48.4m -65° 26 ́ 7.3, 10.0 4.4 ̋ 181 2000 K0III
Data from the Washington Double Star Catalog