SELECTED SOUTHERN DOUBLES and VARIABLES
R.A. 23 Hours
Δ248 / Δ248 AB-C
/ RST 5560 AB (23208-5018) is another brilliant double in
Grus whose primary is again double but is very difficult
to split. Dunlop discovered the main pair of Δ248 in
1826, whose brightness is 6.2v and 8.8v magnitude (6.15V
and 8.89V). The AB-C components were 16.0 arcsec
217° in 1826 but are now presently separated by
16.9 arcsec along PA 212° (2002). This easy pair
can be seen in apertures as low as 7.5cm, and I have see
this pair on a number of occasions - being very impressed
with the view.
Since discovery by Rossiter in 1947, RST 5560 AB has
continued to widen from 0.5 arcsec to 1.3 arcsec in just
52 years - widening in fact by some 0.8/52 arcsec per decade.
Position angle has also decreased from 243° to
233°. Telescopically this is almost perfectly
aligned in between AB and C stars but I could visually
tell this was not the case. This pair will be interesting to
watch in the coming years and it seems likely that the two
are actually attached.
Telescopically the third component is a bit harder to
see than the wide pair. In 20cm the duo was certainly
elongated but at 125× magnification the comes was I
easily spotted. This became clearly separated at 333× -
which was the highest magnification I could usefully employ
with the seeing conditions - at least the last time I
observed it. I suspect something like 10cm to 15cm could
see it with care.
in Grus is an easier and brighter pair than Δ248
(See above) that was remarkably missed among the pages
of Hartung’s southern sky classic “Astronomical
Objects for Southern Telescopes.” It also
remains not commented by Dunlop himself, who simply
names it as ψ Gru / Psi Gruis, which it is not in
fact recognised today as that name among the stars of
the constellation of Grus. Grus actually end with the
star φ Gru / phi Gruis!
ΔΔ249 itself is located about 3.5°S
of the other Dunlop pairs Δ248 and Δ250 -
the latter being in Phoenix. The double also lies some
27′ from the western border of the constellation
of Phoenix, it can be easily found some 3.6°ESE
(PA 109°) from the deep yellow 4.1 magnitude
ζ Gru / Zeta Gruis / HIP 113638 (23009-5245).
Alternatively, the star lies 1.2°SSW (PA
200°) from the much fainter 5.5v magnitude
ο Gru / Omicron Gru (23266-5243). The apparent
field of Δ249 contains one 9th magnitude
yellow-orange colour star to the north-east by some
8.5′ENE (PA 62°) - being HIP 115580
with B-V of +1.439 and spectral class of K3V.
Since discovery in 1826 the pair has continued to decrease
in distance from 27.1 to 26.4 arcsec (1991) but the PA
seemingly continues to remain fixed at 212°.
Magnitudes are given as 6.0v and 7.2v (6.13V & 7.16V) and
I saw the colours as either white and white, though two other
previous observations state both were yellowish without any
variation. Both stars looked wonderful in 20cm and clearly
were divided even at 50× magnification. I suspect even 5cm
apertures could see this duo with ease.
Proper motion between the primary and secondary are similar
suggesting this is might be a true double. Spectral class of
the primary is A4III.
A true gem of the south.
θ Phe /
Δ251 / HJ 5411 / Theta Phoenicis (23395-4638)
is a bright bluish-white / white and yellow duo lies within a
faint Phoenix star field. At 6.6v and 7.2v magnitude (6.07V &
6.89V), Δ251 is now separated by 3.9 arcsec in the due
west PA of 270°. It is split in 7.5cm with care and
easily in 10.5cm.
This truly delightful pair can be quickly found half-way
between the constellations of Phoenix or Grus, either
9.5° WSW of the light orange coloured 2.4 magnitude
α Phe / α Phoenicis / Arkaa / Nair al Zaurak
(00267-4216) or 9.5°E from the 2.1 magnitude red star
Δ248 or β Gru / Beta Gruis /
(22427-4653). Alternatively, as my notes say, Δ251 was
found using the much closer stars of 3.9 magnitude ι
Gru / iota Gruis (23104-4515) being 5.3°E of
Δ251, or by moving from the opposing side some 5.3°W
of 3.9 magnitude ε Phe / epsilon Phe
Hartung (AOST1&2) describes Δ (#1011) as;
“...this elegant unequal white,
which has shown little change in separation and slow
direct motion since John Herschel’s measures
in 1835. The stars are probably in orbit.”
Hartung’s words here are perhaps a bit misleading
as the magnitude difference is about 0.9 magnitude.
Furthermore, since measures were first made in 1839,
the PA has increased by +7° from the due west
270° to 277°, while the separation
has increased by 25% from 3.0 arcsec to 3.9 arcsec. The
last measure or the Hipparcos one of 1991, and like a
number of southern pairs whose measures were made many
decades ago, have found significant change. One that
comes to mind is Gamma Crucis (Δ124) which is some
20% further apart than the 111 arcsec that was held
standard until about a decade ago. Like some of
Hartung’s descriptions in 1968, several pair have
changed leaving his words appear outdated by time.
Some thirty-five measures have been made to date (2007).
Spectral classes are A8V and F0V.
|“Southern Astronomical Delights”
10 Mar 2009