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.

Δ249 (23239-5349) 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 (00098-4542).

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.