Double Star of the Month - January 2009

In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

This months column features two stars which are, nominally at least, the 7th brightest in their respective constellations. Theta Aurigae = STT 545 (05 59 43.24 +37 12 46.0) is one of Otto Struve's discoveries at Pulkovo using the 15-inch refractor in 1852 but there are no measures on record until 1871. Orbital motion is slow but retrograde and covers some 60 degrees over the last 130 years. The separation has very slowly increased from 2".1 to the present 3".8. Whilst this may appear to present no problems to the small telescope user, the difference of magnitude certainly does and this system is one of the classic tests for the small aperture. The WDS gives the magnitudes as 2.6 and 7.2 so its not just the brightness difference but the glare from the primary star that has to be dealt with. However, it is the case that small apertures tend to work better on this star than larger ones as the quality of the atmosphere is also an important consideration when observing it. The writer finds it difficult to measure in an 8-inch refractor when the red field illumination of the micrometer is switched on. Theta Aurigae is about 165 light years from the Sun.

Theta Pictoris = DUN 20 (05 24 46.29 -52 18 58.2) was observed by John Herschel in 1835 and noted to be a `fine' pair. It had been discovered by Dunlop in 1826 and whilst Hartung refers to the two components (visual magnitudes 6.24 and 6.74) as pale yellow, the WDS catalogue lists the spectral types as A0V and A2V. In the last 180 years there has been virtually no relative motion between the two stars and the system is ideal for use as a calibrator for filar or eyepiece micrometers (2010.0: 287.6, 38".14). In 1901 Robert Innes found that A was itself a close pair and the system is a difficult one for any but the largest visual telescopes with the current separation not exceeding 0.2 arc seconds until 2010. It then widens to about 0".46 in 2079, the period being 191 years. There are reasons to believe that theta Pic is in fact a quintuple system. One of the stars in the close pair is a spectroscopic binary and the distant companion is also suspected of variable radial velocity. The group is remote with the revised Hipparcos parallax giving the distance as 512 light years with an uncertainty of 25 light years.

Bob Argyle - Double Star Section Director