Double Star of the Month - August 2012

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.

The two pairs selected this month are almost copies of one another. They have the same separation, and position angle, similar magnitudes and even the colours are close.

Eta Lyrae (19 13 45.49 +39 08 45.5) appears as H 4 2 in the great astronomer's first list of double stars published in 1782, but it was first listed by Christian Mayer in 1779. The primary is a luminous object more than 1,000 light years away. The companion appears to be an F star and with the primary being an early B subgiant, one might have expected colours to be white and yellow when in fact Hartung records yellow and ashy. The WDS gives magnitudes of 4.38 and 8.58 with current distance and PA being 28".5 and 79°. A fainter and more distant star (mag 11.4) can be seen at a distance of 161". Along with theta, also a wide, unequal pair, it follows Vega by about 7 degrees.

Beta1 Sgr (19 22 38.29 -44 27 32.1) is not only not the second brightest star in Sagittarius but it is actually about 12th in the list. The naked eye can make out both beta 1 and its close neighbour beta 2 (mag 4.27) some 20 arc mins away. Dunlop listed beta 1 as a telescopic double in 1826 and and since that time the position angle has increased slowly to 76° and the separation decreased slightly to 28".7. Hartung gives the colours as pale yellow and ashy white - the spectral types are actually B8V and F0V perhaps reversing what one might expect from the colours. However, Ross Gould using a 175-mm refractor disagrees with this judgement and gives the colours as white and yellowish. The stars are magnitudes 3.98 and 7.21 and again this is an optical system. Beta1 lies in the far south-western part of Sagittarius, close to the border with Corona Australis and Telescopium.

Bob Argyle - Double Star Section Director