Double Star of the Month - October 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.
Perhaps to be classed more as a small cluster than a multiple star the system of 8 Lac (22 35 52.3 +39 38 04) was first noted by Herschel.
They form an arch was the comment he added in the Philosophical Transactions paper in 1784. The small telescope user can see four components with relative ease: A=5.7, B=6.3, C=10.4 and D=9.1. AB is 185 degrees, 22".2, AC is 168 degs and 48".3 and CD is 116 degs and 42".4. A fifth star I, is mag 11 and 228 degs and 9".7 from D. It was found by T. Espin in 1906. The remaining four components according to the WDS (where it is known as STF 2922) are either very faint, very close or extremely distant.
Hipparcos had great trouble with the distance to stars A and B which were the only two that it observed, but it seems that they are so remote that the parallax is to all intents too small to measure and means that the main stars are before at least 2000 light years away. The spectral types are early to mid-B so they are probably hot young stars. Thus might indicate that the predominant colour would be white. Chambers makes A and B white but C greenish and D blue. Webb notes that A and B might be tinged with yellow but also makes D blue.
DUN 246 ( 23 07 14.8 -50 41 12) lies in a
thinly sprinkled star field according to Ernst Hartung. The stars are magnitudes 6.3 and 7.1 whilst the separation has been slowly decreasing since discovery by Dunlop in 1825. The current position is about 255 degs and 8".7 and the similar proper motions of both stars indicates a long period binary system. A pair of late type dwarfs is reflected in Hartung's colours of yellow for each component. More recently Gould with 35-cm noted the colours are both pale yellow.
Bob Argyle - Double Star Section Director