February 2018 - Double Star of the Month

Lambda Geminorum (07 18 05.61+16 32 25.7) sits in the ecliptic zone which means it can be occasionally occulted by the Moon. High speed photoelectric measurements of the star's brightness during occultation show that there is another star close in. The star is also known as a spectroscopic binary which may be the same object.

For the visual observer the challenge is to see the faint companion discovered by F. G. W. Struve. The WDS give its magnitude as 10.7 and I can honestly claim never to have seen it. John Nanson, however, finds it slightly easier than delta and kappa Gem, which I don't, so clearly I will have to take another look this Spring.

Lambda is only 101 light years distant and the position angle and separation, currently 36 degrees and 9".3 have changed little since the 1830s. As lambda has a significant proper motion then it seems that the faint star is travelling with it through space.

The duplicity of 5 Pup (07 47 56.71 -12 11 33.8) is also down to Struve, and it is known as STF 1146. During the 19th century the components, of magnitudes 5.7 and 7.3, changed very slowly relative to each other but by 2016 the pair were about 1 arc second apart.

This is a highly-inclined long-period system, like STF 1527, and the current orbit predicts a period of 1331 years and a close approach of 0".7 by 2044. At present it is well separated in 15-20cm but the difference in magnitude and low altitude in the UK sky makes the task of resolving it a little trickier. I have only been able to make two measures in the last five years.

The surrounding area of sky is very rich. Move 3 degrees south and then swing west by 4 or 5 degrees and you will encounter more bright Struve pairs as well as M46 and M47.

Bob Argyle - Double Star Section Director