February 2015 - Double Star of the Month

In mid-February, Gemini passes the meridian at 10pm and offers an excellent opportunity to view some of the fine double and binary stars in this constellation. One of the less-well known systems is 20 Gem (06 32 18.52 +17 47 03.4) which is eaily found about 3 degrees north preceding the bright star gamma Gem. Wil Tirion's Cambridge Star Atlas shows another double star about a degree south of 20. This is STT143 (6.2, 10.4, 103°, 7".6 with a fainter third component at 345° and 47"). Also known as STF924, 20 Gem offers a beautiful pair of stars which Webb noted as topaz yellow and cerulean blue, but as these are the exact hues noted by Smyth in the Bedford catalogue it may be assumed that Webb was merely repeating the Bedford colours. More recently Sissy Haas finds both stars to be gloss-white.

There is little relative motion between the stars and the current situation is that the PA is 211° and the separation 20".2. Hipparcos found the determination of the parallax of star A difficult and finds a distance of 262 light years with an error of 30%. This is no doubt due to the motion induced by the duplicity of A confirmed by both occultation observations and also directly as a spectroscopic binary.

Looking about 1° slightly north preceding the V = 4.4 star q Puppis, one will alight on the wide pairs DUN 67 (08 13 58.31 -36 19 20.2) and DUN 68 about 1.5 arc mins south and west of it. DUN 67 consists of stars of magnitude 5.0 and 6.0 which are currently separated by 66 arc seconds and PA 174°. The separation is slowly decreasing. Hartung chooses not to include them is his book 'Astronomical Objects for Southern Telescopes' but Gould with 175-mm finds both stars to be pale yellow and notes the existence of several fainter pairs in the area. The components of DUN 68 are both mag 7.3 but form a wider pair than its neighbour. The PA is 25° and the separation is 125" and increasing.

The whole region is fine with the large scattered star cluster NGC 2546 about 1.5° further south adding further reason to take in the area with a pair of binoculars as well. Both pairs are at the same distance from us within the stated errors of the Hipparcos parallaxes.

Bob Argyle - Double Star Section Director