January 2025 - Double Star of the Month

About five degrees east of the line joining the two stars which form the 'horns' of the constellation of Taurus, the Bull is a right-angled triangle of 5th magnitude stars. These are 132, 136 and 139 Tauri. If you acquire 132 Tau in the field then about 20 arc-minutes north-east is the wide pair STTA66 (05 47 56.13 +24 41 12). This is an entry in the catalogue of wide pairs which Otto Struve compiled during his survey at Pulkovo in the 1840s.

The stars, which are relatively bright - 7.0 and 7.7 - and are separated by 94" in position angle 166 degrees. Rather surprisingly, they appear to be at the same distance as determined by the Gaia satellite and a search around them reveals a star of magnitude 19 also at that distance although the quoted error is substantial.

132 Tauri itself is recorded in the Washington Double Star catalog (WDS) as a very close visual binary, although only observed on one occasion in 1979. During the Hipparcos survey another component was recorded. The two stars were given as 5.0 and 9.1 at 230 degrees and 3".8. And again the only observation appears to be the first one and such a pair should be visible in 25-cm (the WDS gives magnitudes 5.0, 9.1, 230 degrees, 3".8). It certainly it should have been detected in the recent Gaia observations and does not appear there.

Looking through the earlier entries in this column which occupy the southern sky I find that I have been remiss in not including one of the finest of John Herschel's pairs - HJ 3945 or 145 CMa (07 16 36.84 -23 18 56) known quite widely, especially in the United States, as the 'Winter Albireo'. The stars are not quite bright as those in beta Cygni but form a spectacular pair for the small telescope.

The primary is a K3 supergiant and its companion an F0 dwarf. John Herschel notes Orange and green. Fine contrast of colours. Modern observers also attest to the strong contrast of colours.

This is certainly an optical pair - star B is 346 light years away whilst star A is 2,600. The stars can be found 3 degrees east of the magnitude 3.0 blue supergiant omicron 2 CMa. The 6th magnitude star HIP 35578 which is more than 3 degrees to the south and has almost identical proper motion to B has been formally identified as a physical companion (C). However Gaia DR3 gives its distance as 296 light-years - about 50 light-years closer than B.

Bob Argyle - Double Star Section Director