October 2018 - Double Star of the Month

In Cassiopeia, about 3 degrees west of the magnitude 2.2 star beta Cas (the westernmost of the five in the well-known 'W') is tau Cas. Move a further 3 degrees west and you will alight on SHJ 355 (23 30 01.92 +58 32 56.1).

There are nine components in the Washington Double Star Catalog (WDS), most of which are visible in a 6-inch, and a drawing by John Nanson on the Star Splitters website using that aperture, shows the halo of faint stars around the brightest member of the group which is magnitude 4.9. This B3 star is a well-known eclipsing binary of the Algol type (AR Cas) with a primary dip of 0.14 V magnitudes and a period of 6.06 days.

The small aperture will have no problem in picking out the C component at 269 degrees and 75". Larger apertures may see that both A and C are close, unequal doubles. AB is one of Otto Struve's discoveries (STT 496) and B is some 4.4 magnitudes fainter than A yet now only 0".8 distant. At least 30-cm will probably be needed for this. 20-cm may suffice to show the companion to C discovered by W. R. Dawes in 1841. This pair (DA 2, CD) are magnitudes 7.2 and 9.0, at 213 degrees and 1".3.

John Herschel discovered that delta Sculptoris (23 48 55.48 -28 07 48.1) was double before his journey to South Africa.

The primary is magnitude 4.6 and lies about 12 degrees east of Fomalhaut. He estimated the distance to the magnitude 9.4 companion as 80", but Burnham in his 1906 catalogue suggests that this was a little large.

It appears in that volume because in 1881 Burnham added a close and faint companion to delta (BU 1013) using the 36-inch refractor at Lick. This star now known as B is only 3".4 away and is magnitude 11.6.

Since 1881 there has been but 11 degrees of direct motion between the two stars which are clearly physical, because delta is moving though space at more than 0".1 per year. In fact the distant C, (297 degrees, 74") also possesses the same transverse motion as AB. Gaia DR2 tells us that delta is 139.2 light years away, with an uncertainty of less than 0.1 light year, whilst C is 144.25 +/- 0.01 light years distant.

Bob Argyle - Double Star Section Director