November 2020 - Double Star of the Month

Some 90 arc-minutes to the east of alpha Cas is the open cluster IC 1593 which is inside the nebulosity NGC 287. The cluster is dominated by a group of 9th and 10th magnitude stars which attracted the attention of S. W. Burnham soon after he obtained his 6-inch Clark refractor.

Image of a finder chart for the double star BU 1 in Cassiopeia
A finder chart for the double star BU 1 in Cassiopeia created with Cartes du Ciel.

He found that the brightest star in the cluster (V = 8.6) is a close double. There is a magnitude 9.3 star at 1.5 arc-seconds distance, whilst star C is magnitude 8.9 is 4 arc-seconds away with another 9.7 (D) at 9 arc-seconds distance. It is not clear where this group ends and the cluster begins.

The WDS lists 16 components altogether with most of the stars lying between magnitudes 12 and 16. The accepted distance to the cluster is 2.94 ± 0.15 kiloparsecs whereas the parallax of star A in Gaia DR2 corresponds to a distance of 2.80 ± 0.38 kpc. The bright multiple is known as BU 1 (00 52 49.22 +56 37 39.5) although this was not actually the earliest Burnham discovery.

The southern part of this column in its second appearance in 2006 considered the glorious pair theta Eri or Acamar. Starting at this star and moving three degrees west you will alight upon the closer pair HJ 3527 (02 43 20.36 -40 31 38.8), also in Eridanus (as can be seen from the relevant map in the Cambridge Double Star Atlas, but the WDS catalogue mistakenly has it in Fornax - my thanks to James Whinfrey for pointing this out).

Image of a finder chart for the double star HJ 3527 in Eridanus
A finder chart for the double star HJ 3527 in Eridanus created with Cartes du Ciel.

One of John Herschel's discoveries from Feldhausen, this is a beautiful pair, the primary of which is a late B dwarf. The magnitudes are 7.0 and 7.2 and the current separation of 2".3 appears to be increasing. Whilst observing this star in 2013 with the 67-cm refractor in Johannesburg, the writer found a faint and distant star of magnitude 11.6 at 133", unassociated with the bright pair.

Bob Argyle - Double Star Section Director