June 2020 - Double Star of the Month

In this column for July 2013, I included the difficult pair 26 Dra. One of Burnham's discoveries at the time of writing it was closing and by mid-2020 it will be widening (186 degrees, 0".5) but will still probably require 30-cm and a good night as the stars are almost 3 magnitudes apart.

Attention this month turns to STF2218 (17 40 18.07 +63 40 31.4) which is 2 degrees N of 26 Dra and slightly E. Discovered at Dorpat by F. G. W. Struve this pair has been closing slowly over two centuries but is still within range of 10-cm. The components are magnitudes 7.1 and 8.4 and at 2020.5 they can be found at 307 degrees and 1".4. The orbit is preliminary as the angular motion amounts to just 50 degrees and predicts a period of 2130 years.

R. T. A. Innes used a 7-inch refractor at the Cape Observatory around 1900 to survey the sky for new double stars and to take up work again which he was doing as an amateur astronomer in Australia about 5 years before.

One of his discoveries was I 333 (15 39 55.12 -78 01 38.1), a relatively bright and easy pair with magnitudes 6.9 and 7.5 and 0".8 apart when he happened upon them. He was not the discoverer though. This was Solon Bailey who found the pair from Arequipa in Peru in July 1897 but the observation did not get published until 1908 so Innes got the credit.

When Willem van den Bos made a pair of mean measures from Johannesburg around 1930 the stars were just 0".3 apart and clearly in rapid motion. The WDS Observations Catalogue then reports no further observations until 1990, a stretch of almost 60 years. By then they stars were almost back to where they were discovered and it seems that here is a binary with a period of about 150 years.

I 333 was brought to my attention by Andrew James who had noted the probable binary nature of the pair and at my request Rainer Anton secured a measure in Namibia in mid-2019. His result was 323 degrees 0".85.

Like iota Octantis in April this pair is close to the South Pole but should repay observation with 15 or 20-cm.

Bob Argyle - Double Star Section Director