September 2023 - Double Star of the Month

Four degrees north and slightly east of beta Pegasi is the Herschel pair H N 11 (23 07 27.73 +32 49 3.3) but now known in modern atlases and handbooks as STF 2978. It was found by Herschel on Sep 6th, 1784 and included in his third and last double star compilation. This list was not published until 1821 when it appeared in the first volume of Monthly Notices of the RAS. This is an easy and pretty pair with components having magnitudes 6.4 and 7.5.

Image of a finder chart for the double star H N 11 (STF 2978) in Pegasus
A finder chart for the double star H N 11 (STF 2978) in Pegasus created with Cartes du Ciel.

Over the last two centuries there has been but little motion in position angle and separation. A recent measure in 2018 put the stars at 145 degrees and 8".3. Gaia DR3 gives precise distances for the two stars, 450.31 and 453.31 for A and B respectively. Star A is an Algol variable with an amplitude of 0.22 magnitudes and is known as V343 Peg.

HWE 91 in Piscis Austrinus (22 55 56.89 -32 32 2.9) was also independently discovered by two astronomers. The name represents Herbert Howe, astronomer at Cincinnati Observatory who used an 11-inch Clark refractor on a survey for new double stars. In autumn 1877, using a magnification of x230, he examined the star delta PsA and found a very faint and relatively close companion and estimated the magnitudes as 5.0 and 10.5. Two years later and unaware of Howe's finding, S. W. Burnham came across the star using his 6-inch Clark refractor but observing this time from Mount Hamilton in California. He called it BU 772 but later conceded that Howe had priority of discovery.

Image of a finder chart for the double star HWE 91 in Piscis Austrinus
A finder chart for the double star HWE 91 in Piscis Austrinus created with Cartes du Ciel.

The current position is 250 degrees and 4".9 but there is little motion in this system. The stars are equally distant from us within the errors of the parallaxes and they have similar proper motions. Gaia DR3 also indicates that the stars have G magnitudes of 3.94 and 9.80. Hwe 91 is easily found lying as it does just 3 degrees south of Fomalhaut.

Less than one degree WSW from Hwe 91 is HJ 5367 (also gamma PsA) which presents a similar test. Here the stars are magnitudes 4.5 and 8.2 and they are currently 4".1 apart in PA 255 although this has increased from 2".5 when found by John Herschel in 1836.

Bob Argyle - Double Star Section Director