December 2019 - Double Star of the Month

14 Aurigae (05 15 24.39 +32 41 15.3) sits in a small cluster of naked-eye stars 5 degrees north and a little preceding beta Tauri. It was first found by William Herschel on 24 September 1780, who called it H IV 19. He gave the colours as reddish-white and dusky but by the time that F. G. W. Struve had observed the system the colours had become greenish and blue.

Struve also added a fainter companion B of magnitude 10.9 at 11 degrees and 9".8 whilst the Herschel companion was reclassified as C. There is an additional 10.8 magnitude star at 180". The writer measured AC in early 2016 at 225 degrees and 14".5.

Hubble Space Telescope (HST) observations of star C have shown the existence of a white dwarf companion 2" away but 7 magnitudes fainter than C in the V band. There are no confirming observations to attest that the white dwarf is a physical member of the system, but if it is then 14 Aur is quintuple as both A and C are spectroscopic binaries and C is attached to A.

A low power field also includes 16 Aurigae, a considerably more difficult pair found by Otto Struve in 1848. The Washington Double Star Catalog (WDS) gives magnitudes of 4.8 and 10.6 whilst the position of B has changed little relative to A. In 2009 it was at 55 degrees and 4".1. This implies a physical relation as the primary star, a K3 giant has a proper motion well in excess of 0".1 per year. Gaia DR2 shows the B star but is coy about it's parallax and proper motion.

BU 311 (04 26 56.93 -24 04 52.8) lies in an extensive sparse area of Eridanus about 10 degrees west of Lepus. It was found by S. W. Burnham with his 6-inch refractor on 24 October 1874, who estimated the distance at 1". Burnham noted that there was uncertain change by the time he compiled his General Catalogue (1906).

Since then the companion has passed through periastron and is now on the opposite side of the apparent orbit at half the discovery distance. The period is 596 years and the stars will remain close for some years to come. The ephemeris for 2020.0 gives 162 degrees and 0".4. The magnitudes are 6.7 and 7.1, so about 30-cm will be needed to divide these stars.

Bob Argyle - Double Star Section Director