February 2019 - Double Star of the Month

Epsilon Hya (08 46 46.51 +06 25 07.7) is one of the most observed double stars in the catalogue. According to the Washington Double Star Catalog (WDS) it has been measured 432 times since it was discovered by Wilhelm Struve in 1825. It seems to have escaped the attention of William Herschel although it would have been within the capability of his telescope.

Since 1825 the companion, known as C, has moved 120 degrees in position angle with little change in separation. A measurement at Cambridge in 2017 showed it at 309 degrees and 2".93. It should be resolvable in 10-cm; the stars have visual magnitudes of 3.8 and 7.8.

In 1860 Otto Struve, using the 15-inch refractor at Pulkovo suspected that the primary star was elongated, an impression he received again in 1864. In April 1888 Giovanni Schiaparelli, observing with the 15-inch refractor at Milan, noted a clear elongation and subsequent follow-up observations allowed him to say that the primary star was a close binary of short period.

Eight revolutions have been traced out since discovery and the period of AB is close to 15.05 years. The stars are never wider than 0".27 and at the start of 2019 they are 0".22 apart and closing.

A more distant star D (V = 12.5) at 210 degrees, 18" is also physical, and C is a spectroscopic binary of period 9.9 days meaning this is a quintuple system.

The Struve pair is a fine sight on a good night - the stars are given as yellow and purple by Smyth but I saw them more as yellow and light blue.

In the visually barren but telescopically interesting area between Sirius and Procyon there are a number of fine double stars and clusters.

About 5 degrees west and a little north of 5 Pup (see the column for February 2018) is STF 1097 (07 27 56.66 -11 33 24.7), an easy 6.3 and 8.2 magnitude pair with colours of yellow and bluish.

I came across it in Spring of last year and obtained 311 degrees and 20".8. I did not see the close companion to A that Dembowski had suspected in 1865, and Burnham confirmed nine years later with his 6-inch Clark. It should be visible in 20-cm although the low altitude of the star would have been a factor.

BU 332, as it is known, is currently at 0".7 and may be closing; the stars are magnitudes 6.2 and 7.4. There are two faint comites. D is 9.7 at 157 degrees 23" (distance decreasing) and E is 12.4 at 43 degrees and 32".

Espin noted that A varied between 6 and 6.8 with a period of 14 days, whilst Otero, more recently, suggests that it is the Burnham component which is likely varying by around 0.6 magnitude to produce the small observed variation in AB (0.13 magnitude) found by Hipparcos.

Bob Argyle - Double Star Section Director