Double Star of the Month: Struve 747
Position: 05 hr 35 min 03 sec -06 degrees 00 min 07 sec
Due south at 19:51 (GMT) on 15 February 2023
[Double Star for Feb 2023 - #1]
Struve (STF) 747
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
There are not many top-rate doubles which completely lack a name, Bayer letter, or at least a Flamsteed number. Struve (STF) 747 is one such double. It is immediately below and slightly to the right of Hatysa (aka Na’ir al Saif) or Iota Orionis, which is the bottom star of Orion’s sword. The two main stars are magnitude 4.70 and 5.5 and the separation is an easy 36 arcseconds, in fact very similar to Albireo. However, the colours are seen as both white or perhaps blue-white, as they are hot B stars; they are also young. As with most of the other stars in Orion the binary pair is about 1,300 light years distant. STF 747 is surrounded by nebulosity from the Orion Nebula and is less easy to see than one might expect because of the glare from nearby Hatysa. There is a third companion which is magnitude 9 and is almost twice as distant from the main star with a separation of 68 arcseconds and is physically connected to the other two stars. If one thinks of the AB pair being at 4 o’clock, the AC pair is at 10 o’clock. It was first observed by William Herschel on 7 October 1779 along with Iota Orionis.
Hatysa (Iota Orionis) is itself a triple. Surprisingly it is not clear if it is a physically bound group. The primary is magnitude 2.8 but the secondary is a much dimmer 7.7, and the separation is only 12.5 arcseconds. It is thus more difficult to split and glare (and nebulosity) is a major problem. The primary is an O9 star and the secondary a B8, which means that they should be seen as both white or blue white. The main star has been seen by some observers as yellow-white and the secondary is best described as “ghostly”.