Double Star of the Month:
Theta Serpentis
AKA: Alya; STF 2417
Position: 18 hr 56.2 min +04 degrees 12 min 12.9 sec
Due south at 22.19 (BST) on 15 August 2021.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Before we look at this bright easy double in detail, we need to say something about the constellation it is found in. With all other constellations, there just one single constellation to consider. However the constellation of Serpens (the Serpent) has two separate parts. The reason for this is that the Serpent is being held by the Serpent Holder, Ophiuchus. In one hand, he is holding the head of the serpent and in the other hand he is holding the tail. The body of the serpent is behind his back (in theory of course, not in practice). Hence the two parts of the constellation, which are about 25 degrees apart, are called Serpens Caput (the Head of the Serpent) in the east and Serpens Cauda (the Tail of the Serpent) in the west. Theta Serpentis is in Serpens Cauda and another famous double Delta Serpentis is in Serpens Caput. Theta Serpentis has the now official name of Alya which comes from “tail” in Arabic (dhayl), which would logically refer to the fact it is in the tail of the serpent, but has been explained as meaning the “fat tail of the sheep”.
Theta Serpentis has an overall apparent magnitude of 4.0 which mean it should be visible to the naked eye even in Havering and it lies roughly two-fifths of the way between Altair and the mag. 2.8 Beta Ophiuchi (Cebalrai) in a fairly barren part of the sky just above the constellation of Scutum. The main star is mag. 4.6 and the secondary is only slightly dimmer at mag. 4.9. With a separation of 22.4 arcseconds, it should be easily split even in a small telescope. There is a dimmer (mag. 6.8) third star (C) at a distance of 421 arcseconds from A and at a roughly sixty degree angle from the AB pair. William Herschel first observed on 17 October 1779 describing it as equal and white, but it had been seen by earlier astronomers. The two main stars are about 162 light years away (about the same distance as the double-double from July 2021) and about 1000 astronomical units apart (roughly the distance of the solar system’s Oort Cloud), with a period of about 16,000 years. They are both hot A type stars on the main sequence. By contrast C is very similar to the Sun and lies only 86 light years away, so it is only an optical double. As A type stars, the main pair should be white and indeed they are, but many observers have seen tints in these stars ranging from yellow and green to pink. What tints can you see?