Double Star of the Month
145 (Gould) Canis Majoris
AKA: HJ 3945, “Winter Albireo”. Position: 07 hr 16.6 min -23 deg 19 min.
Due south at 21.40 (GMT) on 14 February 2020.
Image credit: Simbad (http://simbad.u-strasbg.fr/simbad/)
This star is very far south as seen from Havering, and is best seen when due south (assuming no houses or trees are in the way). It is also dim, the apparent magnitude is 4.8, but because of its low altitude and hence extinction, the actual apparent magnitude is closer to 5.5. Hence it is best found using go-to, but if you do not have go-to, either form a straight line between bright Epsilon Canis Majoris (Adhara) and yellowish Delta Canis Majoris (Wezen) and go on as far again to the north-east, but Wezen and Adhara are even further south and may be difficult to see. Alternatively it forms a right angle triangle with Procyon and Sirius (with Sirius at the right-angle corner) but this is a rather hit and miss method.
It is a very pretty almost equal yellow and blue double, which was described by the Rev. Thomas Webb as “magnificent”. The primary is magnitude 5 and the secondary is 5.8 and the separation is 26.8 seconds, so it is an easy spilt even in a small telescope. It is an optical double like its summer namesake. It is not the only “Winter Albireo”; this nickname has also been used for Iota Cancri and ignores the fact that Albireo itself is visible (and easily seen) at least in the earlier part of the winter. The name is of recent origin, although it is difficult to pin down who first used it. The first person to record the double was John Herschel during his stay in Cape Town, South Africa, in 1837. Doubles found by John Herschel on his own (as opposed to his earlier catalogue with James South) have the prefix italic small h to distinguish them from double stars recorded by his father William Herschel, but in modern computer readable notation it is called HJ 3945. The main star is 145 Canis Majoris, but this 145 is not a Flamsteed number; Flamsteed’s numbers hardly if ever go up to three digits. It is a Gould number as it is recorded in the Uranometrica Argentina of 1879 compiled by Benjamin Apthorp Gould, an American astronomer who worked at the Argentine National Observatory between 1871 and 1885. Gould is best known today for being instrumental in the breaking up of the Ptolemaic constellation Argo Navis.