Double Star of the Month:
k Puppis
AKA: H 3 27. Position: 07 hr 38.8 min -26 degrees 48 min 14 sec
Due south at 21:55 (GMT) on 15 February 2021.
Image credit: Simbad
There can be few double stars with more nomenclature complications than k Puppis. Many people assume that it must be Kappa Puppis, but there is no Kappa Puppis. To understand why, we have to remember that Puppis was once part of the huge ancient constellation of Argo Navis. When Argo Navis was broken up in the late nineteenth century, largely at the behest of Benjamin Althorp Gould (of 145 Gould fame, see the double star for February 2020), the Bayer letters were left untouched rather than new sets of Bayer letters being allocated to the new constellations. So Alpha Argûs Navis (usually just Alpha Argûs) or Canopus became Alpha Carinae, so there is no Alpha Puppis. Perhaps the most famous of the old Argûs Bayer stars was Eta Argûs which became Eta Carinae. Kappa Argûs became Kappa Velorum and hence there cannot be a Kappa Puppis. The same problem exists with another double, n Puppis, which is often mistakenly called Eta Puppis, but that is a story for another time. Furthermore the double star designation for k Puppis is unusual, being a William Herschel designation, namely H 3 27. The three refers to the degree of separation of the two stars, being the 27th star of the third class, but in modern computer-based catalogues, the 3 has been (confusingly in my view) tacked on to the 27 to make it H 327. Herschel first observed it on 15 February 1781, simply noting that the two stars were equal.
But enough of these complications! k Puppis is a fine double but difficult to see in Havering, as it is one of the most southerly doubles we can easily see. Even when it crosses the meridian, it is only 11.5 degrees above the horizon or roughly the height of the house at the bottom of your garden so you will need a pretty clear view to the south. It lies directly beneath Procyon and on the same level as the yellow star Wezen (Delta Canis Majoris) in southern Canis Major, thus forming a right angle triangle with these two bright stars. The two stars in k Puppis are about the same magnitude (4.4 and 4.6) and have a moderate separation of 9.9 arc seconds. They are both B stars so they should be the same colour (either white or blueish). I see them as white but with possible bluish and reddish tints which may be a result of the low altitude. They are nice because they are bright and equal, with a separation which is not too wide but also not difficult to split. The secondary is an odd star with a strong magnetic field and a peculiar chemical composition with a lack of helium, but an overabundance of iron and rare earths. It is also variable. Although they are both about 350 light years away, they may be an optical double.