Double Star of the Month:
n Puppis
AKA: H N 19
Position: 07 hr 34.3 min 18.62s -23 degrees 28 min 25 sec
Due south at 21.52 (GMT) on 15 February 2022
n Puppis above and Howe 19 below marked with a cross
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
We are in Puppis again and once again we descend into a nomenclatural nightmare. This is because many amateur astronomers assume that Puppis has a complete set of Bayer letters, but it does not because of the break-up of Argo Navis. We have seen previously (Double Star of the Month for February 2021) that k Puppis is often assumed to be Kappa Puppis. Leaving aside the possibility of confusion with N Puppis, a somewhat brighter star which is too far south to be seen in Havering, there does not seem to be any reason to become muddled as n Puppis cannot be confused with any Bayer letter. Alas, no, it is often called Eta Puppis! The reason for this seemingly bizarre confusion is the old version of the Norton Star Atlas which showed the Greek letter Eta in a way which makes it look a small n with a long tail. Hence when observers saw the n they assumed it was in fact Eta, ugh! But it gets worse. Instead of being a nice simple Struve (STF) double star or even a South-Herschel (SHJ) double, it is a fiddly Herschel double star, namely H N 19, the nineteenth double in William Herschel’s “new” catalogue, which is sometimes incorrectly written as HN 19 or H 19.
But the fault lies in us, not in these two stars, so let us now turn to the double star. It is useful that they are almost the same magnitude – 5.82 and 5.85 – and it has a good separation of 9.9 arcseconds. The biggest problem is its low declination of almost 23.5 degrees south, level with the lower part of Canis Major. So try to get a clear view of the southern horizon and observe it when it is near the meridian. The two stars are almost the same spectral class, being F5 and F6, and they are indeed a binary pair. Their spectral classes would suggest that they are both yellow and this is how they are usually seen or perhaps as yellowish-white. Herschel first observed it on 20 November 1784, probably around 3 am. It can be found on a line between Sirius and magnitude 3.3 Xi Puppis, but about 4/5th of the way to Xi Puppis. It forms an equilateral triangle with two stars mag. 4.4 HD 60532 and mag. 4.9 HD 59612, n Puppis being in the bottom left-hand corner. Just below n Puppis one can observe another double star just 15 minutes away and hence probably in your telescope’s field of view, namely Howe 18 discovered by Herbert Alonzo Howe at the Cincinnati Observatory in Ohio in 1876. This is a dimmer and a much tighter double, so you will need a high magnification, about 100x. The pair is magnitude 8.1 and 8.9 with a separation of 1.9 arcseconds and are usually seen as being white.