Double Star of the Month: Tau-1 Hydrae
AKA: HJ 1167, 31 Hydrae. Position: 09 hr 29.1 min -02 46 08.2
Due south at 20.53 (BST) on 15 April 2020.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
There are many yellow and blue double stars, Albireo being the classic example (as I mentioned last month), and this is usually the result of the main star being a yellow star (for example K) and the secondary being hotter (B in the case of Albireo). But what happens if the order is reversed? One should see a blue and yellow pair, but in practice this does not seem to happen. It is as if the eye refuses to accept a blue-yellow pairing. Instead, the secondary is seen as various colours, grey, purple and even blue—a complete reversal of the proper order. Tau1 Hydrae is such an example (although not the best one). The main star is F6 which should be white or at the very most slightly yellowish and the secondary is K0 which is yellow or even orange. But in practice, the main star is seen as white, yellowish or yellow, and the secondary as grey, blue-grey or as in my own case, purple. See what colours you find.
This double was first observed by William Herschel in January 1782 and described by him as pretty unequal. The main star is a fairly bright 4.6 and the secondary is 7.3. The separation is very wide being 67.5 arcsec and it has widened slightly since Herschel’s own observation. It is a true binary rather than an optical double. Hydra as you will recall from the Christmas quiz is the largest constellation in the sky and much of it lies far below the celestial equator. Fortunately Tau1 Hydrae is near the top not very far from its “head” and it is well above the horizon in the April evenings. To find it draw a right angle triangle with Regulus in Leo and Procyon. The right-angle corner is Tau1 Hydrae although you may first spot a pair of fourth-magnitude stars just above it, they are Iota and Tau2 Hydrae, which form a small right-angle triangle with Tau1 Hydrae. Herschel described the three stars as being just in front of Hydra’s neck. Alternatively you can draw a line from Castor and Pollux and stop just below Regulus.