Double Star of the Month:
Epsilon Lyrae
AKA: the Double-Double, STFA 37; STF 2382 (AB); STF 2823 (CD).
Position: 18 hr 44.3 min +39 degrees 40 min 12.4 sec
Due south at 00:09 (BST) on 16 July 2020.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Back in May 2020, I discussed the issue of resolving the widest double stars with the naked eye and reported that in the laboratory the eye can resolve a distance of roughly one arc-minute (60 arc-seconds). The distance between the two main stars (AB and CD) in Epsilon Lyrae is 209.5 arc-seconds, so in theory easily resolved with the naked eye and certainly in even small binoculars or a finderscope. Furthermore they are almost the same magnitude (4.7 and 4.6). However the main interest of Epsilon Lyrae—as its nickname implies—is that the two main stars are themselves double. The secondary stars in these doubles are dimmer (6.10 for B in AB and 5.4 for C in CD), but the difference is not as great as it seems as the A star is dimmer than the combined AB (5.2 rather than 4.7) and the C star in CD is also dimmer (5.3 vs. 5.4). However, they are very tight doubles: the separation of AB is 2.3 arc-seconds and the separation of CD is an almost identical 2.4 arc-seconds. The remarkable similarities in magnitudes and separations sets up a very nice experiment. As it happens the AB pair is at right angles to the CD pair, so if one pair appears horizontal to the observer, the other pair will be vertical. As the separations are virtually the same, this partly becomes a test of whether a horizontal pair is easier to resolve (see apart) than a vertical pair. I find the ease of separation is very different, but another factor is the difference in magnitudes, which is almost a magnitude for AB but only a tenth of a magnitude for CD, which will make CD easier to resolve. The whole system is physically bound (there is a fifth member which is not directly visible) and lies at a distance of 162 light years away. The two main stars are a sixth of a light year apart (about 10,500 AU, roughly the distance of the Oort Cloud from the Sun) and they will take many thousands of years to orbit each other, The A and B stars are separated by 116 AU and have an orbital period of about 1800 years. By contrast the B and C stars are 121 AU apart with a period of 724 years. Three (A, C and D) of the four stars are very similar: they are all A class stars, being all just over twice the mass of our Sun, and they are very hot (about 7900K). The other star (B) is cooler (a mere 7000K) and smaller, being 1.6 solar masses; it is spectral type F0. They all look white. It is an easy double star to spot: just focus your binoculars or finderscope on Vega, the brightest star in the Summer Triangle, and the double-double will be an obvious pair just to the left of Vega. William Herschel observed it on 29 August 1779 and described as “a very curious double-double star”. It had been observed somewhat earlier by the Moravian astronomer Christian Mayer.