Epsilon (ε) Lyrae & Struve (Σ) 2470 & 2474
Epsilon (ε) Lyrae, HIP 91919 - The Double Double
Along with Albireo in Cynus, the Double Double in Lyra is probably one of the most viewed multiple star systems in amateur astronomy; it is relatively easy to find, makes a good test for one's optics and scope and is rather beautiful to behold. It is estimated that the star system is some 162 light years away from Earth, separated by billions of miles and orbiting each other over a period of hundreds of thousands of years.
ε1 (to the right in the sketch) and ε2 (to the left) can be split themselves into two further binary star systems which again are orbiting each other. The component stars of ε1 have magnitudes of about 4.5 and 6, and ε2 about 5 and 5.5 and again are separated by billions of miles, each orbiting their partner over a period between 1,200 and 600 years respectively.
The two binary pairs are probably Type A, dwarf stars, something similar to Sirius, Deneb or Vega, with a mass of about 1.5 to 2 times that of the Sun. Typically, dwarf stars are young stars with just a few hundred million years of age. It is understood that there are also a number of other stars which could be part of the Epsilon star system. ε2, for example, might have another star orbiting its binary pair and collectively, ε1 and ε2 could have a total of ten other stars held by the same gravitational pull.
A City Observation
Although Epsilon Lyrae is easy to find and split into the two distinct components, splitting these again is very difficult. As can be seen from the sketch below with the 4” at about 140x, a clean separation wasn’t possible.
Perhaps I will need to re-observe and re-draw the Double Double over the winter months when seeing conditions in the city are better, but, perhaps, this isn’t the real answer. I feel that resolving this particular double doesn’t only depend upon atmospheric steadiness and dark skies, but also on my own vision which these days may not be up to scratch.
Struve (Σ) 2470 & 2474 – The Other Double Double
Σ 2470, the more northern one to the right, appears to consist of a bluish-white primary and a fainter blue companion at about 6 and 8 magnitude whilst Σ 2474, again at about a 6 and 8 magnitude, appears to consist of a reddish-yellow primary with a fainter, lighter yellow companion. Σ 2474 is said to be another binary star system in itself, whilst a 11 magnitude orbital partner, known as the C component, can be seen just left of it at about 120º.
Σ 2474, like the sun, is believed to be a Type G star about 160 light years away. Σ 2470, on the other hand, is thought to be a Type B star with about 16 times the mass of the sun and over 1,300 light years away. It follows that collectively, this ‘Other Double Double’ in Lyra, although a stunning visual binary, are not physically related and so are not true double binary systems.
A City Observation
Although not nearly as well know as their Epsilon partners, aesthetically speaking I find Struve Σ 2470 and Σ 2474 far more appealing. The double is an easy, low power split, lying parallel rather than perpendicular to each other and being less a magnifying challenge than Epsilon Lyrae, the double appears somewhat brighter whilst offering the observer a richer field of stars as a gorgeous backdrop to the colourful binary system.