Epsilon Lyrae: The Double-Double Test

Jack Kramer

For as long as I can recall, the famous "Double-Double" -- Epsilon (å) Lyrae -- has stood as a benchmark. This striking multiple star system in Lyra provides an excellent test of a telescope's optical performance based on the ability to split the close components of the system.

First, something about the star system itself. Even through a finder, it appears as two stars separated by about 3.5 arcminutes. In fact, it is claimed that a few sharp-eyed observers have been able to split the pair with the naked eye. Upon closer inspection with a telescope, each of these stars shows that it is in turn composed of two components of almost equal magnitude and color. The northern pair is designated as å1> and the southern pair as å2. The northern pair of stars are 2.8 arcseconds apart and the southern stars have a separation of 2.6 arcseconds. The brighter star of the northern pair is also a spectroscopic binary. The fact that all the stars share a common proper motion through space and are at a distance of 180 LY from Earth shows that all the components of this system are physically related.

The Double-Double has the added virtue of being easy to locate in even the worst of skies. If you can find Vega, then look just a bit to the east to find Epsilon.

What you see in your telescope tells a lot about the quality of the optics. Some sources say it's possible to split the close components in a 3-inch scope, yet others have said that it could not be split in a 41/2-inch. The question then arises whether it can be split cleanly. Is there an area of black separating the close components or do the stars just seem to run into each other? Adding slightly to the challenge is the fact that the components all appear to be about the same color -- white -- so there's no color contrast to set one off from the other. If your telescope provides high definition, such as in a refractor or highly-corrected reflector, then Epsilon Lyrae should look something like the drawing shown below (which is not exactly to scale). Use moderately high magnification. As an example, in my 4-inch refractor, a magnification of 150x is able to split the system nicely. (All these illustrations are oriented as Epsilon would appear in a Newtonian reflector, with south up.)

Typical commercially-made mirrors will show that there are two pairs of stars, but they may not be cleanly separated or just barely show some black sky between them, as illustrated here. This is about what they look like in my 10-inch reflector. The greater the aperture, the better the resolution, so Epsilon becomes easier to split in larger scopes, even with less than perfect optics. However, larger scopes also show the stars as brighter, which may be a detriment in itself.

With poorer quality optics that do not resolve to the Dawes limit, star images appear bloated, and in the case of Epsilon Lyrae, the close pairs would appear as single, elongated stars, as shown in the following illustration. But make sure to observe on a night when the conditions are good. Atmospherics can cause bloated images even in the best of scopes. Obviously, if you have a reflector, make sure it's properly collimated, and any telescope should have been outside long enough to reach ambient temperature.

Epsilon Lyrae is a beautiful sight when it's well resolved and it's a great test object. Those who have a telescope with very good optics take special delight in showing it off. When experienced observers see the constellation Lyra, like everyone else, they immediately think of the famous Ring Nebula (M57), but then they're sure to say, "Let's take a look at the Double-Double!"

Published in the July 1999 issue of the NightTimes