January 15, 2019

Astro-Event: Spot the “Double-Double.”



Epsilon Lyrae finder chart. (Created by Author in Starry Night).

Epsilon Lyrae finder chart. (Created by Author in Starry Night).



Looking to expand your star-party repertoire beyond Saturn & Albireo? Let me introduce you to a sure northern hemisphere crowd pleaser, the famous “Double-double” star Epsilon Lyrae in the constellation Lyra. Located about 1 ½ degrees to the northeast of the bright star Vega, this pair is easily resolved in binocs or by the keen eyed observer. The constellation Lyra lies high to the west during the Fall at dusk. But wait, there’s more; each pair is resolvable via moderate sized telescope into a pair of stars, making for a quadruple system. Now for the geometry of what you are seeing; the system is about 162 light years distant. Visible separation of 208” hasn’t changed appreciably since discovery by William Herschel in 1779, as the pair spans a distance of about 0.16 of a light year and orbits once every hundreds of thousands of years. The northern-most component consists of a 4.7 and 6.2 magnitude stars at a separation of about 2” arc seconds. This computes to about a 140 AU physical separation and a 1200 year orbit. The southern pair has a magnitude contrast of 5.1 and 5.5 respectively, for a visual separation of 2.5” arc seconds and an orbit perhaps half as long; separation is slowly increasing until 2100 until the pair dips towards a mutual periastron in 2229 AD. If you were on a Tatooine-like planet orbiting either pair, you would see the distant sister pairing at about a 0.5 degree separation (the apparent size of a Full Moon) and a magnitude of about -8 to -10, definitely noticeable. And if that isn’t mind bending enough, the brighter star in the northern pair is a confirmed spectroscopic binary, and the southern pair has another component star detected by speckle interferometry in 1985… that brings the entire system menagerie up to a grand tally of six stars!

The astro-term for this week is Common Proper Motion. So, just how do astronomers know that all those stars in the Epsilon Lyrae system are related, and not just a chance alignment? By measuring the tiny changes in position over the years, gravitational relationships can be ascertained. And we are talking tiny; the CPM of the Epsilon Lyrae system is about 0.06” arc seconds per year. The CPM of nearby stars can heavily lend evidence to a mutual attraction and a shared history. It must be stressed that common proper motion is only an apparent motion from our solar system-bound vantage point, which is itself moving about the center of the Milky Way galaxy. This also combines the transverse velocity across our line of sight, as well as the radial velocity towards or away from us. Stellar motion through space is indeed complex stuff!


  1. [...] and Struve 2725 are also sometimes referred to as “the Dolphin’s Double-Double” in reference to Epsilon Lyrae, but I like the Halloween meme of “The Ghost Double” as a system that seems to be stalking the [...]

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