The Demon's Eyes

"What you look hard at seems to look hard at you."

(Gerard Manley Hopkins)

The other night at Carrs Mill, Richard Orr asked me whether I had ever looked at the "Demon Eyes". I had to admit that I had never heard of them. The Demon Star (Algol), yes. But Demon Eyes? Nope. 

Well, we had just been talking about the Water Jar over in Aquarius, which is to me to only recognizable asterism in that entire constellation. The rest of it is just a giant 4th magnitude mess - the worst place in the sky to locate anything by star hopping.

The Water Jar consists of 4 relatively bright stars, 3 of which (Eta, Pi, and Gamma Aquarii) form a tight little triangle with a fourth (Zeta Aquarii) right in the center. All 4 stars are approximately 4th magnitude, making the asterism naked eye visible, but not particularly prominent. (Some people, myself included, consider Alpha Aquarii, a 3rd magnitude star somewhat to the east of the other four, to be part of the asterism. But that's not important here.)

To the naked eye or through binoculars, Zeta Aquarii appears to be a run of the mill solitary star of a nondescript white color. Even when I turned my 90mm refractor over to it the other night (using a 13mm Nagler eyepiece), there was nothing worth mentioning about it. But when I swapped eyepieces for a 3.5mm Nagler... Wow! There was a stunning, easily splittable double star, the two components of apparently equal brightness. They looked eerily like a pair of (rather sinister) eyes staring back at me. Just who was observing whom?

The A and B components are a true double star, with an orbital period of approximately 540 years, and the system itself is about 92 light years away. Impossible to see telescopically, the A component is itself a double star, composed of an F-type main sequence star with a companion about one 3th its mass. They orbit each other every 26 years, so they must be extremely close to each other. As for the system's main components (A and B), their separation is thought to be about 140 AU.

It's that last figure which intrigues me the most, since Neptune's orbital diameter is 60 AU. So if you can imagine one half of the distance between A and B Zeta Aquarii, that's pretty much what our own solar system would look like at a distance of 92 light years.

As an experiment, I observed the pair with my 5mm Nagler eyepiece. I could still split the double, but it wasn't as "clean" as with the 3.5mm. So if you're thinking of taking a look yourself (highly recommended),  use all the power you have. It's worth it!