This double-star system is remarkable for more than one reason.
Cassiopeia is renowned for its many clusters and I’m sure you’ve spent some quality time with them. But I would like to tell you about a remarkable double star: Eta (η) Cassiopeiae.
Eta is an easy naked-eye star, just off an imaginary line between Schedar (α), Cassiopeia’s brightest member, and Navi (γ), the middle star in the constellation’s familiar W. Eta is among the prettiest doubles in the sky and, at a distance of just under 20 light years, a neighbour of the Sun. Eta’s two main components are separated by about 80 astronomical units and revolve about their common centre of gravity every 500 years. In a telescope they appear about 13 arc seconds apart. That makes them an easy, low-power split in any telescope. But that’s just the beginning.
According to double-star enthusiast John Nanson, Eta Cass is much more than just a pretty pair. Nanson and Greg Stone, run a web site called Star Splitters, a major resource for double-star observers. In an article on the site, Nanson notes that Eta Cassiopeiae actually has ten component suns. Yes, ten! If he’s correct, this system is unique.
I urge you to read the full article, but here’s the key point. In other multi-star systems, such as Castor in Gemini, some members can only be detected spectroscopically or in Hubble or Keck telescopic images. This is where Eta Cass is different. All ten components can be seen in medium-sized telescopes. In fact, all ten can be seen in a single field of view. Nanson detected them in his 152-millimetre f/10 refractor telescope with a 14-millimetre TeleVue Radian eyepiece that provides 109×. In his article, Nanson presents helpful finder charts for identifying the various stars in the Eta system, the faintest of which is magnitude 12.
As for me, I’m still working on spotting all ten. Using my 120-millimetre f/7.5 Orion EON refractor and a 13mm Ethos eyepiece it’s theoretically possible, though urban light pollution may defeat me. Magnitude 10 is usually my limit with this scope. Nevertheless, my plan is to prepare a sketch showing every star in the field of view around Eta Cass, and compare it with Nanson’s chart.
Give it a try yourself and see how many of the Eta Ten you can track down.
David A. Rodger is an editor, writer, broadcaster and amateur astronomer living in North Vancouver, British Columbia. From 1967 to 1980, he served as the first director of Vancouver’s H.R. MacMillan Planetarium.