Sky Object of the Month – December 2014
Eta (η) Cassiopeiae – Double Star in Cassiopeia
by Glenn Chaple
The spotlight this month goes to the binary star Struve 60, better known as eta (η) Cassiopeiae (or Achird, if you prefer its Arabic name). Details first.
William Herschel discovered the duplicity of eta Cassiopeiae in 1779. At the time, the magnitude 7.4 companion was 11.3 arc-seconds east-northeast (position angle 62o) of the magnitude 3.5 primary. Since then, it has traveled ¾ of the way around the main star to its current location 13.2” to its northwest (P. A. 323o). A complete orbit takes about 480 years. Eta Cas A is a main sequence GO-type star – a virtual twin of our sun, similar in mass and size. Eta Cas B is a M0-type star with about 2/3 the sun’s mass and size. The pair lies 19.4 light years away.
Now for the neat stuff. Eta Cas sports a color contrast so stunning that James Mullaney and Wallace McCall included it in their 1960s Sky and Telescope series “The Finest Deep-Sky Objects.” I have no reason to argue the selection. After viewing eta Cas with a 90 mm f/11 refractor at 167X, I wrote,” Striking colors! Primary golden yellow; companion pale red.” Sissy Haas, in her book Double Stars for Small Telescopes, considers it a showcase pair. Check it out and see if you agree.
Before putting your telescope away, stand back and take a good naked eye look at eta Cas and its neighbor gamma () Cas. Visually a magnitude brighter than eta Cas, gamma appears to be much closer. It’s an illusion. Gamma Cas is actually nearly 30 times farther away.. Move this blue-white subgiant next to eta Cas, and it would shine at a dazzling magnitude -5.5. Remember, eta Cas is a twin to the sun. If it (or our sun) were moved to the same distance as gamma Cas, it would be a 12th magnitude speck. First diagram of location as function of period courtesy: www.dibonsmith.com