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9 CET (9 Ceti). Stars that are at extremes fascinate: high massones that may be ready to blow up, likeBetelgeuse,Antares,EtaCarinae, and so many others; low mass feeble red dwarfs likeProxima Centauri that produce occasionalflares; those holding neutron stars likeXPersei, or black holes such asCygnus X-1. But little fascinates as much as those of modest characterthat are like theSun, some so much so thatare known as solar analogues, even solar clones. The closest matchis probably18 Scorpii, though16 Cygni,Zeta-2Reticuli, and the pair that make53Aquarii come close. Here's another, the G2 dwarf (same classas the Sun) sixth magnitude (6.39, at the limit of human vision) 9Ceti. Not that there are no differences. We've never foundperfection. At a distance of 68 light years, Flamsteed's number 9ofCetus shines with an absolutemagnitude (apparent mag it would have if 32.6 light years away) of4.79, just four percent brighter than the Sun. That, with atemperature of 5760 Kelvin (10 degrees cooler than the Sun), givesa total luminosity consistently four percent higher than solar anda radius just three percent higher. The mass is, as expected,closely the same as our very own star. Oddly the star's color ismore consistent with a cooler G4 class, which gives one a bit ofpause regarding the other parameters. The age seems to be around5.5 billion years: not far from that of the 4.6 billion year Sun,though some sources suggest much younger. The greatest deviances,however, are in chemical composition and rotation, and they aresubstantial. Nine Ceti carries 50 percent more iron (relative tohydrogen) than the Sun (which might be part of the color problem). Of equal interest is the relatively rapid spin. A projectedrotation speed of 6 kilometers per second gives a rotation periodof under 8.6 days (as opposed to the 25-day solar rotation). However, being a solar-type star, 9 Ceti presents us with magneticactivity and is consistently spotted. The activity and spots causevariations in emissions from spectrum lines and in visualbrightness, since the active areas swing in and out of view becauseof rotation and as a result of long-term activity cycles (whichgive 9 Ceti the alternativevariable star name of BE Ceti). The actual rotation period is 7.7 days, which suggests an axialtilt to the line of sight of 63 degrees. Activity cycles of 9.1and 6.7 years (similar to the 11-year solar cycle) have both beenreported, on which might be superimposed a longer 25-year cycle. Some 3.6 minutes of arc away is an 11th magnitude possiblecompanion. If real, from its brightness, it would have to be aclass M4 red dwarf orbiting at a distance of at least 4500Astronomical Units with a period greater than a quarter millionyears. From the companion (if indeed it is one), 9 Ceti wouldappear 30 times brighter than our Venus at her best, while from 9Ceti proper, the red dwarf would glow just a couple times brighterthan Sirius does in our own skies. Sadly, there is no evidence ofa planet on which someone might have such a view. Written byJim Kaler 12/04/09. Return toSTARS.
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