9 CET (9 Ceti). Stars that are at extremes fascinate: high mass ones that may be ready to blow up, like Betelgeuse, Antares, Eta Carinae, and so many others; low mass feeble red dwarfs like Proxima Centauri that produce occasional flares; those holding neutron stars like X Persei, or black holes such as Cygnus X- 1. But little fascinates as much as those of modest character that are like the Sun, some so much so that are known as solar analogues, even solar clones. The closest match is probably 18 Scorpii, though 16 Cygni, Zeta-2 Reticuli, and the pair that make 53 Aquarii come close. Here's another, the G2 dwarf (same class as the Sun) sixth magnitude (6.39, at the limit of human vision) 9 Ceti. Not that there are no differences. We've never found perfection. At a distance of 68 light years, Flamsteed's number 9 of Cetus shines with an absolute magnitude (apparent mag it would have if 32.6 light years away) of 4.79, just four percent brighter than the Sun. That, with a temperature of 5760 Kelvin (10 degrees cooler than the Sun), gives a total luminosity consistently four percent higher than solar and a radius just three percent higher. The mass is, as expected, closely the same as our very own star. Oddly the star's color is more consistent with a cooler G4 class, which gives one a bit of pause regarding the other parameters. The age seems to be around 5.5 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 are substantial. Nine Ceti carries 50 percent more iron (relative to hydrogen) than the Sun (which might be part of the color problem). Of equal interest is the relatively rapid spin. A projected rotation speed of 6 kilometers per second gives a rotation period of under 8.6 days (as opposed to the 25-day solar rotation). However, being a solar-type star, 9 Ceti presents us with magnetic activity and is consistently spotted. The activity and spots cause variations in emissions from spectrum lines and in visual brightness, since the active areas swing in and out of view because of rotation and as a result of long-term activity cycles (which give 9 Ceti the alternative variable star name of BE Ceti). The actual rotation period is 7.7 days, which suggests an axial tilt to the line of sight of 63 degrees. Activity cycles of 9.1 and 6.7 years (similar to the 11-year solar cycle) have both been reported, on which might be superimposed a longer 25-year cycle. Some 3.6 minutes of arc away is an 11th magnitude possible companion. If real, from its brightness, it would have to be a class M4 red dwarf orbiting at a distance of at least 4500 Astronomical Units with a period greater than a quarter million years. From the companion (if indeed it is one), 9 Ceti would appear 30 times brighter than our Venus at her best, while from 9 Ceti proper, the red dwarf would glow just a couple times brighter than Sirius does in our own skies. Sadly, there is no evidence of a planet on which someone might have such a view.
Written by Jim Kaler 12/04/09. Return to STARS.