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2 AND T CEN (2 and T Centauri). While cool semiregular variablestars are not that unusual, they are not that common either. WhatIS unusual is to find two of them visible to the naked eye so closetogether, just 1.75 degrees apart (T to the northwest of muchbrighter 2), innorthern Centaurus. Ifyou can't find one, you can probably spot the other, making them(it's irresistible) "T for 2 and 2 for T." Then we diverge, as thetwo are really quite different, 2 Cen an ill-defined "SRb" star, Tan "SRa," which behaves more like a regular, though shorter-period,Mira-type variable. And their closeness isjust line-of-sight, as 2 Cen is 183 light years away (give or taketwo), whereas fainter T lies at a whopping 1370 light years distant(give or take a rather large 280).

At mid-fourth magnitude (4.2), 2 Cen (which like 3 Cen carries oneof the fewFlamsteed numbersin this southern constellation) is a fine, red, class M (M5)giant that does not vary all thatmuch, between 4.16 and 4.26 over about a possible 12 day period. The temperature of this reddish star is not all that well knowneither, around 3400 Kelvin as estimated from its class. When itslarge amount of infrared radiation is taken into account, the starshines at a rough luminosity 625 times that of theSun, which gives it a radius of 72 solar (0.34Astronomical Units, just short of the size of Mercury's orbit). Direct measure of angular diameter, however, gives a divergent andlarger radius of 89 times that of the Sun, which with temperaturewould imply a larger luminosity of 950 solar, so something is wrongsomeplace. Just under half a magnitude of interstellar dustabsorption could reconcile things, but given the color of the star,that seems rather unlikely. While the variation is reallyinsensible to the naked eye, one can at least admire the fine coloras seen through binoculars or a telescope.

One CAN, however, watch T Cen go through its cycle, as it variesbetween about magnitudes 5.5 and 8.0 over a well-determined 91 dayperiod. The star, a "bright giant," is then visible to the nakedeye for a brief duration (and easily watchable through a smalltelescope).

Over a 900 day period we watch T Centauri go through nearly ninecycles as it varies between visual magnitudes 5.5 and 8. The greendots are photometric measures, while the others are eye estimates. Tha gaps are caused by the Sun getting in the way. The lower axisgives theJulian Day Number, a running countsince January 1, 4713 BCE on the Julian Calendar. JD 2454549 isMarch 23, 2008, JD 2455225 January 28, 2010. Courtesy of theAmerican Association of Variable Star Observers, AAVSO, the datacontributed by observers worldwide.

As it varies in brightness, so does its class, from K0 to M4(averaging K6), the former unusually warm for such a star, thelatter rivalling its seeming neighbor, 2 Cen. The relation betweenthe visual cycle and that of temperature and class is not clear, soneither is the luminosity, but using averages it is in theneighborhood of 600 Suns, the radius (because of highertemperatures) smaller than that of 2 Cen, more like 50 solar. Bothstars have most likely recently given up helium fusion and arebrightening asadvancedgiants. Their difference seems mostly to be caused bydifference in mass, 2 Cen carrying perhaps 1 to 1.25 times that ofthe Sun, T weighing in maybe two Suns, the actual number quiteunclear. So T for 2 (sorry) is not such a bad description afterall. (Thanks to Jerry Diekmann for suggesting 2 Cen, which led toT.)

Written byJim Kaler 6/11/10. Return toSTARS.

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