2 AND T CEN (2 and T Centauri). While cool semiregular variable stars are not that unusual, they are not that common either. What IS unusual is to find two of them visible to the naked eye so close together, just 1.75 degrees apart (T to the northwest of much brighter 2), in northern Centaurus. If you 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 the two are really quite different, 2 Cen an ill-defined "SRb" star, T an "SRa," which behaves more like a regular, though shorter-period, Mira-type variable. And their closeness is just line-of-sight, as 2 Cen is 183 light years away (give or take two), 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 one of the few Flamsteed numbers in this southern constellation) is a fine, red, class M (M5) giant that does not vary all that much, between 4.16 and 4.26 over about a possible 12 day period. The temperature of this reddish star is not all that well known either, around 3400 Kelvin as estimated from its class. When its large amount of infrared radiation is taken into account, the star shines at a rough luminosity 625 times that of the Sun, which gives it a radius of 72 solar (0.34 Astronomical Units, just short of the size of Mercury's orbit). Direct measure of angular diameter, however, gives a divergent and larger radius of 89 times that of the Sun, which with temperature would imply a larger luminosity of 950 solar, so something is wrong someplace. Just under half a magnitude of interstellar dust absorption could reconcile things, but given the color of the star, that seems rather unlikely. While the variation is really insensible to the naked eye, one can at least admire the fine color as seen through binoculars or a telescope.

One CAN, however, watch T Cen go through its cycle, as it varies between about magnitudes 5.5 and 8.0 over a well-determined 91 day period. The star, a "bright giant," is then visible to the naked eye for a brief duration (and easily watchable through a small telescope).
Over a 900 day period we watch T Centauri go through nearly nine cycles as it varies between visual magnitudes 5.5 and 8. The green dots are photometric measures, while the others are eye estimates. Tha gaps are caused by the Sun getting in the way. The lower axis gives the Julian Day Number, a running count since January 1, 4713 BCE on the Julian Calendar. JD 2454549 is March 23, 2008, JD 2455225 January 28, 2010. Courtesy of the American Association of Variable Star Observers, AAVSO, the data contributed 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, the latter rivalling its seeming neighbor, 2 Cen. The relation between the visual cycle and that of temperature and class is not clear, so neither is the luminosity, but using averages it is in the neighborhood of 600 Suns, the radius (because of higher temperatures) smaller than that of 2 Cen, more like 50 solar. Both stars have most likely recently given up helium fusion and are brightening as advanced giants. Their difference seems mostly to be caused by difference in mass, 2 Cen carrying perhaps 1 to 1.25 times that of the Sun, T weighing in maybe two Suns, the actual number quite unclear. So T for 2 (sorry) is not such a bad description after all. (Thanks to Jerry Diekmann for suggesting 2 Cen, which led to T.)
Written by Jim Kaler 6/11/10. Return to STARS.