19 PSC (19 Piscium). As large as it is, dim Pisces, the Fishes, has few stars that carry proper names. It does, however, possess one of the more famed asterisms, its "Circlet," and just to the east of the Circlet (and sometimes actually included in it) is one of the more studied stars of the sky, a fifth magnitude (5.04) carbon star with the Flamsteed number19 Piscium. While not as red as the more extreme carbon stars, its reddish color still makes it stand out from its surroundings. Also slightly variable, it carries the variable-star name TX Piscium. The Sun is twice as rich in oxygen than in carbon (oxygen third in the list of abundances, carbon fourth, both following hydrogen and helium). Carbon stars reverse the ratio, making carbon much more abundant than oxygen. The spectrum of the 19 Psc is filled with absorptions of molecular carbon (C-2), cyanogen (CN), CH, carbon monoxide (CO), HCN, and a variety of other molecules. The mass of absorptions cuts out the blue part of the spectrum, turning the star red as seen with the eye. At a distance of 760 light years and classed as a C5 "bright giant" with a temperature of only 3050 Kelvin (which causes it to emit most of its radiation in the invisible infrared), the star radiates with a total luminosity 4700 times greater than the Sun, which implies a radius 245 times solar, or 20 percent larger than the Earth's orbit! Direct measures of angular diameter coupled with distance give a satisfyingly close value of 240 solar radii. Nineteen Psc is in an advanced state of evolution. Like classic Mira, it is brightening as a giant for the second time, now with a dead carbon core (first timers have dead helium cores), its mass, though greater than the Sun, not well known. The star varies by only a few tenths of a magnitude. While classed as an "irregular" variable, recent studies indeed show it to have some regularity to it, consistent with other stars in its situation. Carbon stars have changed their surface chemical compositions from oxygen-rich to carbon-rich by dredging up fresh carbon made by internal nuclear reactions (wherein three atoms of helium collide to form one atom of carbon with the release of energy). Proof of such transformation is provided by stars such as our "19," as it displays absorptions caused by the element technetium (Tc, number 43 in the periodic table). All isotopes of Tc are radioactive with short lives. There is none on Earth except that manufactured. But there it is in 19 Psc. The only way it can be there is to be manufactured by the star itself as part of a complex network of nuclear reactions. Here we have a fine demonstration that stars make the heavy elements of nature. Indeed, through their evolutionary processes, including the explosions of massive stars, they make them all.
Written by Jim Kaler. Return to STARS.