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.