AL TARF (Beta Cancri). Surely, if Cancer
(the Crab) were anywhere but on the ecliptic, the apparent pathway
of the Sun, it never would have been made into a constellation by
the ancients. Its brightest star, Al Tarf, is fourth magnitude,
though in its favor (at 3.59) it is just over the line from third.
Though the luminary of the constellation, it still received the
Beta designation from Bayer. Marking one of the celestial Crab's
legs, Al Tarf -- from Arabic meaning "the End" -- is rather distant
from the most prominent part of the constellation, the four-star
"manger" at the center, which contains the naked-eye Beehive
cluster. Al Tarf is a mid-temperature (4040 Kelvin) orange giant
of class K. At a distance of 290 light years (which accounts for
its relative faintness), it actually shines (accounting for a fair
bit of infrared radiation) 660 times brighter than our Sun, from which we find a true giant diameter
53 times solar, 65 percent the size of Mercury's orbit. A direct
measure of angular diameter gives a similar true diameter 48 times
solar, showing that the measures of the star's properties are
consistent. This large star takes almost two full
Earth-years to make a full rotation.
Though about the same color and temperature as much
better known Aldebaran, Al Tarf has
twice the luminosity, and is thus on the bright side of gianthood.
It began life as a star of about three solar masses, though it is
quite difficult to tell, as in its stage of life, stars of
different masses rather gang together at similar temperatures and
luminosities. Similarly, though the star is clearly dying, it is
hard to tell just what state it is in, though most likely it is now
fusing its core helium into carbon and oxygen. Al Tarf has a
rather distant, very faint, fourteenth magnitude companion, that
from its actual luminosity must be an ordinary red dwarf of class
M. At an angular distance of 29 seconds of arc, the companion is
at least 2600 astronomical units (65 times the distance of Pluto
from the Sun) from the bright giant. The two move through space
together and are almost certainly real companions, though no
orbital motion can be seen, no surprise given that the period is at
least 76,000 years! From the companion, Al Tarf would shine with
30 times the brilliance of our full Moon. Al Tarf is a mild
"barium star" that has a barium abundance six times solar. Barium
stars are thought have been contaminated with by-products of
nuclear fusion by companions that have already evolved and died and
are now white dwarfs (like Sirius B),
though no such star has ever been found.