CHI CNC (Chi Cancri). Though only fifth magnitude (5.14), Chi Cancri
nevertheless is part of the outline (at least that used here; others
differ) of Cancer, the Crab, whose
fame comes not from its bright stars but from its position in the
ancient Zodiac, situated between
Gemini and Leo (though its harboring of the Beehive cluster does not hurt).
Don't confuse it with Chi Geminorum, which lies across the border in
Gemini just 3.7 degrees to the west and is an ordinary orange K2 giant 256 light years away.
Chi Cancri on the other hand is very different; as a class F (F6)
it is not all that dissimilar from the Sun. And as a lower mass dwarf, it has to
be much closer than Chi Gem, just 60 light years away (give or take
under 1). From its point of view, our Sun would be a full magnitude
fainter, mixed in with the stars of deep southern Capricornus. In spite of Chi Cnc's
relative faintness, it's pretty popular. More than 20 temperature
measures average 6270 Kelvin. With no significant correction for
infrared or ultraviolet light, the star radiates at a rate 2.4 times
that of the Sun, which gives it a radius of 1.30 times solar. Direct
measure of angular diameter through interferometry yields a radius of
1.39 times that of the Sun, just seven percent higher, not all that
bad. The mass is about 20 percent greater than that of the Sun, a more
precise study giving 1.07 solar. From other studies, the age is close
to that of our Sun, which because of its higher mass means that Chi
Cnc is relatively older when compared with its hydrogen fusing lifetime
of around 7.5 billion hears. Chi Cancri is just under the "rotation
break" at class F5, hotter than which stars rotate much more rapidly
as a result of a decline in their outer convective layers, which
generate magnetic fields that, coupled with the stars' winds, slow them
down. Chi Cnc rotates with a projected equatorial speed of just 5
kilometers per second, giving it a rotation period under 13 days. Yet
the star seems to have no magnetic activity, which may have died away
as a result of age. Bright dwarfs like Chi Cnc are prime targets for
planet searches. Yet the star seems devoid. Especially telling, there
is no evidence for any debris disk as seen around so many stars,
including hotter ones like Vega.
Perhaps that is related to the lower metal content, which is about half
(relative to hydrogen) that of the Sun, planets rather liking higher
values. The lower metal content may be related to a somewhat higher
velocity through space relative to the Sun, about triple normal. Not
only are there no planets (at least found), there are no companions either, the
star quite alone in a lonely spot in the Zodiacal Crab.
Written byJim Kaler 4/18/14. Return to STARS.