21 LMi (21 Leonis Minoris). North of Leo,
the celestial Lion, find the much fainter modern version, Leo Minor, the Smaller Lion. It's made
mostly of three fourth magnitude stars laid out in a flat triangle,
from east to west Praecipua (46 LMi),
Beta LMi, and the faintest of the trio
(magnitude 4.48) 21 LMi. Oddly, only Beta carries a Greek letter and only second-ranked 46
carries a proper name, though other Flamsteed numbers abound, going
from 7 to 41 with a number of gaps (the result of shifting constellation boundaries). A class A star,
at A7 a bit down the temperature range (8020 Kelvin), 21 LMi is
noted for its fast equatorial spin of at least 157 kilometers per
second, which makes the temperature a bit problematic, since the
flattened poles of rapid rotators are hotter than their equators.
Nevertheless, the metal content seems pretty much solar. The rapid
rotation also keeps things so stirred up that there are none of the
all-too-common abundance anomalies caused by diffusion, in which
some chemical elements settle downward by gravity while others are
lofted upward by radiation (Chi Lupi, Alpha Cancri, Delta Capricorni, and others coming to
mind). From a well-determined distance of 92 light years, give or
take just a half, 21 LMi shines just 10 times more brightly than
our Sun, which gives it a radius of 1.6 times solar and thus a
rotation period of under half a day (as opposed to the 25-day
rotation period of our own Sun). From that distance, our Sun would appear as a dim 7th magnitude star in
Pisces Austrinus southwest of first
magnitude Fomalhaut that would be
invisible to the naked eye. The mass then comes in at 1.75 times
solar, theory suggesting that the star is very young (though one
source puts it at 900 million years, half way to the end of its
hydrogen-fusing life). The star is listed as a Delta Scuti type of chattering variable,
with a range of 0.05 magnitudes that can take it just barely into
fifth magnitude and a poorly-determined period of a tenth of a day.
Oddly, there have been no follow-up observations to nail down the
variable character. As we find for a large number of class A stars
(Vega in the lead), 21 LMi is surrounded by
a disk, more likely a ring, of cool infrared-radiating dust that
may indicate some sort of planetary system,
though none has ever actually been detected either through stellar
motion or directly (as has been the case for, again, Fomalhaut).
Indeed, there seems to be no companion at all, at least not a
stellar one that has ever been found.
Written by Jim Kaler 6/04/10. Return to STARS.