ETA LEO (Eta Leonis). Eta Leo first distinguishes itself by being
the only star in Leo the Lion's head --
indeed in the whole traditional outline -- not to have a proper
name, even though at the bright end of fourth magnitude (3.51) it
is brighter than Rasalas (Mu Leonis),
which holds the Beast's faintness record. The reason for Eta's
obscurity is not unto itself, but because of its great distance of
2100 light years. It is so far that the errors of measurement are
huge, and allow a distance from 1400 to 4300 light years. Given
the distance, that we can even see the star tells of its
magnificence, the star a class A (A0) supergiant (though a lesser
one that is not quite on the scale of, say, Deneb). Temperature measures yield an
uncertain 9900 Kelvin, and the 2100 light year distance gives a
luminosity 16,400 times that of the Sun,
which in fact places it just where its class says it should be, so
the distance is probably pretty close to the mark. The luminosity
and temperature combine to give a radius 44 times solar. Like most
supergiants, it is losing mass, this one about five hundred-
millionths of a solar mass per year (over 100,000 times the flow
rate of the solar wind). Eta Leo is close enough to the ecliptic
that it is occasionally hidden, or occulted, by the Moon. Stars
are so angularly tiny that they wink out behind the lunar limb
nearly instantly. Eta was seen to wink out twice, showing it to be
double, one star approximately 60 percent brighter than the other
(though there is no confirmation of duplicity). If truly double,
both have masses around 9 times that of the
Sun, are at least 60
Astronomical Units apart, and orbit at least every 120 years. Now
about 17 million years old, the brighter (the class A star) seems
(with a dead helium core) to be in transition to becoming a much
larger supergiant. The dimmer is a hot class B hydrogen-fusing
star that will shortly follow. The two are right on the edge of
the mass range at which stars will blow up. Most likely they will
someday make a pair of massive white dwarfs. If
evolutionary processes (great expansion as red supergiants) can
draw them together, they might someday merge and then still
explode. We can only watch over the next several million years.