ETA LEO (Eta Leonis). Eta Leonis 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 1275 light years (second Hipparcos reduction). Given the distance, that Eta Leo is still a part of the constellation's actual outline tells of its magnificence, the star a class A (A0) supergiant, though a lesser one that is not on the scale of Deneb. A temperature of 9730 Kelvin plus the distance gives a luminosity 5600 times that of the Sun (really making it more a "bright giant"), while the luminosity and temperature combine to give a radius 27 times solar. Direct measure of angular diameter yields a close 28 solar radii, suggesting that the parameters are correct. Theory then shows the star to carry a mass just over 7 times that of the Sun. Like most such massive stars, Eta Leo 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 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, the masses come in at just under 7 and about 5 solar. If so, the components are at least 40 Astronomical Units apart and complete an orbit at least every 65 years. However, then the radius of the brighter star no longer fits well with that derived from direct measure, implying the binary concept is not valid. Assuming that Eta Leo is just one star, it is now about 35 million years old, has a dead helium core, and is in transition to becoming a much larger supergiant. If it is two, the age of the brighter is somewhat greater, while the dimmer should be a hot class B hydrogen-fusing star that will shortly follow its mate. Whatever the case, the result will be one or two massive white dwarfs.
Written by Jim Kaler 3/30/03; revised 12/11/09. Return to STARS.