ALGIEBA (Gamma Leonis), a double giant with a planet. Double stars dot the sky. Many are the favorites of amateurs and observatory nights. Some, like those that make Mizar in Ursa Major, are plain white, while others like Albireo exhibit beautiful color contrasts. Even when the colors are fairly similar, the eye enhances them, rendering close unequal pairs like second magnitude Algieba (Gamma Leonis, the two components just 4.7 seconds of arc apart) quite lovely, Smyth and Chambers in the nineteenth century calling Gamma "a splendid double star...bright orange and greenish yellow" and a "most beautiful object." Named after its place in the foreparts of Leo the Lion, the Arabic name Algieba means "the forehead," and was originally applied to several of the stars of Leo's famed "Sickle." Algieba marks the radiant of the renowned Leonid meteor storm (the debris of Comet Temple-Tuttle), the storm returning with varying degrees of success every 33 years, the last in 1998. In 1833, the storm produced a fall at a fantastic rate of 100,000 per hour.

Precise magnitudes for the components are a problem, as each star affects the measure of the other. From the best-agreeing pair of sources, second magnitude Gamma-1 Leo, the western and brighter of the two (also called Gamma Leonis A), shines at magnitude 2.29, while fourth mag Gamma-2 (Gamma Leo B) comes in at 3.55, the two together glowing at a near-perfect mid-second (1.99). The brighter, the more orange of the two, is a class K (K1) giant with a somewhat uncertain temperature of 4410 Kelvin, while the fainter is a warmer (4870 Kelvin) class G (G7) giant, making it the yellower one. Such giant pairings (like Capella) are rather unusual. The orbital period is so long, estimated at 510 years, that only a fraction of the full path has been seen since discovery. At the star's distance of 131 light years (second Hipparcos reduction, give or take 3), the calculated orbit shows that the two components average 170 Astronomical Units apart, more than four times the average distance of Pluto from the Sun. A very high eccentricity, however, takes the two between 26 and 313 AU apart, the pair closest in 1671. But there are problems: see below.
Algieba The relatively short interval of observation is insuffient to constrain Algieba's orbit very well. From the best fit to the data, Gamma-2 Leo orbits Gamma-1 (at the cross, in reality the pair orbiting each other) over a 510 year period at an average separation of 170 AU with a high eccentricity. The major axis of the orbit (the dot-dash line) is offset because of the high (76 degree) orbital tilt to the plane of the sky. The orbital parameters, however, give a sum of masses far too high, showing a far longer period of observation is needed. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
The stars are quite luminous, Gamma-1 285 times brighter than the Sun, the other 72 times (when invisible infrared is taken into account), leading to respective diameters of 29 and 12 times solar. Theory then gives Gamma-1 and Gamma-2 masses of 3.0 and 2.5 Suns. With the constraint that both were born at the same time and have the same ages of about 500 million years, they are best understood as being in different stages of gianthood. Gamma-2 may just be starting to fuse its internal helium into carbon and oxygen, while more massive Gamma-1 seems to have finished the job and is brightening further. Both were born as blue-white class B stars, roughly B8 and B9.5. Kepler's laws applied to the orbit, however, give a much higher sum-of-masses, 19 Suns, showing that the orbit is just approximate, a fit being secured by a much smaller mean separation between the two of 112 AU. (In answering a query, an orbital expert said "Ask me again in a couple centuries.") Both stars are deficient in metals, carrying only about a third the iron content (relative to hydrogen) as the Sun. Consistently, the stars are both weak in the cyanogen (CN) molecule, implying lower abundances of lighter elements as well. The pair also moves with rather high speed relative to the Sun, 74 kilometers per second, some five times normal, that and the low metal content suggesting that the binary comes from a different part of the Galaxy. Some six minutes of arc away are ninth and tenth magnitude "companions," neither of which actually belongs to the system (though one of the "line-of- sighters," Gamma Leo C, is itself a tight orbiting binary).

THE PLANET: In 2010, astronomers announced the discovery of a planet orbiting the brighter giant, Gamma-1 (Gamma Leo A), yet another large one with a minimum mass of 8.8 times that of Jupiter in an Earth-like orbit averaging 1.2 AU in size with a period of 1.2 years. A modest eccentricity takes it between 1.0 and 1.4 AU from Gamma-1. The planet is in modest violation of the general trend of planet-holding stars having higher metal contents than does the Sun. However, these figures are for a much lower stellar mass of 1.2 Suns. Raising the mass of Gamma-1 to 3 Suns increases the orbital radius of the planet to 1.6 AU and the planetary mass to at least 16 Jupiters, which places it into the realm of possible brown dwarfs (failed stars that cannot run full hydrogen fusion). But note that the evolutionary masses of the stars are also suspect, as they depend on the uncertain visual magnitudes. (Thanks to Jason Pero, who helped research this star and to Bill Hartkopf for expert commentary.)
Written by Jim Kaler 6/07/99; revised 8/18/06 and 3/04/11. Return to STARS.