Gamma Leonis (γ Leonis, abbreviated Gamma Leo, γ Leo), also named Algieba, is a binary star system in the constellation of Leo, made up of two red giants. In 2009, a planetary companion around the primary was announced.
γ Leonis (Latinised to Gamma Leonis) is the star's Bayer designation. The A and B components of the binary are often referred to as γ1 Leonis and γ2 Leonis, respectively.
It also bore the traditional name Algieba or Al Gieba, which originated from the Arabic الجبهة Al-Jabhah, meaning 'the forehead' (despite this meaning, the star actually appears in the mane of Leo). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Algieba for this star.
The star's traditional Latin name was Juba. It is known as 軒轅十二 (the Twelfth Star of Xuanyuan) in Chinese (Xuanyuan is the name of the Yellow Emperor).
Algieba, along with Zeta Leonis, Regulus, Mu Leonis, Epsilon Leonis and Eta Leonis, have collectively been called the Sickle, which is an asterism that marks the head of Leo.
The bright binary system in Leo with orange-red and yellow or greenish-yellow components is visible through a modest telescope under good atmospheric conditions. To the naked eye, the Algieba system shines at mid-second magnitude, but a telescope easily splits the pair. The double nature of Gamma Leonis was discovered by William Herschel in 1792.
The brighter component (named Gamma1 Leonis) has an apparent magnitude of +2.28 and is of spectral class K0III. This giant K star has a surface temperature of, 250 times the luminosity of the Sun, and 26 times the Sun's diameter. The companion (named Gamma2 Leonis) has an apparent magnitude of +3.51 and belongs to the spectral class G7III. This giant G star has a temperature of, is 63 times more luminous and 10 times larger than the Sun. Both are more likely red clump giants, evolved stars that have initiated helium ignition at their core. They are estimated to be 2 billion years old and have subsolar metallicites.
The orbital parameters of Gamma Leonis are still uncertain due to the very long orbit. The orbital period is thought to be between 400 and 700 years. Preliminary data show that the stars have an orbital period of 554 years, are visually separated by 3.1' in the sky and have a highly-eccentric (0.93) orbit. Nonetheless, these parameters are contradictory with the physical parameters, by the Kepler's third law, it would imply that the combined mass is of . Another data set show parameters that imply a combined mass of, even more contradictory with the observed combined mass .
γ Leonis was a suspected variable star, with a visual magnitude range of 1.84 to 2.03. It is not known which of the two components is variable. The American Association of Variable Star Observers mention that it is a RS Canum Venaticorum variable with a smaller range of 1.98 to 2.02. In 1959, the star was mistakenly published as an eclipsing binary due to a typographical error when referring to .
The flare star AD Leonis lies just 5' from . It is unrelated to the pair, and much closer to Earth at a distance of 4.97pc.
On November 6, 2009, the discovery of a planetary companion around primary star γ1 Leonis (γ Leonis A) was announced. The radial velocity measurements suggest two additional periodicities of 8.5 and 1,340 days. The former is likely due to stellar pulsation, whereas the latter could be indicative of the presence of an additional planetary companion with 2.14 Jupiter masses, moderate eccentricity (e=0.13) and located at 2.6 AU away from the giant star. Nevertheless, the nature of such a signal is still unclear and further investigations are needed to confirm or rule out an additional substellar companion.
A new study revised the minimum mass for Gamma1 Leonis b to about (increased by 20%), based on a larger mass for the host star (instead of).