Gamma Leonis
Binary red giant system in Leo with one exoplanet.
Gamma Leonis, commonly called Algieba, is a binary star system located in the constellation Leo. Its two components are both red giants, and the brighter of the two has one confirmed exoplanet in orbit.
The system’s Bayer designation is γ Leonis, Latinized to Gamma Leonis. The two stars are individually labeled γ¹ Leonis and γ² Leonis. The traditional name Algieba (also spelled Al Gieba) comes from the Arabic Al-Jabhah, meaning “the forehead,” though the star actually lies in the lion’s mane. In 2016, the International Astronomical Union’s Working Group on Star Names officially approved Algieba as the proper name. Its older Latin name was Juba. In Chinese astronomy, it is known as the Twelfth Star of Xuanyuan, a constellation representing the Yellow Emperor. Algieba is also part of the Sickle asterism, which outlines the head of Leo, along with Zeta Leonis, Regulus, Mu Leonis, Epsilon Leonis, and Eta Leonis.
The system appears as a mid-second-magnitude point of light to the naked eye, but a modest telescope under good conditions reveals two stars: an orange-red primary and a yellow or greenish-yellow secondary. William Herschel discovered that Gamma Leonis is a double star in 1792. The primary, Gamma¹ Leonis, has an apparent magnitude of +2.28 and a spectral class of K0III. It is a giant star with a surface temperature that gives it about 250 times the Sun’s luminosity and a diameter 26 to 32 times larger. The secondary, Gamma² Leonis, shines at magnitude +3.51 and is a G7III giant. It is 63 times more luminous than the Sun and about 10 times its diameter. Both stars are likely red clump giants—evolved stars that have undergone a helium flash and now fuse helium in their cores. They are estimated to be about 2 billion years old and have lower-than-solar metallicities.
The orbital parameters of the pair remain uncertain because the orbital period is very long—only a fraction of the orbit has been observed. Estimates range from 400 to 700 years, with a highly eccentric orbit. One preliminary solution gives a period of 554 years and an angular semi-major axis of 3.1 arcseconds; another gives 510 years and 4.24 arcseconds. These estimates are questionable because, using Kepler’s third law, they imply total masses that conflict with values derived from stellar evolution models.
Quick Facts
- Bayer designation
- γ Leonis
- Apparent magnitude (combined)
- 2.16 to 2.21
- Spectral types
- K0III (primary), G7III (secondary)
- Number of known exoplanets
- 1
- Orbital period of planet
- 429 days
- Minimum mass of planet
- about 8.78 Jupiter masses (revised)
Facts from the source article.
Lore & Background
The traditional name Algieba (or Al Gieba) comes from the Arabic 'Al-Jabhah', meaning 'the forehead', though the star actually appears in the mane of Leo. In 2016, the International Astronomical Union's Working Group on Star Names approved Algieba as the proper name. The star also bore the Latin name Juba and is known in Chinese as the Twelfth Star of Xuanyuan. Algieba, together with Zeta Leonis, Regulus, Mu Leonis, Epsilon Leonis and Eta Leonis, forms the asterism called the Sickle, which marks the head of Leo.
The binary nature of Gamma Leonis was discovered by William Herschel in 1792. The primary component (γ¹ Leonis) has an apparent magnitude of +2.28 and is a K0III giant, while the secondary (γ² Leonis) has magnitude +3.51 and is a G7III giant. Both are likely red clump giants that have undergone a helium flash and are now fusing helium at their cores. They are estimated to be 2 billion years old with subsolar metallicities. The orbital parameters remain uncertain due to the very long orbital period, estimated between 400 and 700 years, with a highly eccentric orbit. A 2025 revision using 190 years of data gave a period of 553 years and a semi-major axis of 3.1 arcseconds, though the implied sum of masses is inconsistent with stellar evolution models.
The secondary component is a variable star, with the combined magnitude varying from 2.16 to 2.21 over a period of either 1.65289 or 1.6245 days; the variable type is unknown. The flare star AD Leonis lies just 5 arcminutes from the system but is unrelated and much closer to Earth.
Reader's Guide
Gamma Leonis holds significance as a bright binary system that has been studied for over two centuries, with its double nature first resolved by William Herschel. Its traditional name Algieba connects it to Arabic astronomical heritage, and its inclusion in the Sickle asterism ties it to the iconic shape of Leo. The system's long orbital period and uncertain parameters make it a continuing subject of study in stellar dynamics. The discovery in 2009 of a gas giant exoplanet around the primary star added to its importance, as it is one of the relatively few planets known to orbit a red giant star. The planet's minimum mass was later revised upward, and radial velocity measurements suggest possible additional periodicities that may indicate a second substellar companion, though this remains unconfirmed. The system's variability and the presence of the nearby flare star AD Leonis further contribute to its interest for stellar and exoplanetary research. Its legacy is that of a well-known naked-eye star that reveals complexity upon closer examination, from its binary nature to its planetary system and variability.
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