Mu Leonis

μ Leonis (Latinised to Mu Leonis, abbreviated Mu Leo, μ Leo) is the star's Bayer designation.

It bore the traditional names Rasalas and Alshemali, both abbreviations of Ras al Asad al Shamaliyy (‘north lion's head’). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Rasalas for this star on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.

Mu Leonis is an evolved K-type red giant star with a stellar classification of K2 IIIb CN1 Ca1. It is believed to be on the red giant branch, where it is fusing hydrogen into helium in a shell surrounding an inert helium core. The trailing notation indicates that, for a star of its type, it has stronger than normal absorption lines of cyanogen and calcium in its spectrum. It has around 1.5 times the Sun's mass and is estimated to be 5 billion years old, older than the Sun's age of 4.6 billion years. Using interferometry with the CHARA Array, its diameter was determined to be 11.74 times that of the Sun. Mu Leonis shines with 57 times the luminosity of the Sun from an outer atmosphere that has an effective temperature of 4,606 K.

In 2014 it was announced that Mu Leonis has a planetary companion that is at least 2.4 times as massive as Jupiter and orbits with a period of 358 days. This planet was detected by measuring radial velocity variations caused by gravitational displacement from the orbiting body.

Later in 2024, a study using astrometry from the Gaia spacecraft found a mass of 12.6 M_J, which the authors interpret as a likely upper limit, as the large level of RUWE in the astrometric solution, which could be caused by a companion around the star, might be just the result of systematic calibration errors. This indicates that Mu Leonis b lies in the planetary-mass regime and is not a brown dwarf.