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Mu Leonis
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Leo
Right ascension	09h 52m 45.81654s
Declination	+26° 00′ 25.0319″
Apparent magnitude (V)	3.88
Characteristics
Evolutionary stage	Red giant branch
Spectral type	K2 IIIb CN1 Ca1
U−B color index	+1.38
B−V color index	+1.23
Astrometry
Radial velocity (Rv)	14.03±0.19 km/s
Proper motion (μ)	RA: −217.31 mas/yr ; Dec.: −54.26 mas/yr
Parallax (π)	26.28 ± 0.16 mas
Distance	124.1 ± 0.8 ly ; (38.1 ± 0.2 pc)
Absolute magnitude (MV)	+0.83
Details
Mass	1.5±0.1 M☉
Radius	11.89±0.11 R☉
Luminosity	53±1 L☉
Surface gravity (log g)	2.43±0.06 cgs
Temperature	4,519±23 K
Metallicity [Fe/H]	0.27±0.03 dex
Rotational velocity (v sin i)	4.5 km/s
Age	3.35±0.70, ~5.0 Gyr
Other designations
	Rasalas, Alshemali, μ Leo, 24 Leonis, BD+26°2019, HD 85503, HIP 48455, HR 3905, SAO 81064
Database references
SIMBAD	data

Mu Leonis (μ Leonis, abbreviated Mu Leo, μ Leo), also named Rasalas /ˈræsəlæs/,^[10]^[11] is a star in the constellation of Leo. The apparent visual magnitude of this star is 3.88,^[2] which is bright enough to be seen with the naked eye. Based upon an annual parallax shift of 0.02628 arc seconds as measured by the Hipparcos satellite,^[1] this system is 124 light-years (38.1 parsecs) from the Sun. In 2014, an exoplanet was discovered to be orbiting the star.

Nomenclature

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

It bore the traditional names Rasalas and Alshemali, both abbreviations of Ras al Asad al Shamaliyy.^[12] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[13] 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.^[11]

Properties

Mu Leonis is an evolved K-type red giant star with a stellar classification of K2 IIIb CN1 Ca1.^[4] 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.^[3] 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.^[14] It has around 1.5 times the Sun's mass^[7] and is estimated to be 5 billion years old,^[3] older than the Sun's age of 4.6 billion years. Using interferometry with the Navy Precision Optical Interferometer, its diameter was determined to be 11.8 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.^[15]

Planetary system

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.^[7]

Later in 2024, a study using astrometry from the Gaia spacecraft find 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 indicate that Mu Leonis b lies in the planetary-mass regime and is not a brown dwarf.^[16]

The Mu Leonis system
Companion	Mass	Semimajor axis (AU)	Orbital period	Eccentricity	Discovery year
b	≥2.4±0.4 and ≤12.6 M_J	1.1±0.1	357.8±1.2 days	0.09±0.06	2014

References

^ ^a ^b ^c ^d ^e ^f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
^ ^a ^b ^c ^d Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
^ ^a ^b ^c ^d Howes, Louise M.; Lindegren, Lennart; Feltzing, Sofia; Church, Ross P.; Bensby, Thomas (2019-02-01). "Estimating stellar ages and metallicities from parallaxes and broadband photometry: successes and shortcomings". Astronomy & Astrophysics. 622: A27. arXiv:1804.08321. Bibcode:2019A&A...622A..27H. doi:10.1051/0004-6361/201833280. ISSN 0004-6361.
^ ^a ^b Keenan, P.; McNeil, R. (October 1989), "The Perkins catalog of revised MK types for the cooler stars", Astrophysical Journal Supplement Series, 71: 245–266, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
^ ^a ^b Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209, S2CID 121883397.
^ Cardini, D. (January 2005), "Mg II chromospheric radiative loss rates in cool active and quiet stars", Astronomy and Astrophysics, 430: 303–311, arXiv:astro-ph/0409683, Bibcode:2005A&A...430..303C, doi:10.1051/0004-6361:20041440, S2CID 12136256.
^ ^a ^b ^c ^d Lee, B.-C.; et al. (2014). "Planetary Companions in K giants β Cancri, μ Leonis, and β Ursae Minoris". Astronomy & Astrophysics. 566: 7. arXiv:1405.2127. Bibcode:2014A&A...566A..67L. doi:10.1051/0004-6361/201322608. S2CID 118631934. A67.
^ ^a ^b ^c ^d ^e Soubiran, C.; Creevey, O. L.; Lagarde, N.; Brouillet, N.; Jofré, P.; Casamiquela, L.; Heiter, U.; Aguilera-Gómez, C.; Vitali, S.; Worley, C.; de Brito Silva, D. (2024-02-01), "Gaia FGK benchmark stars: Fundamental Teff and log g of the third version", Astronomy and Astrophysics, 682: A145, Bibcode:2024A&A...682A.145S, doi:10.1051/0004-6361/202347136, ISSN 0004-6361 Mu Leonis' database entry at VizieR.
^ "* mu. Leo". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2016-09-28.{{cite web}}: CS1 maint: postscript (link)
^ Rumrill, H. B. (June 1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. 48 (283). San Francisco, California: 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681. S2CID 120743052.
^ ^a ^b "Naming Stars". IAU.org. Retrieved 16 December 2017.
^ Allen, Richard Hinckley (1899), Star-names and Their Meanings, G. E. Stechert, retrieved 2016-09-28.
^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
^ Keenan, Philip C. (August 1987), "Spectral types and their uses", Publications of the Astronomical Society of the Pacific, 99: 713–723, Bibcode:1987PASP...99..713K, doi:10.1086/132036.
^ Baines, Ellyn K.; Armstrong, J. Thomas; Schmitt, Henrique R.; Zavala, R. T.; Benson, James A.; Hutter, Donald J.; Tycner, Christopher; Van Belle, Gerard T. (2018), "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer", The Astronomical Journal, 155 (1): 30, arXiv:1712.08109, Bibcode:2018AJ....155...30B, doi:10.3847/1538-3881/aa9d8b, S2CID 119427037.
^ Wallace, A. L.; Casey, A. R.; Brown, A. G. A.; Castro-Ginard, A. (2024-11-10). "Detection and Characterisation of Giant Planets with Gaia Astrometry". Monthly Notices of the Royal Astronomical Society. 536 (3): 2485. arXiv:2411.06705. Bibcode:2025MNRAS.536.2485W. doi:10.1093/mnras/stae2769.