Rasalas (Mu Leonis)

Rasalas, Mu Leonis (μ Leo), is a K-type red giant star located 124.1 light-years away in the constellation Leo. It has an apparent magnitude of 3.88. It is one of the stars that form the Sickle of Leo. The star hosts an exoplanet, Mu Leonis b, discovered in 2014.

Star type

Rasalas is an evolved orange giant of the spectral type K2 IIIb CN1 Ca1. The spectral class indicates that the star has enhanced absorption lines of calcium and cyanogen in its spectrum.

Mu Leonis is currently on the red giant branch (RGB). It has an inert helium core and is fusing hydrogen into helium in a shell around the core.

Rasalas has a mass of 1.5 solar masses and has expanded to a size of 11.89 solar radii as it evolved away from the man sequence. With a surface temperature of 4,519 K, it is around 53 times more luminous than the Sun. The giant star spins at 4.5 km/s. It has an estimated age of 3.35 or 5 billion years.

Rasalas (Mu Leonis), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Planet

Rasalas has an orbiting planet, Mu Leonis b, discovered using the radial velocity method in 2014. Upon discovery, astronomers reported a minimal mass of 2.4 Jupiter masses (763 Earth masses) and an orbital period of 357.8 days for the planet. Mu Leonis b orbits the host star at a distance of 1.1 astronomical units (Earth – Sun distances).

In 2024, astrometric data obtained by the Gaia spacecraft allowed astronomers to calculate a maximum mass of 12.6 Jupiter masses.

Artist’s impression of Mu Leonis b, image credit: Wikimedia Commons/KKolaczynski (CC BY-SA 4.0)

Facts

Rasalas is one of the bright stars of Leo that form the Sickle, an asterism that corresponds to the head, mane and shoulders of the celestial Lion. The asterism looks like a backward question mark in the sky. The bright Regulus appears at the base of the Sickle and marks the Lion’s heart. The Sickle is formed by Rasalas with Regulus (Alpha Leonis), Eta Leonis, Algieba (Gamma Leonis), Adhafera (Zeta Leonis), and Epsilon Leonis.

Name

The name Rasalas (pronunciation: /ˈræsəlæs/) comes from the Arabic Ras al Asad al Shamaliyy or ra’s alasad (al-shamali), meaning “the northern part of the Lion’s head.” It is the traditional name of Mu Leonis and refers to the star’s position in the constellation Leo.

The star was historically also known as Ras Elased Borealis (Rasalasad Borealis), while its brighter neighbour Epsilon Leonis was called Ras Elased Australis, “the southern (star) of the Lion’s head.”

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) approved the name Rasalas for Mu Leonis on September 12, 2016.

Location

Rasalas is easy to find because it is part of a conspicuous spring asterism, the Sickle of Leo. It is the star at the top of the Sickle.

The Sickle can be found using the bright stars of the Big Dipper. A line extended from Megrez through Phecda, the inner stars of the Big Dipper’s bowl, leads to Regulus, the bright star at the base of the Sickle.

The location of Rasalas (Mu Leonis), image: Stellarium

Constellation

Rasalas is located in the zodiac constellation of Leo. The celestial Lion is one of the ancient Greek constellations, catalogued by the Greco-Roman astronomer Claudius Ptolemy of Alexandria in his Almagest in the 2nd century CE. In Greek lore, Leo is associated with the myth of Heracles and the Nemean Lion, a mythical monster defeated by the hero as part of his twelve labours.

Leo occupies an area of 947 square degrees and is the 12th largest constellation in the sky. It lies on the celestial equator and is visible from virtually anywhere for at least part of the year. It is also one of the zodiac constellations, which lie on the ecliptic (the Sun’s apparent path across the sky). When the Sun appears to pass through Leo, the constellation does not rise in the night sky for about a month each year.

Leo hosts Regulus, the 21st brightest star in the sky. The hot blue star shines at magnitude 1.40 from a distance of 79.3 light-years. Other bright stars in Leo include the orange giants Algieba (Gamma Leonis) and Alterf (Lambda Leonis), the A-type stars Denebola (Beta Leonis), Zosma (Delta Leonis) and Chertan (Theta Leonis), the yellow-white giant Adhafera (Zeta Leonis), and the blue supergiants Shaomin (Rho Leonis) and Eta Leonis.

Leo constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Leo is also home to the red dwarf Wolf 359, one of the nearest stars to the solar system, Icarus, one of the most distant stars known, the carbon star CW Leonis, and Caffau’s Star, one of the oldest known stars in our galaxy.

Deep sky objects in Leo include the Leo Triplet, consisting of the spiral galaxies Messier 65, Messier 66, and the Hamburger Galaxy (NGC 3628), the Leo I Group (Messier 95, Messier 96 and Messier 105), the Owl Galaxy (NGC 3758), the Frosty Leo Nebula, and the Cosmic Horseshoe, a gravitationally lensed system of two distant galaxies.

The best time of the year to see the stars and deep sky objects of Leo is during the month of April, when the constellation rises higher above the horizon in the early evening. The entire constellation is visible from locations north of the latitude 65° S.

The 10 brightest stars in Leo are Regulus (Alpha Leo, mag. 1.40), Algieba (Gamma Leo, mag. 2.08), Denebola (Beta Leo, mag. 2.113), Zosma (Delta Leo, mag. 2.56), Epsilon Leonis (mag. 2.98), Chertan (Theta Leo, mag. 3.324), Adhafera (Zeta Leo, mag. 3.33), Eta Leonis (mag. 3.486), Subra (Omicron Leo, mag. 3.52), and Shaomin (Rho Leo, mag. 3.9).

Rasalas – Mu Leonis