Zeta Centauri (Leepwal)

Leepwal, Zeta Centauri (ζ Cen), is the primary component in a binary system located approximately 382 light-years away in the southern constellation of Centaurus. With an apparent magnitude of 2.55, it is the seventh brightest point of light in Centaurus. It appears in the body of the celestial Centaur.

Star system

Zeta Centauri is a double-lined spectroscopic binary star with an orbital period of 8.024 days. The two components are separated by only 1.4 milliarcseconds and cannot be resolved even in the largest of telescopes.

The primary component, Zeta Centauri A (formally known as Leepwal), has the spectral type B2.5 IV, indicating a hot blue subgiant star. The star has a mass 7.8 times that of the Sun and a radius of 5.80 solar radii. It has an effective temperature of about 23,561 K and an estimated age of 39.8 million years. The young star is thousands of times more luminous than the Sun.

Leepwal is moving away from us with a radial velocity of +6.5 km/s. The star is an exceptionally fast spinner, with a projected rotational velocity of 235 km/s. The fast rotation has flattened the star into an oblate shape. Zeta Centauri has an equatorial bulge and its polar radius is smaller than it radius at the equator.

Leepwal (Zeta Centauri), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Facts

Shining at magnitude 2.55, Zeta Centauri is the seventh brightest star in Centaurus. It is, on average, the 95th brightest star in the night sky. It is only slightly fainter than Aljanah in the constellation Cygnus, Markab in Pegasus, and Menkar in Cetus. It is about as bright as Zeta Ophiuchi in Ophiuchus and Zosma in Leo, and it just outshines Acrab in Scorpius, Arneb in Lepus, and its Centaurus neighbour Delta Centauri.

In traditional Chinese astronomy, Leepwal was known as 庫樓一 (Kù Lóu yī), the First Star of Arsenal. It formed the Arsenal asterism with Eta Centauri, Menkent (Theta Centauri), 2 Centauri, d Centauri, f Centauri, Gamma Centauri, Tau Centauri, D Centauri, and Sigma Centauri. The asterism represented the armory. It was part of the larger Horn mansion, which represented the horns of the Azure Dragon.

The star Iota Centauri, now formally known as Kulou, was originally the main star of the Arsenal (Kulou) asterism.

Name

Leepwal (pronunciation: leyepwal) is the traditional name for Zeta Centauri in the Marshall Islands. The original spelling of the name in Marshallese is Ļeepwal.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) approved the name on July 18, 2024.

Zeta Centauri was historically also known as Alnair (pronunciation: /æˈnɛər/). The name comes from the Arabic phrase nayyir baṭan qanṭūris (Al Nā᾽ir al Baṭn al Kentaurus), meaning “the bright (star) of the body of the Centaur.”

However, R. H. Allen points out in his Star Names: Their Lore and Meaning (1899) that the early designation Zeta Centauri referred to one of the stars of the Southern Cross in the constellation Crux, as did the designations Epsilon, Nu and Xi² Centauri. The name Alnair now formally applies to Alpha Gruis, the brightest star in the constellation Grus (the Crane).

Zeta Centauri was also called Baten Kentaurus, “the belly of the Centaur.”

Location

Zeta Centauri is relatively easy to find because it is bright and part of the familiar constellation figure of Centaurus. It is the brightest point of light in the region between Rigil Kentaurus (Alpha Centauri) and Kulou (Iota Centauri).

Omega Centauri, the brightest and largest globular cluster in the sky, lies about 5 degrees almost directly west of Zeta Centauri, roughly halfway between Menkent in Centaurus and Gacrux in Crux. The barred spiral galaxy NGC 4945 appears in the same region of the sky. The bright galaxy Centaurus A appears about 4 degrees north of Omega Centauri.

At declination -47°, Zeta Centauri is best seen from the southern hemisphere. It never rises for observers north of the latitude 42° N.

Location of Leepwal (Zeta Centauri), image: Stellarium

Zeta Centauri, Omega Centauri, Centaurus A and NGC 4945, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Constellation

Leepwal is located in the constellation Centaurus. Centaurus is the ninth largest constellation in the sky and the fifth largest southern constellation. It stretches across 1,060 square degrees of the far southern sky.

Centaurus is one of the 48 Greek constellations, catalogued by the Greek astronomer Claudius Ptolemy in his Almagest in the 2nd century CE. It is one of the brightest constellations in the sky, along with Orion, as well as one of the oldest. It is associated with the Babylonian Bison-man (MUL.GUD.ALIM).

Centaurus is home to Rigil Kentaurus and Hadar (Alpha and Beta Centauri), the third and 11th brightest stars in the sky. Alpha Centauri is the closest star system to the Sun. It contains the Sun-like stars Rigil Kentaurus and Toliman and the red dwarf Proxima Centauri, the nearest individual star to the Sun.

Other bright stars in the constellation include the orange giant Menkent (Theta Centauri), the binary star Gamma Centauri, the Beta Cephei variable Epsilon Centauri, the massive blue stars Eta Centauri and Delta Centauri, and the white main sequence star Kulou (Iota Centauri).

Centaurus also hosts the chemically peculiar Przybylski’s Star, the Mira variable R Centauri, the hypergiant V766 Centauri, one of the largest stars discovered to date, and the white dwarf BPM 37093, popularly known as Lucy or the Diamond Star.

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

Centaurus is home to many bright and well-known deep sky objects. These include the massive globular cluster Omega Centauri (NGC 5139), the active galaxy Centaurus A (NGC 5128), the barred spiral galaxy NGC 4945, the Running Chicken Nebula (IC 2944), the Pearl Cluster (NGC 3766), and the Blue Planetary Nebula (NGC 3918). Other deep sky objects in the constellation include the preplanetary Boomerang Nebula, the blue dwarf galaxy NGC 5253, and the Backward Galaxy (NGC 4622).

The best time of the year to observe the stars and deep sky objects of Centaurus is during the month of May, when the constellation appears higher above the horizon in the early evening. The entire constellation can be seen from locations south of the latitude 25° S. From the southern hemisphere, Centaurus is visible throughout the year.

The 10 brightest stars in Centaurus are Alpha Centauri (mag. -0.27), Hadar (Beta Cen, mag. 0.61), Menkent (Theta Cen, mag. 2.06), Gamma Centauri (mag. 2.17), Epsilon Centauri (mag. 2.30), Eta Centauri (mag. 2.35), Leepwal (Zeta Cen, mag. 2.55), Delta Centauri (mag. 2.57), Kulou (Iota Cen, mag. 2.73), and Lambda Centauri (mag. 3.13).

Leepwal – Zeta Centauri