Beta Ophiuchi

Lore & Background

The star bears the traditional names Cebalrai, Celbalrai, Cheleb, and Kelb Alrai (or sometimes just Alrai), all derived from the Arabic كلب الراعي kalb al-rā‘ī, meaning 'the shepherd's dog'. In 2016, the International Astronomical Union's Working Group on Star Names approved the name Cebalrai for this star. In Chinese astronomy, β Ophiuchi and γ Ophiuchi form the asterism Zongzheng, transliterated as Tsung Ching by R. H. Allen.

Beta Ophiuchi is a giant star with a stellar classification of K2 III. Although only 2.75 billion years old—60% of the solar age—it has expanded to about 12 times the Sun's radius and radiates 58 times the Sun's luminosity. Its outer envelope has an effective temperature of 4,559 K, giving it the orange hue typical of K-type stars. Like some other K-type giants, it varies very slightly in brightness, by 0.02 magnitudes.

Cebalrai is a member of the thin disk population of the Milky Way, following a low-eccentricity orbit that carries it between 27.3 and 30.9 kly from the Galactic Center and up to 0.62 kly above or below the galactic plane. Radial velocity variations with a period of 142 days are likely explained by periodic radial pulsations from intrinsic stellar variability.

Reader's Guide

Beta Ophiuchi, known as Cebalrai, holds significance as a bright naked-eye star in the constellation Ophiuchus, visible even from urban skies. Its traditional name, derived from Arabic for 'the shepherd's dog', reflects its historical role in Arabic astronomy, and the name was formally recognized by the IAU in 2016. The star also appears in Chinese astronomy as part of the asterism Zongzheng. Its physical properties as a K-type giant—cooler, larger, and more luminous than the Sun—illustrate a later stage of stellar evolution for a star somewhat younger than the Sun. The star's slight brightness variations and radial pulsations contribute to the understanding of intrinsic variability among K-type giants. Additionally, its membership in the thin disk population and its orbit through the Milky Way provide context for the dynamics of stars in the galactic disk. The star also served as the namesake for the United States Navy cargo ship USS Cheleb (AK-138), a Crater class vessel, linking it to maritime history.