Rigel is labelled beta yet usually outshines Orion's alpha star
Johann Bayer's 1603 lettering normally gives beta to a constellation's second-brightest star, yet Orion's beta, Rigel, nearly always beats Betelgeuse. One astronomer suspects Bayer happened to look during a rare spell when variable Betelgeuse was brighter; another explanation is that he ranked equal-class stars from north to south.
Rigel is a blue supergiant roughly 850 light-years away, generally the seventh-brightest star in the night sky. Its surface burns at about 12,100 kelvin and its radius exceeds seventy times the Sun's. Estimates of its power vary widely with method, from 61,500 to 363,000 times the Sun's luminosity, and of its mass from 18 to 24 suns. Only seven to nine million years old, it has already burned through its core hydrogen and swelled. A fierce stellar wind strips away mass at about ten million times the Sun's rate, and it is expected to die as a Type II supernova, leaving either a neutron star or a black hole.
The single blue-white point seen by eye actually hides at least four stars. Separated from Rigel by 9.5 arcseconds lies a triple system of three blue-white stars, each three to four solar masses. Two of them circle each other every 10 days, the third orbits that pair every 63 years, and the whole trio swings around Rigel roughly once every 24,000 years.
Rigel flickers slightly, between magnitudes 0.05 and 0.18, because its unstable atmosphere pulsates, earning it a place among the Alpha Cygni variables. Its hydrogen-alpha spectral line behaves erratically. In 1933 astronomers noticed a P Cygni profile, the signature of a star shedding material; the line now switches unpredictably between normal absorption, double peaks, that profile and its inverse. The changes point to looping, magnetically driven structures rising from the surface.
Rigel culminates at midnight on 12 December, so it graces northern winter and southern summer evenings. It marks a corner of the Winter Hexagon and is a key navigation star, visible from every ocean except the waters north of the 82nd parallel.
Source: Rigel
