Look at the constellation Orion on a winter evening and two bright stars stand out: orange Betelgeuse at one shoulder and blue-white Rigel at the opposite foot. Its name comes from the Arabic for “the left foot” of the giant. Rigel is usually the brightest star in Orion and the seventh-brightest in the whole night sky.
A young giant
Rigel is a blue supergiant, a star that has run through the hydrogen in its core while still young. At about 8 million years old, it is less than a five-hundredth of the Sun’s age. It holds about 21 times the Sun’s mass, is about 74 times wider, and shines around 120 000 times more brightly. Its surface is about 12 100 K (11 830 °C), twice as hot as the Sun’s, which is why it looks blue-white.
It is also restless. Rigel’s brightness varies slightly as its outer layers pulsate unevenly, and it loses mass in a strong wind.
Astronomers class it as an Alpha Cygni variable: it wanders between about magnitude 0.05 and 0.18 with no obvious period. When the MOST satellite watched it for 28 days in 2009, Ehsan Moravveji and colleagues combined that light curve with more than six years of spectra and found nineteen separate pulsations, with periods from about 1.2 to 75 days. Its wind reaches about 300 km/s. Estimates of how much mass it carries away depend on which spectral line is used, from about 1.5 × 10⁻⁷ to 9.4 × 10⁻⁷ Suns a year; even the lower figure is about ten million times the rate of the Sun’s own wind.
How far, how wide
Two independent methods agree on Rigel’s distance, and a third does not. The Hipparcos satellite’s parallax, as revised in 2007, gives about 860 light-years. A 2022 study led by E. S. G. de Almeida observed Rigel in the red light of hydrogen with intensity interferometry, fitted the result with spectra and models of the star and its wind, and found about 850 light-years, though that result assumes a luminosity of 123 000 Suns, which is itself disputed. Gaia’s parallax for the companion Rigel B suggests about 1 000 light-years, but that measurement may be unreliable.
The width comes from interferometry too. In 2018 the Navy Precision Optical Interferometer measured an angular diameter of 2.606 milliarcseconds; at the Hipparcos distance that is about 74 times the Sun’s radius. Its true power is less certain: estimates from evolutionary models range from 83 000 to 363 000 Suns.
Rigel also lights up its surroundings. The Witch Head Nebula, IC 2118, is a dusty cloud that glows mainly by reflected Rigel light. It looks strikingly blue, partly because Rigel is blue and partly because dust grains reflect blue light more efficiently than red.
More than one star
Rigel is not alone. In 1781 William Herschel saw a faint companion beside it, now called Rigel B. That companion is itself a pair of stars, Ba and Bb, circling each other every 9.86 days, and a third star, Rigel C, orbits that pair. Together they make Rigel a system of at least four stars. A fifth, faint star nearby may or may not belong to the group.
Its future
A star as heavy as Rigel lives fast. It is expected to end its life as a supernova, leaving behind a neutron star or a black hole. At its distance of about 850 light-years, the explosion would be harmless to Earth, but spectacular: it would shine in our sky far brighter than any star, at roughly magnitude −11, fainter only than the Moon.