Rigel

• 1. Introduction

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  Rigel burns in the constellation Orion as a blue-white point of light so intense it ranks among the seven brightest stars in the night sky. From a distance of roughly 850 light-years, it shines between 61,500 and 363,000 times more brightly than the Sun, a range that wide because astronomers have struggled for decades to pin down exactly how far away it is. It is heavier than the Sun by a factor of 18 to 24, and its radius stretches more than seventy times wider. This is a star near the end of an extraordinary life, burning through its last reserves of fuel, swelling and pulsating, expelling mass into space at a rate ten million times greater than our own Sun loses matter. How did a star this violent get named for a human foot? Why does it carry the designation beta when it is almost always brighter than the star that got the alpha? And what happens when it finally dies, close enough that its death flash would light up the night like a quarter Moon? Those are the questions Rigel carries with it as it blazes above us, quietly changing from one night to the next.

• 2. A Puzzling Greek Letter

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  Johann Bayer made his fateful decision in 1603, assigning Rigel the Greek letter beta in his star catalog. The convention was to give the brightest star in a constellation the designation alpha. Yet Rigel is almost always brighter than its neighbor Betelgeuse, the red giant that received the alpha designation for Orion. Astronomer James B. Kaler has offered one plausible explanation: Bayer may have catalogued Orion during a rare interval when the variable star Betelgeuse temporarily outshone Rigel, reversing their usual order. There is a further wrinkle. Bayer did not sort strictly by brightness. Instead he grouped stars by magnitude class, and within each class he ordered Orion's stars from north to south. Both Rigel and Betelgeuse were placed in the first magnitude class, and Betelgeuse sits farther north. That geographic logic, combined with an unlucky moment in Betelgeuse's brightness cycle, may explain why the most luminous candidate within a thousand light-years carries the second letter of the Greek alphabet. When the International Astronomical Union formally catalogued star names in 2016, it confirmed that the name Rigel applies specifically to the primary star, component A, and described the companion designations Rigel B, C, and D as useful but unofficial nicknames.

• 3. The Hidden Companions

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  William Herschel discovered on the 1st of October 1781 that Rigel was not a single star but a visual double, cataloguing it as star 33 in his second class of double stars. What looked like a companion turned out over the following decades to be considerably more complicated. The object Herschel noted, now called Rigel B, sits 9.5 arc seconds south of Rigel at a position angle of 204 degrees. Its visual magnitude of 6.7 makes it about 440 times fainter than Rigel proper, a gap large enough that telescopes smaller than 15 centimeters cannot split the pair. At Rigel's estimated distance, that 9.5 arc-second gap translates to a projected separation of more than 2,200 astronomical units. The orbital period of the pair is estimated at 24,000 years. Sherburne Wesley Burnham suspected in 1871 that Rigel B was itself a binary, and by 1878 he had resolved it into two components. Speckle interferometry in 2009 found those two components, designated Rigel B and Rigel C, separated by just 0.124 arc seconds, with visual magnitudes of 7.5 and 7.6. Their mutual orbital period is estimated at 63 years. Meanwhile, Rigel B is also a spectroscopic binary, showing two sets of spectral lines that indicate an inner orbital period of 9.86 days for components Ba and Bb. Burnham additionally flagged a star of roughly 13th magnitude in 1878 as a possible fifth member, though whether it is genuinely bound to the system or merely aligned by chance remains uncertain.

• 4. A Restless Atmosphere

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  As early as 1888, astronomers noticed that the radial velocity of Rigel, measured from Doppler shifts in its spectral lines, refused to stay constant. By 1933, something stranger had been documented: the hydrogen alpha line in Rigel's spectrum appeared unusually weak and shifted toward shorter wavelengths, while a narrow emission spike sat about 1.5 angstroms to the long-wavelength side of the main absorption line. This profile, now recognized as a P Cygni profile after a star that displays the feature prominently, signals mass loss: emission from a dense stellar wind close to the star combines with absorption from material already blowing outward. The hydrogen alpha line does not stay in any one configuration. It is a normal absorption line roughly a third of the time. About a quarter of the time it doubles into an absorption line with an emission core, or the reverse. Another quarter of the time it takes on the P Cygni form; most of the remainder shows an inverse P Cygni profile, where the emission component falls on the short-wavelength side. Occasionally the line shifts to pure emission. These changes are interpreted as varying quantities and velocities of material ejected from the star's surface. Observations from 2006 to 2010 using optical and infrared spectroscopy together with interferometry from the VLTI found loop and arm structures within the stellar wind itself. The overall picture is of large looping structures rising from the photosphere and driven by magnetic fields.

• 5. Measuring an Uncertain Giant

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  A 2018 study using the Navy Precision Optical Interferometer measured Rigel's angular diameter as 2.526 milli-arc seconds; after correcting for limb darkening, the figure becomes 2.606, yielding a radius that depends critically on which distance estimate one adopts. Old estimates placed the star 166 parsecs away. The 2007 Hipparcos new reduction gave a parallax of 3.78, placing Rigel at 863 light-years with a margin of error of roughly 9 percent. Gaia Data Release 3 found a parallax for Rigel B suggesting a distance of around 310 parsecs, though those measurements are flagged as potentially unreliable. An older measurement of the angular diameter gave 2.75 milli-arc seconds, equivalent to a different radius at the Hipparcos distance of 264 parsecs. A 1922 estimate by John Stanley Plaskett pegged Rigel's diameter at 25 million miles, smaller than its neighbor Aldebaran, a figure that modern interferometry has rendered obsolete. The uncertainty ripples through every derived quantity. Luminosity estimates range from a low near 61,515 solar luminosities to values several times higher, depending on the distance adopted and the modeling approach used. Rigel oscillates simultaneously in at least 19 non-radial modes with periods ranging from about 1.2 to 74 days. The Canadian MOST satellite watched it for nearly 28 days in 2009 and detected milli-magnitude variations, along with gradual flux changes suggesting the presence of long-period pulsation modes.

• 6. A Star's Ending

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  Rigel has already spent its core hydrogen fuel, expanded, and cooled, crossing the upper portion of the Hertzsprung-Russell diagram to reach its current blue supergiant state. When it sat on the main sequence, its effective temperature would have been around 30,000 Kelvin; today's surface temperature is measured at 12,100 Kelvin. Helium has built up at the surface, rising from 26.6 percent when the star formed to 32 percent now, a signature of the convective mixing that has occurred over the star's seven-to-nine-million-year lifetime. It is estimated to have lost about three solar masses since it began life as a star of 24 solar masses. The pulsations Rigel currently displays are thought to be powered by nuclear reactions in a hydrogen-burning shell that is at least partially non-convective. Models suggest those pulsations are stronger and more numerous in stars that have already passed through a red supergiant phase and then warmed again. When Rigel eventually exhausts its remaining fuel, it is expected to explode as a Type II supernova, one of the closest known potential supernova progenitors to Earth. At its peak, that explosion would reach an apparent magnitude of around -11, comparable to a quarter Moon, or about 300 times brighter than Venus ever gets. What remains afterward would be either a neutron star or a black hole, depending on the original mass of the star.

• 7. Names Across Cultures

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  The earliest known written record of the name Rigel appears in the Alphonsine tables of 1521, derived from the Arabic Rijl Jauzah al Yusra, meaning the left leg or foot of the figure called Jauzah, the Arabic proper name for Orion. An alternative Arabic name, rijl al-jabbar, translated as the foot of the great one and gave rise to the rare variant names Algebar and Elgebar. Alternate spellings proliferated in the 17th century: Giovanni Battista Riccioli used Regel, Wilhelm Schickard wrote Riglon, and Edmund Chilmead recorded Rigel Algeuze or Algibbar. In Norse mythology, Rigel is presumed to be the star identified as Aurvandil's toe. In the Caribbean, it represented the severed leg of the folkloric figure Trois Rois, cut off with a cutlass by the maiden Bįhi, herself identified with Sirius. The Lacandon people of southern Mexico called it tunsel, meaning little woodpecker. The Wotjobaluk koori of southeastern Australia knew Rigel as Yerrerdet-kurrk, the mother-in-law of Totyerguil (Altair); the distance between the two stars in the sky encoded the cultural taboo against a man approaching his mother-in-law. The Wardaman people of northern Australia identified Rigel as the Red Kangaroo Leader Unumburrgu, who conducts ceremonies when Orion stands high. The Minamoto clan of Japan adopted Rigel and its white color as their symbol, naming it Genji-boshi, while their rivals the Taira clan took Betelgeuse and its red color; the two stars were seen as facing off across the three stars of Orion's Belt during the Genpei War. The Maori of New Zealand called Rigel Puanga, a daughter of Rehua (Antares), and its heliacal rising signals the approach of the Maori New Year in late May or early June.

• 8. Rigel Beyond the Sky

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  On Earth, Rigel's name has traveled into geography, naval history, and military technology. The MS Rigel began its life as a Norwegian ship, built in Copenhagen in 1924, before being requisitioned by German forces during World War II and sunk in 1944 while transporting prisoners of war. Two US Navy ships have carried the name USS Rigel. The SSM-N-6 Rigel, a cruise missile program developed for the US Navy, was cancelled in 1953 before reaching deployment. In the far south, the Rigel Skerries are a chain of small islands in Antarctica, renamed after originally being called Utskjera; they received the name because Rigel was used as an astrofix during the survey work there. Mount Rigel, rising to 1,910 meters, also stands in Antarctica. Meanwhile, Rigel remains a practical celestial tool for navigation, serving as a prominent equatorial navigation star visible across virtually all of the world's oceans, with the only exception being the area north of the 82nd parallel north.