Rigel

beta orionis

Rigel, Beta Orionis (β Ori), is a blue-white supergiant star located in the constellation Orion, the Hunter. With an apparent magnitude of 0.13, it is the brightest star in Orion and the seventh brightest star in the sky. It is slightly fainter than Vega in the constellation Lyra and Capella in Auriga, but outshines Procyon in Canis Minor, Achernar in Eridanus and (most of the time) its Orion neighbour Betelgeuse. Like Betelgeuse, it is slightly variable, showing fluctuations in brightness from magnitude 0.05 to 0.18. Rigel lies at an estimated distance of 860 light years from Earth.

beta orionis
Rigel (Beta Orionis), image: Wikisky

Star system

Even though it appears as a single star to the unaided eye, the Beta Orionis system consists of at least four components, designated Rigel A (or β Ori A), Rigel Ba, Rigel Bb and Rigel C. Rigel B and Rigel C are commonly referred to as Rigel BC. The three fainter components (Rigel Ba, Bb and C) are sometimes simply called Rigel B. There is another star, fainter and at wider separation, that is a suspected component of the system.

Rigel A is a luminous supergiant, separated from the fainter triple star system by 9.5 arcseconds. It is 400 times brighter than Rigel BC. Rigel B is a spectroscopic binary star system, which means that its binary nature can only be detected through the stars’ spectral lines because the two components – Rigel Ba and Rigel Bb – are too close together and cannot be resolved visually, even with the largest telescopes. Rigel B and Rigel C, on the other hand, can be resolved, but only with telescopes with very high resolving power.

Rigel, as viewed from the Rutherfurd Observatory. Image: Wikimedia Commons/Haktarfone (CC BY-SA 3.0)

Rigel BC takes about 24,000 years to complete an orbit around Rigel A, while the components B and C orbit each other with a period of about 63 years. Rigel Ba and Rigel Bb have a considerably shorter period, completing an orbit every 9.860 days.

The properties of the components Rigel Ba, Ba and C are difficult to determine because of the stars’ proximity to each other. All three appear to be blue-white main sequence stars of the spectral type B9 V, with similar temperatures. Their estimated masses are also similar at 3.84, 2.94, and 3.84 solar masses respectively.

Rigel A

Rigel A is a massive, luminous star of the spectral type B8 Ia, indicating a bright supergiant appearing blue or blue-white in colour. The star has a radius almost 79 times that of the Sun and an estimated mass of 21 solar masses. With a temperature of 12,100 K, it shines with about 120,000 solar luminosities. (Its exact luminosity is uncertain, but estimates have been in the range from 61,500 to 363,000 L☉.)

Rigel A is classified as an Alpha Cygni variable, which means that, like Deneb (Alpha Cygni), it exhibits small variations in brightness as a result of non-radial pulsations, with some parts of its surface contracting while others simultaneously expand. A number of well-known stars belong to this class, including Alnilam in Orion, Aludra and Omicron2 Canis Majoris in Canis Major, Rho Leonis in Leo, Sigma Cygni in Cygnus, and possibly Naos in Puppis, among others.

Rigel has exhausted the supply of hydrogen in its core and, as it evolved from the main sequence, it started to cool and expand to its current size. Based on the star’s pulsations, it is possible that it has already been a blue supergiant once, before evolving into a red supergiant and then increasing its temperature and becoming a blue supergiant again. Even though Rigel’s physical properties are uncertain, the star will face the same fate as other extremely massive stars. It will keep fusing increasingly heavier elements in its core until the core can no longer counter gravitational compression and implodes, triggering a violent Type II supernova explosion.

Rigel is one of the nearest known supernova candidates. When it explodes, the supernova will likely reach a visual magnitude of about -11, which is about as bright as a quarter Moon, but it will not affect Earth because the star is too distant.

Rigel B

Rigel B has an apparent magnitude of 6.7, which would make the star easily visible in small telescopes if it were not so close to Rigel A. As it is about 440 times fainter than its neighbour, Rigel B is difficult to see in telescopes with apertures smaller than 6-inch.

The binary system is separated from the primary component by 9.5 arcseconds, or more than 2,200 astronomical units. It has a similar proper motion to Rigel A. The two have an orbital period of at least 18,000 years.

Rigel B is a double-lined spectroscopic binary (SB2), which means that spectral lines from both stars are visible. Based on the periodic changes in the lines, the two components are believed to have an orbital period of 9.86 days. Both stars are hot and have a spectral classification of about B9. They are believed to form a triple star system with Rigel C.

The Gaia Data Release 2 gave a parallax of 2.9186 ± 0.0761 milliarcseconds for Rigel B, which would place the star at an approximate distance of 1,100 light years, or 340 parsecs, from Earth. The distance would be consistent with membership in the Taurus-Orion R1 association, but would place Rigel B considerably further away than Rigel A. However, the Gaia parallax is considered to be unreliable.

Rigel C

Rigel C is separated from Rigel B by less than 0.1 to 0.3 arcseconds. Observations in 2009 revealed that Rigel B and Rigel C had almost identical properties and were separated by only 0.124 seconds of arc. The stars have apparent magnitudes of 7.5 and 7.6 and orbit each other with a period of 63 years.

Luminosity

Rigel is classified as a class Ia luminous supergiant based on its hydrogen spectral lines. With an absolute magnitude of -7.84, if it were located at the same distance from us as Sirius, it would have an apparent magnitude of about -10 and be a little dimmer than the full Moon (mag. -12.74).

Even though Rigel is often referred to as the most luminous star within 1,000 light years of Earth, the star’s luminosity is still uncertain. Based on the currently accepted distance of 860 light years, its estimated luminosity is about 120,000 times that of the Sun. However, a recent study gives a distance of 1,170 ± 130 light years, which yields a luminosity of 218,000 solar luminosities.

Calculations using stellar evolutionary models give luminosities in the range from 83,000 to 363,000 solar luminosities, while estimates based on angular diameter measurements from the Navy Precision Optical Interferometer, using the Hipparcos distance of 860 light years, indicate an energy output of 61,515 ± 11,486 solar luminosities.

Brightness

Rigel is usually slightly fainter than Capella, but not always. With Rigel’s brightness varying from magnitude 0.05 to 0.18 and Capella’s from 0.03 to 0.16, Rigel occasionally outshines Auriga’s brightest star, briefly becoming the sixth brightest star in the sky.

Rigel’s variability has been known since at least 1930. However, the variations in the star’s brightness have a small amplitude and can only be detected photoelectrically or with CCD photometry. The variations do not have a clear period. A study conducted in 1984 revealed variations of up to 0.13 magnitudes over several hours to a few days, but with no apparent period.

In 1998, a study of B stars observed with the Hipparcos satellite presented a classification of new variable stars discovered by Hipparcos and classified Rigel as an Alpha Cygni variable. Alpha Cygni stars generally seem to have irregular variations because they are caused by pulsations, which can appear irregular because there are multiple pulsation periods, typically lasting from several days to several weeks. The mechanism of pulsations is not entirely understood.

Mass 

Rigel’s mass is also uncertain. A comparison of evolutionary tracks yielded a mass of 21 ± 3 solar masses at an age of 8 ± 1 million years. However, calculations based on atmospheric modelling from the star’s spectrum produced a value of 24 ± 8 solar masses.

Rigel’s H-alpha emissions show a P Cygni profile, a spectroscopic feature that indicates the existence of an expanding shell of gas and a dense stellar wind, both associated with mass loss. However, the star shows a P Cygni profile only a quarter of the time and its line profiles transform every few days, sometimes indicating mass loss and at other times infalling material. The changes have been explained as a result of varying quantity and velocity of the material lost from the star. A study published in 1997 suggests that the changes are due to Rigel having extended rotating magnetic structures emerging from its photosphere.

Based on observations of Rigel’s variable H-alpha line, the star is estimated to lose mass at a rate of (1.5 ± 0.4) × 10−7 solar masses per year, or 10 million times faster than the Sun. Later studies gave a mass loss rate of (9.4 ± 0.9) × 10−7 solar masses per year (2006-2007) and (7.6 ± 1.1) × 10−7 solar masses per year (2009-2010), based on observations of Rigel’s H-gamma line. Rigel is believed to have lost about 3 solar masses from its initial mass of 24 ± 3 solar masses, which it had when it began its life 7 to 9 million years ago.

Colour

Like all class B supergiants, Rigel is very luminous and appears distinctly bluish in colour, in stark contrast to the red supergiant Betelgeuse.

Rigel serves as a spectral standard for its class, B8 Ia. Its spectrum shows neutral helium lines and strong hydrogen absorption lines, characteristic of its class, as well as heavier elements, including oxygen, magnesium and calcium.

Photo taken by Rogelio Bernal Andreo in October 2010 of the Orion constellation showing the surrounding nebulae of the Orion Molecular Cloud Complex. Also captured is the red supergiant Betelgeuse (top left) and the famous belt of Orion composed of the OB stars Alnitak, Alnilam and Mintaka. Rigel can be found at the bottom right. The red crescent shape is Barnard’s Loop. Image: Rogelio Bernal Andreo (CC BY-SA 3.0)

Distance

Rigel’s currently accepted distance from the Sun is about 863 light years, or 265 parsecs, based on the 2007 Hipparcos reduction of the star’s parallax (3.78 ± 0.34 milliarcseconds). The margin of error is about 9%.

However, the Gaia Data Release 2 gave a parallax of 2.9186 ± 0.0761 milliarcseconds for one of Rigel’s fainter companions, which translates into a distance of 1,100 light years, or 340 parsecs. As the companion is a close binary system, the value may not be reliable.

Rigel’s distance has also been estimated through indirect measurements. The star illuminates the nearby Witch Head Nebula (IC 2118), which extends for about 5 degrees at an angular separation of 2.5 degrees (or 39 light-years) from the star. Distance measurements of the stars embedded within the nebula yielded a value of 949 ± 7 light-years, or 291 ± 2 parsecs.

Supergiant Rigel and IC 2118 in Eridanus, image: European Southern Observatory, credit: Robert Gendler (CC BY 4.0)

Size  

The measurements of Rigel’s angular diameter and size have also produced different results. An older value given for the star’s angular diameter is 2.75 ± 0.01 milliarcseconds, which translates into a radius 78.9 times solar. However, a study published in 2017 that used the Navy Precision Optical Interferometer to measure the star’s apparent size gave a value of 2.526 milliarcseconds for Rigel’s angular diameter and derived a radius of 74.1 solar radii (allowing for a margin of error in the range from 66.8 to 80.2 solar radii), close to the orbit of Mercury.

UY Scuti compared to other stars, image: Wikimedia Commons/IStoleThePies (CC BY-SA 4.0)

Temperature

Rigel’s estimated surface temperature is around 12,100 K based on the star’s colour and spectral class. That’s less than 50% compared to its temperature before it evolved off the main sequence (about 30,000 K).

Facts

Rigel has the Bayer designation Beta Orionis even though most of the time it is brighter than Betelgeuse, which got the designation Alpha. This is not the only case of the brightest star in a constellation not being designated Alpha (e.g. the brightest stars in Sagittarius, Ursa Major, Pegasus, Hercules and Gemini have the designations Epsilon Sagittarii, Epsilon Ursae Majoris, Epsilon Pegasi, Beta Herculis, and Beta Geminorum). However, it is possible that when German uranographer Johann Bayer assigned Greek letters to stars around the year 1600, Betelgeuse was in fact brighter than Rigel. Bayer was guided by magnitude class, not the exact order of brightness, and did not use the same scheme for every constellation, so it is also possible that he used a different logic when assigning the letters to Orion stars.

Rigel is one of the 58 stars selected by Her Majesty’s Nautical Almanac Office and the US Naval Observatory for navigation. It is one of four navigational stars in Orion. The other three are Betelgeuse and Bellatrix, the bright stars marking Orion’s shoulders, and Alnilam, the central star of Orion’s Belt.

Rigel has been known to be a multiple star system for several centuries. The German-born British astronomer William Herschel discovered it to be a double star on October 1, 1781. He entered it as star 33 (H II 33) in his Catalogue of Double Stars.

In 1871, American astronomer Sherburne Wesley Burnham suspected Beta Orionis B to be a binary star and was able to resolve it into two stars in 1878. The second star is now designated as Rigel C. Burnham entered Rigel as β 555 (the modern BU 555) in his catalogue of double stars.

In 1878, Burnham also observed another suspected companion, a 13th magnitude star, and catalogued it as component D of β 555. The star was separated by 44.5 arcseconds from Rigel in 2007. Whether or not it is a member of the Beta Orionis system is still uncertain. The Gaia Data Release 2 identified it as a sun-like star, likely an orange dwarf, of 12th magnitude, located at about the same distance as Rigel. If it is a member of the Rigel star system, its estimated orbital period would be about 250,000 years.

Rigel is considered to be a member of the Taurus-Orion R1 Association, a loose group of stars that share a common origin and motion through space, located about 1,200 light years, or 360 parsecs, from the solar system. Some sources classify the star as an outlying member of the Orion OB1 association, but since Rigel is much closer to us than the stars in the Orion OB1 group, lying at a similar distance as Betelgeuse and Saiph, its membership is unlikely.

Variations in Rigel’s radial velocity (the speed at which it moves relative to Earth) were first detected in 1888 and later confirmed to vary by about 10 km/s.

In images of the Orion constellation, Rigel is commonly depicted as the foot of Orion. The nearby Cursa, Beta Eridani, represents Orion’s footstool.

Like many other exceptionally bright stars, Rigel had cultural and historic significance in many countries around the world. In Japan, it was chosen to represent the Genji clan, one of the four major clans during the Heian period (794-1185). They called the star Genji-boshi (源氏星). The rivalling Taira clan chose Betelgeuse and its red colour as their symbol. The two clans fought the Genpei War and Rigel and Betelgeuse were seen as facing off against each other in the sky, with only the Orion’s Belt stars keeping them apart. Rigel was also called Gin-waki (銀脇), meaning “the silver star beside Mitsu-boshi (Orion’s Belt).”

Rigel may be the star marking “Orwandil’s toe” in Norse mythology. In the myth, Orwandil, the Giant, was frostbitten and Thor broke off his frozen toe and threw it into the sky. Since the broken off toe is more commonly associated with Alcor in Ursa Major, Richard Hinckley Allen suggests that Alcor is the frost-bitten toe and Rigel, the other toe in his Star Names: Their Lore and Meaning (1889).

In Caribbean lore, Rigel marked the severed leg of Trois Rois, a man who loved the daughter of a woman who hated him. As the lovers took flight, the mother ran after them and managed to cut off one of his legs. The daughter is represented by Aldebaran, Trois Rois by the stars of Orion’s Belt, and the mother by Sirius.

Two places in Antarctica were named after Rigel: the Rigel Skerries and Mount Rigel.

Rigel has been used in countless works of fiction, including multiple episodes of Star Trek and the films Star Trek: The Motion Picture (1979) and Fantastic Four: Rise of the Silver Surfer (2007). Notable uses of the star in literature include M. A. R. Barker’s Tékumel novels and games, the novels Next of Kin (1959) by Eric Frank Russell, Jack Vance’s Demon Princes series (1964-1981), and Timothy Zahn’s Night Train to Rigel (2005).

Name

The name Rigel (pronunciation: /ˈraɪdʒəl/) comes from the Arabic phrase Rijl Jauzah al Yusrā, meaning “the left leg of the central one,” and refers to the star’s position at the foot or leg of Orion. Jauzah was the Arabic name for the constellation Orion, representing a female figure whose identity is uncertain.

The less common names for the star were Algebar or Elgebar, derived from the Arabic riǧl al-ǧabbār, meaning “the foot of the great one.” The name Rigel was also spelled as Regel, Riglon, Algibbar and Rigel Algeuze.

The name dates back to the 10th century. The earliest known written mention of it is in the Alfonsine Tables of 1521.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) officially approved the name for Rigel A on June 30, 2016. Even though the name formally applies only to Rigel A, it is commonly used for the entire star system and its individual components.

Rigel has been known by many different names across different cultures. The Chinese know it as the Seventh Star of Three Stars, referring to the Chinese Three Stars (參宿) asterism, which originally consisted of the three stars of Orion’s Belt, but was later expanded to include four other bright stars of Orion.

The Lacandon people of Mexico called the star tunsel, meaning “the little woodpecker.”

The Wotjobaluk koori of Victoria in Australia knew Rigel as Yerrerdet-kurrk. It represented the mother-in-law of Totyerguil, marked by Altair. The distance between the two stars symbolized the taboo of a man approaching his mother-in-law.

The Boorong people of Victoria called the star Collowgullouric Warepil, while the Wardaman people of the Northern Territory knew it as the Red Kangaroo Leader Unumburrgu, who led ceremonies when the constellation was high in the sky. The other bright stars of Orion represented his tools and followers.

The Māori people of New Zealand knew the star as Puanga, a daughter of Rehua, the chief of all stars, represented by Antares. Rigel’s heliacal rising heralded the appearance of the Pleiades, known as Matariki, in the morning sky in late May or early June, symbolizing the Māori New Year.

Location

Rigel is easy to find because it is part of one of the most recognizable constellations in the sky. Marking the left foot of Orion, the star appears as the bottom right star of Orion’s hourglass figure to northern observers and as the top left star to observers in the southern hemisphere.

Rigel marks one of the vertices of the Winter Hexagon (or Winter Circle), a large asterism that dominates the evening sky during the northern hemisphere winter. Other vertices of the Winter Hexagon are at Aldebaran in Taurus, Capella in Auriga (which is part of a smaller hexagon formed by the brightest stars in Auriga and Elnath in Taurus), Pollux in Gemini, Procyon in Canis Minor and Sirius in Canis Major, the brightest star in the sky.

The Winter Triangle and the Winter Hexagon, image: Wikisky

Rigel is located in the vicinity of several notable deep sky objects. The nearest one is the Witch Head Nebula (IC 2118), a faint reflection nebula illuminated by Rigel, located in the region between Rigel and Cursa, in the neighbouring constellation Eridanus. The faint H II region Sharpless 278 lies just north of Rigel. The Orion Nebula (Messier 42), which lies a bit further away and appears as the central star of Orion’s Sword, forms a triangle with Rigel and Saiph, Orion’s other foot.

Rigel, the Orion Nebula and the Witch Head Nebula, image: Wikisky

Rigel can be seen from all locations on Earth except those within 8 degrees of the North Pole. It is at its highest point in the sky at midnight on December 12 and at 9 pm on January 24. The best time of year to see Rigel and other stars in Orion is during the month of January, when the constellation is prominent in the evening sky. From northern locations, Rigel is visible from late summer to early spring at some point in the night.

As the name implies, this reflection nebula associated with the star Rigel looks suspiciously like a fairytale crone. Formally known as IC 2118 in the constellation Orion, the Witch Head Nebula glows primarily by light reflected from the star. The color of this very blue nebula is caused not only by blue color of its star, but also because the dust grains reflect blue light more efficiently than red. A similar physical process causes Earth’s daytime sky to appear blue. Image: NASA/STScI Digitized Sky Survey/Noel Carboni

Constellation

Rigel is located in the constellation Orion, the Hunter. Named after the Greek mythical hunter, Orion is one of the most conspicuous constellations in the sky. Rigel marks the giant huntsman’s left foot, possibly the one stung by the scorpion in the myth, outlining the constellation’s hourglass figure with six other bright stars: Betelgeuse and Bellatrix marking the shoulders, Alnitak, Alnilam and Mintaka forming the Belt of Orion, and Saiph marking the other foot.

Orion constellation, image: Roberto Mura

The constellation is known for its two familiar asterisms, Orion’s Belt and Orion’s Sword. Orion’s Sword contains the bright, large Orion Nebula (M42) with the young Trapezium Cluster, the neighbouring De Mairan’s Nebula (M43), and the Running Man Nebula (Sharpless 279). Orion is also home to the reflection nebulae Messier 78 and NGC 1999, the dark Horsehead Nebula, and the emission nebulae NGC 2174 (the Monkey Head Nebula), NGC 2024 (the Flame Nebula), and Barnard’s Loop (Sharpless 276).

Orion dominates the evening sky from January to March. The 10 brightest stars in the constellation are Rigel (Beta Ori, mag. 0.05 – 0.18), Betelgeuse (Alpha Ori, mag. 0.0 – 1.3), Bellatrix (Gamma Ori, mag. 1.59 to 1.64), Alnilam (Epsilon Ori, mag. 1.64 – 1.74), Alnitak A (Zeta Ori A, mag. 2.00), Saiph (Kappa Ori, mag. 2.09), Mintaka AB (Delta Ori AB, mag. 2.23), Hatysa (Iota Ori, mag. 2.77), Tabit (Pi3 Ori, mag. 3.16), and Eta Orionis (mag. 3.31 – 3.6).

Rigel – Beta Orionis

Absolute magnitude-7.84
Distance860 ± 80 light years (260 ± 20 parsecs)
Parallax3.78 ± 0.34 mas
Radial velocity17.8 ± 0.4 km/s
Proper motionRA: +1.31 mas/yr
Dec.: +0.50 mas/yr
ConstellationOrion
DesignationsRigel, Beta Orionis, β Ori, ADS 3823, H II 33, BU 555, STF 668, WDS J05145-0812, CCDM J05145-0812

 

Rigel A

Spectral classB8 Ia
Variable typeAlpha Cygni
U-B colour index-0.66
B-V colour index-0.03
Apparent magnitude0.13 (0.05 – 0.18)
Mass21 ± 3 M
Luminosity120,000 L(99,000 – 145,000 L)
Radius78.9 ± 7.4 R
Temperature12,100 ± 150 K
Metallicity−0.06 ± 0.10
Age8 ± 1 million years
Rotational velocity25 ± 3 km/s
Surface gravity1.75 ± 0.10 cgs
Right ascension05h 14m 32.27210s
Declination-08° 12′ 05.8981”
Designations19 Orionis, HR 1713, HD 34085, HIP 24436, FK5 194, BD-08°1063, SAO 131907, CSV 100463, GC 6410, GCRV 3110, IRAS 05121-0815, AAVSO 0509-08, 2MASS J05143226-0812060, PPM 187839, TYC 5331-1752-1

 

Rigel BC

Spectral classB9V + B9V
Apparent magnitude6.67 (7.5/7.6)
Mass (Rigel Ba, Bb, C)3.84 M, 2.94 M, 3.84 M
Right ascension05h 14m 32.049s
Declination-08° 12′ 14.78”
Designations (Rigel B)Rigel B, GCRV 3111