Rigel

Rigel /ˈraɪdʒəl/, designated β Orionis (Latinized to Beta Orionis, abbreviated Beta Ori, β Ori), is generally the seventh-brightest star in the night sky and the brightest star in the constellation of Orion. Its brightness varies slightly, and it is occasionally outshone by Betelgeuse, itself a semi-regular variable star. Rigel is a blue-white colour to the naked eye, contrasting with orange-red coloured Betelgeuse. Although appearing as a single star to the naked eye, Rigel is actually a multiple star system composed of at least four stars: Rigel A, Rigel Ba, Rigel Bb, and Rigel C.

The name Rigel strictly refers to only the primary star (A), although it is commonly applied to the whole system. The primary has a companion star 9.5″ away with an apparent magnitude of 6.7, 400 times fainter than the primary. The companion is actually a triple star system, including the stars Rigel Ba, Rigel Bb, and Rigel C. Rigel Ba and Bb form a spectroscopic binary, while Rigel B and C, together called "Rigel BC" can only be resolved using very large telescopes. Historically, the whole triple system has been referred to as "Rigel B".

Rigel is a massive blue supergiant estimated to be anywhere from 61,500 to 363,000 times as luminous as the Sun, depending on the method used to calculate its properties and assumptions about its distance, estimated to be about 860 light-years (260 pc). Rigel's radius is over 70 times that of the Sun. Pulsations cause Rigel's small intrinsic brightness variations, and it is classified as an Alpha Cygni variable. Rigel's physical parameters are poorly known, and its rapid complex evolution is not well understood, though the star's likely fate in the future is to end as a supernova.

Nomenclature

The traditional name Rigel is derived from Arabic, meaning the leg or foot of Orion. In 2016, the International Astronomical Union (IAU) included the name Rigel in the IAU Catalog of Star Names.

Rigel was designated β Orionis (Latinized to Beta Orionis) by Johann Bayer in 1603. The "beta" designation is usually given to the second-brightest star in each constellation, but Rigel is almost always brighter than Alpha Orionis (Betelgeuse). Astronomer James B. Kaler has speculated that Rigel was designated by Bayer during a rare period when it was outshone by the variable star Betelgeuse, resulting in the latter star being designated "alpha" and Rigel designated "beta". Rigel is included in the General Catalogue of Variable Stars, but since it already has a Bayer designation, β Orionis, it has no separate variable star designation.

Rigel has several alternate stellar designations taken from various catalogues, including the Flamsteed designation 19 Orionis (19 Ori), the Bright Star Catalogue entry HR 1713, and the Henry Draper Catalogue number HD 34085. These designations appear in the scientific literature, but rarely in popular writing.

The naked-eye star Rigel is now known to have several fainter companions. According to the IAU Catalog of Star Names, the proper name "Rigel" applies only to the supergiant primary component β Orionis A. In historical astronomical catalogs, the system is listed variously as H II 33, Σ 668, β 555, or ADS 3823. For simplicity, Rigel's companions can be referred to as Rigel B, C, and D; the IAU describes such names as "useful nicknames" that are "unofficial". In modern comprehensive catalogues, the whole multiple star system is known as WDS 05145-0812 or CCDM 05145-0812.

Observation

Rigel is an intrinsic variable star with an apparent magnitude ranging from 0.05 to 0.18. It is typically the seventh-brightest star in the celestial sphere excluding the Sun, although occasionally fainter than Betelgeuse. It is usually fainter than Capella, which also varies slightly in brightness. Rigel appears slightly blue-white, and has a (B–V) color index of −0.06. It contrasts strongly with reddish Betelgeuse.

Culminating at midnight on 12 December, and at 9 PM on 24 January, Rigel is visible in winter evenings in the northern hemisphereand summer in the southern. In the southern hemisphere, Rigel is the first bright star of Orion visible as the constellation rises. The star is a vertex of the "Winter Hexagon", an asterism that includes Aldebaran, Capella, Pollux, Procyon, and Sirius. Rigel is a prominent equatorial navigation star, being easily located and readily visible in all the world's oceans (the exception is the area within 8° of the North Pole).

Spectroscopy

Rigel's spectral type is a defining point of the classification sequence for supergiants. The overall spectrum is typical for a late B class star, with strong absorption lines of the hydrogen Balmer series together with neutral helium lines and some of heavier elements such as oxygen, calcium, and magnesium. The luminosity class for B8 stars is estimated from the strength and narrowness of the hydrogen spectral lines, and Rigel is assigned to the bright supergiant class Ia.

As early as 1888, the radial velocity of Rigel, as estimated from the Doppler shifts of its spectral lines, was seen to vary. This was confirmed and interpreted as due to a spectroscopic companion with a period of about 22 days. The radial velocity has since been measured to vary by about 10 km/s around a mean of 21.5 km/s.

In 1933, the Hα spectral line was seen to be unusually weak and shifted 0.1 nm towards shorter wavelengths, while there was a narrow emission spike about 1.5 nm to the long wavelength side of the main absorption line. This is now known as a P Cygni profile after a star that shows this feature strongly in its spectrum. It is associated with mass loss where there is simultaneously emission from dense wind close to the star and absorption from circumstellar material expanding away from the star.

The unusual Hα line profile is observed to vary unpredictably: around a third of the time it is a normal absorption line; about a quarter of the time it is a double-peaked line, that is an absorption line with an emission core or an emission line with an absorption core; about a quarter of the time it has a P Cygni profile; most of the rest of the time the line has an inverse P Cygni profile, where the emission component is on the short wavelength side of the line; rarely there is a pure emission Hα line. The line profile changes are interpreted as variations in the quantity and velocity of material being expelled from the star. Occasional very high-velocity outflows have been inferred, and, more rarely, infalling material. The overall picture is one of large looping structures arising from the photosphere and driven by magnetic fields.

Variations in the spectrum have resulted in the assignment of different classes to Rigel, such as B8 Ia, B8 Iab, and B8 Iae.

Variability

Rigel has been known to vary in brightness since at least 1930. The small amplitude of Rigel's brightness variation requires photoelectric or CCD photometry to be detected. These brightness changes have no obvious period. Observations over 18 nights in 1984 showed variations at red, blue, and yellow wavelengths of up to 0.13 magnitudes on timescales of a few hours to several days, but again no clear period. Rigel's colour index varies but is not strongly correlated with its brightness variations.

From analysis of Hipparcos satellite photometry, Rigel is identified as belonging to the Alpha Cygni class of variable stars, defined as "non-radially pulsating supergiants of the Bep–AepIa spectral types". The 'e' indicates that it displays emission lines in its spectrum, while the 'p' means it has an unspecified spectral peculiarity. Alpha Cygni type variables are generally considered to be irregular or have quasi-periods. Rigel was added to the General Catalogue of Variable Stars in the 74th namelist of variable stars on the basis of the Hipparcos photometry, which showed variations with a photographic amplitude of 0.039 magnitudes and a possible period of 2.075 days. Rigel was observed with the Canadian MOST satellite for nearly 28 days in 2009. Milli-magnitude variations were observed, and gradual changes in flux suggest the presence of long-period pulsation modes.

Mass loss

From observations of the variable Hα spectral line, Rigel is estimated to lose (1.5±0.4)×10⁻⁷ solar masses per year (M_☉/yr), around 10 million times more than the mass loss rate from the Sun. More detailed optical and K band infrared spectroscopic observations, together with VLTI interferometry, were taken from 2006 to 2010. Analysis of the Hα and Hγline profiles, and measurement of the regions producing the lines, show that Rigel's stellar wind varies greatly in structure and strength. Loop and arm structures were also detected within the wind. Calculations of mass loss from the Hγ line give (9.4±0.9)×10⁻⁷ M_☉/yr in 2006-7 and (7.6±1.1)×10⁻⁷ M_☉/yr in 2009–10. Calculations using the Hα line give lower results, around 1.5×10⁻⁷ M_☉/yr. The terminal wind velocity is 300 km·s⁻¹. It is estimated that Rigel has lost around 3 solar masses since beginning life as a star of 24±3 solar masses 7 to 9 million years ago.

Distance

Rigel's distance from the Sun is somewhat uncertain, with different distance estimates obtained with different methods. The 2007 Hipparcosreduction of Rigel's parallax is 3.78±0.34 mas, giving a distance of 863 light-years (265 parsecs) with a margin of error of about 9%. A companion star to Rigel, usually considered to be physically associated and at the same distance, has a Gaia Data Release 2 parallax of 2.9186±0.0761 mas, suggesting a distance around 1,100 light-years (340 parsecs). However, the measurements for this object may be unreliable, possibly because it is a close double star.

Indirect distance estimation methods have also been employed. For example, Rigel is believed to be in a region of nebulosity, with its radiation illuminating several nearby clouds. Most notable of these is the 5°–long IC 2118 (Witch Head Nebula), located at an angular separation of 2.5° from the star, or a distance of 39 light-years (12 parsecs) away. From measures of other nebula-embedded stars, IC 2118's distance is estimated to be 949 ± 7 light-years (291 ± 2 parsecs).

Rigel is an outlying member of the Orion OB1 Association, which is located at a distance of up to 1,600 light-years (500 parsecs) from Earth. It is a member of the loosely-defined Taurus-Orion R1 Association, somewhat closer at 1,200 light-years (360 parsecs). Rigel is thought to be considerably closer than most of the members of Orion OB1 and the Orion Nebula. Betelgeuse and Saiph lie at a similar distance to Rigel, although Betelgeuse is a runaway star with a complex history and might have originally formed in the main body of the association.

Stellar system

Rigel

Separation=9.5″ Period=24,000 y

Ba

Separation=0.58 mas Period=9.860 d

Bb

Separation=0.1″ Period=63 y

C

Hierarchical scheme for Rigel's components

The Rigel star system has at least four components. The blue supergiant primary has a visual companion, which is likely a close triple star system. A fainter star at wider separation might also be a component of the Rigel system.

William Herschel discovered Rigel to be a visual double star on 1 October 1781, cataloguing it as star 33 in the "second class of double stars" in his Catalogue of Double Stars, usually abbreviated to H II 33, or as H 2 33 in the Washington Double Star Catalogue. Friedrich Georg Wilhelm von Struve first measured the relative position of the companion in 1822, cataloguing the visual pair as Σ 668. The secondary star is often referred to as Rigel B or β Orionis B. The angular separation of Rigel B from the primary star is 9.5 arc seconds to its south along position angle 204°. Although not particularly faint at visual magnitude 6.7, the overall difference in brightness from the primary (about 6.6 magnitudes or 440 times fainter) makes it a challenging target for telescope apertures smaller than 15 cm (6 in).

At Rigel's estimated distance, Rigel B's projected separation from its primary is over 2,200 AU. Since its discovery, there has been no sign of orbital motion, although both stars share similar common proper motion. The pair would have a minimum orbital period of around 18,000 years. Gaia Data Release 2 (DR2) contains a somewhat unreliable parallax for Rigel B, placing it at about 1,100 light-years (340 parsecs), further away than the Hipparcos distance for Rigel, but similar to the Taurus-Orion R1 association. There is no parallax for Rigel in Gaia DR2. The Gaia DR2 proper motions for Rigel B and the Hipparcos proper motions for Rigel are both small, although not quite the same.

In 1871, Sherburne Wesley Burnham suspected Rigel B to be double, and in 1878, he resolved it into two components. This visual companion is designated as component C (Rigel C), with a measured separation from component B that varies from less than 0.1″ to around 0.3″. In 2009, speckle interferometry showed the two almost identical components separated by 0.124", with visual magnitudes of 7.5 and 7.6 respectively. Their estimated orbital period is 63 years. Burnham listed the Rigel multiple system as β 555 in his double star catalogue or BU 555 in modern use.

Component B is a double-lined spectroscopic binary system, which shows two sets of spectral lines combined within its single stellar spectrum. Periodic changes observed in relative positions of these lines indicate an orbital period of 9.86 days. The two spectroscopic components Rigel Ba and Rigel Bb cannot be resolved in optical telescopes but are known to both be hot stars of spectral type around B9. This spectroscopic binary, together with the close visual component Rigel C, likely form a physical triple star system, although Rigel C cannot be detected in the spectrum which is inconsistent with its observed brightness.

In 1878, Burnham found another possibly associated star of approximately 13th magnitude. He listed it as component D of β 555. Its 2017 separation from Rigel was 44.5″ almost due north at a position angle of 1°, although it is unclear whether it is physically related or a coincidental alignment. Gaia DR2 finds it to be a 12th magnitude sunlike star at approximately the same distance as Rigel. Likely an orange dwarf, this star would have an orbital period of around 250,000 years, if it is part of the Rigel system.

A spectroscopic companion to Rigel was reported on the basis of radial velocity variations, and its orbit was even calculated, but subsequent work suggests that the star does not exist and that observed pulsations are intrinsic to Rigel itself.

Physical characteristics

Estimation of many physical characteristics of Rigel and other blue supergiant stars are difficult due to their rarity and uncertainty about how far they are from the Sun. As such, much of our understanding about their characteristics is based on theoretical stellar evolutionmodels.

Although Rigel is often considered the most luminous star within 1,000 light-years of the Sun, its energy output is poorly known. For example, using the Hipparcos distance of 860 light-years (264 parsecs), the estimated relative luminosity for Rigel is about 120,000 times that of the Sun, but another recently published distance of 1,170 ± 130 light-years (360 ± 40 parsecs) suggests an even higher luminosity of 218,000 times that of the Sun. Other calculations based on theoretical stellar evolutionary models of Rigel's atmosphere give luminosities anywhere between 83,000 L_☉ and 363,000 L_☉, while summing the spectral energy distribution from historical photometry with the Hipparcos distance suggests a luminosity as low as 61,515±11,486 L_☉.

A 2018 study using the Navy Precision Optical Interferometer measured the angular diameter as 2.526 mas. After correcting for limb darkening, the angular diameter is found to be 2.606±0.009 mas, yielding a radius of 74.1+6.1
−7.3 R_☉. An older measurement of the angular diameter gives 2.75±0.01 mas, equivalent to a radius 78.9 times the radius of the Sun (R_☉) at 264 pc.

A mass of 21±3 M_☉ at an age of 8±1 million years has been determined by comparing evolutionary tracks, while atmospheric modelling from the spectrum gives a mass of 24±8 M_☉. From the spectral type and colour, Rigel's surface temperature is estimated to be about 12,100 K.

Rigel is a blue supergiant that has exhausted the hydrogen fuel in its core, expanded and cooled as it moved away from the main sequence across the upper part of the Hertzsprung–Russell diagram. When it was on the main sequence, its temperature would have been around 30,000 K. Rigel's pulsation properties suggest it may have already passed through a red supergiant phase and then increased its temperature to become a blue supergiant for a second time, something that is expected for some sufficiently massive stars. The surface abundances seen in the spectrum are compatible with this only if its internal convection zones are modelled using non-homogeneous chemical conditions known as the Ledoux Criteria. Rigel is expected to eventually end its stellar life as a supernova, in the process ejecting material that will serve to seed future generations of stars. It is one of the closest known potential supernova progenitors to Earth, and would be expected to have an apparent magnitude of around −11 (similar to a quarter moon) at its peak.

Rigel's complex variability at visual wavelengths is caused by stellar pulsations similar to those of Deneb. Additional observations of radial velocity variations indicate that it simultaneously oscillates in at least 19 non-radial modes with periods ranging from about 1.2 to 74 days. Recent stellar evolution models suggest the pulsations are powered by nuclear reactions in a hydrogen-burning shell that is at least partially non-convective. The star may also be fusing helium in its core.

Due to their closeness to each other and ambiguity of the spectrum, little is known about the individual intrinsic properties of the members of the Rigel BC triple system. All three stars seem to be near equally hot B-type main-sequence stars that are 3 to 4 times as massive as the Sun.

Etymology and cultural significance

The earliest known recording of the modern name Rigel is in the Alfonsine Tables of 1521. It is derived from the Arabic name Rijl Jauzah al Yusrā, "the left leg (foot) of Jauzah" (i.e. rijl meaning "leg, foot"), which can be traced to the 10th century. "Jauzah" was a proper name of the Orion figure, an alternative Arabic name was رجل الجبار riǧl al-ǧabbār, "the foot of the great one", which is the source of the rarely used variant names Algebar or Elgebar. The Alphonsine Tables saw its name split into "Rigel" and "Algebar", with the note, et dicitur Algebar. Nominatur etiam Rigel. Alternate spellings from the 17th century include Regel by Italian astronomer Giovanni Battista Riccioli, Riglon by German astronomer Wilhelm Schickard, and Rigel Algeuze or Algibbar by English scholar Edmund Chilmead.

In the constellation of Orion as the mythological Greek huntsman, Rigel represents his knee or (as its name suggests) foot; with the nearby star Beta Eridani marking Orion's footstool. Rigel is presumably the star known as "Aurvandil's toe" in Norse mythology. In the Caribbean, Rigel represented the severed leg of the folkloric figure Trois Rois, himself represented by the three stars of Orion's Belt. The leg had been severed with a cutlass by the maiden Bįhi (Sirius). The Lacandon people of southern Mexico knew it as tunsel ("little woodpecker").

Rigel was known as Yerrerdet-kurrk to the Wotjobaluk koori of southeastern Australia, and held to be the mother-in-law of Totyerguil (Altair). The distance between them signified the taboo preventing a man from approaching his mother-in-law. The indigenous Boorong people of northwestern Victoria named Rigel as Collowgullouric Warepil. The Wardaman people of northern Australia know Rigel as the Red Kangaroo Leader Unumburrgu and chief conductor of ceremonies in a songline when Orion is high in the sky. Eridanus, the river, marks a line of stars in the sky leading to it, and the other stars of Orion are his ceremonial tools and entourage. Betelgeuse is Ya-jungin "Owl Eyes Flicking", watching the ceremonies. The Māori people of New Zealand named Rigel as Puanga, said to be a daughter of Rehua (Antares), the chief of all stars. Its heliacal rising presages the appearance of Matariki (the Pleiades) in the dawn sky, marking the Māori New Year in late May or early June. The Moriori people of the Chatham Islands, as well as some Maori groups in New Zealand, mark the start of their New Year with Rigel rather than the Pleiades. Puaka is a local variant used in the South Island. In Japan, the Minamoto or Genji clan chose Rigel and its white color as its symbol, calling the star Genji-boshi (源氏星), while the Taira or Heike clan adopted Betelgeuse and its red color. The two powerful families fought the Genpei War; the stars were seen as facing off against each other and only kept apart by the three stars of Orion's Belt. Rigel was also known as Gin-waki, (銀脇), "the Silver (Star) beside (Mitsu-boshi)".

In modern culture

The MS Rigel was originally a Norwegian ship, built in Copenhagen in 1924. It was requisitioned by the Germans during World War II and sunk in 1944 while being used to transport prisoners of war. Two US Navy ships have borne the name USS Rigel.

The SSM-N-6 Rigel was a cruise missile program for the US Navy that was cancelled in 1953 before reaching deployment.

The Rigel Skerries are a chain of small islands in Antarctica, renamed after originally being called Utskjera. They were given their current name as Rigel was used as an astrofix. Mount Rigel, elevation 1,910 m, is in Antarctica.