Scorpii

V915 Scorpii (HR 6392, HD 155603) is an orange hypergiant variable star in the constellation Scorpius.

Surroundings

V915 Scorpii is surrounded by the sparse OB association Moffat 2. It is also surrounded by an envelope of dust and gas, producing a significant infrared excess.

V915 Sco has been classified as a triple star. 15" away is the Wolf-Rayet star WR 85, one of the most luminous stars known, but still visually four magnitudes fainter than V915 Sco. Component C is a 10th magnitude K class star 17" away. There is also a 14th magnitude star 22" away. Photometry and space motions suggest that only V915 Sco and WR 85 lie at the same distance, while the other two stars are foreground objects. Assumptions about the brightness of each star suggest a distance of 2,600 parsecs, and a projected separation of 0.2 pc.

Four arc minutes distant are two other assumed members of the association, a 10th magnitude B0 giant and an 11th magnitude OB star. Fitting the association members to a main sequence gives a highly uncertain distance of 1.8 kpc. A kinematical distance has been calculated for the bubble around WR 85 at 2.8 kpc. The distance to V915 Scorpii derived assuming minimal interstellar extinction is 7,300 pc. However, the star is considerably reddened and this results in a distance of 2,630 pc. Analysis of WR 85 as a luminous hydrogen-rich star gives a distance of 6,600 pc.

Variable

V915 Scorpii is variable over nearly half a magnitude, but the nature of the variations is not known. Any period associated with the variation is longer than 600 days.

Properties

The distance to V915 Sco is highly uncertain, and it has hardly been observed in the last 20 years, but its absolute magnitude is consistently determined between −8 and −9, making it an extremely luminous supergiant. The spectral type was assigned as G5Ia in 1954, G5Ia-0 in 1973, G8Ia in 1977, K0Ia in 1982, and K0Ia-0 in 1989.

Amphitrite

Amphitrite (/ˌæmfɪˈtraɪtiː/ am-fi-TRY-tee; minor planet designation: 29 Amphridite) is one of the largest S-type asteroids, approximately 200 kilometers (124 miles) in diameter, and probably third largest after Eunomia and Juno, although Iris and Herculina are similar in size.

Discovery

Amphitrite was discovered by Albert Marth on March 1, 1854, at the private South Villa Observatory, in Regent's Park, London. It was Marth's only asteroid discovery. Its name was chosen by George Bishop, the owner of the observatory, who named it after Amphitrite, a sea goddess in Greek mythology.

Characteristics

Amphirite's orbit is less eccentric and inclined than those of its larger cousins; indeed, it is the most circular of any asteroid discovered up to that point. As a consequence, it never becomes as bright as Iris or Hebe, especially as it is much further from the Sun than those asteroids. It can reach magnitudes of around +8.6 at a favorable opposition, but usually is around the binocular limit of +9.5.

Asteroid 29 Amphitrite will again be visible at 13:24 on Sunday, the 13th of October 2019. It will be convenient to observe 29 Amphitrite from the constellation Pisces, well above Sydney's horizon for the greater part of the night. 

In 2007, James Baer and Steven R. Chesley estimated Amphitrite to have a mass of 1.9×10¹⁹ kg. A more recent estimate by Baer suggests it has a mass of 1.18×10¹⁹ kg.

A satellite of the asteroid is suspected to exist, based on lightcurve data collected by Edward F. Tedesco. In 1988 a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Mauna Kea Observatories, but the effort came up empty.

Hektor

624 Hektor (/ˈhɛktɔːr/ HEK-tor) is the largest Jupiter trojan and the namesake of the Hektor family, with a highly elongated shape equivalent in volume to a sphere of approximately 225 to 250 kilometers diameter. It was discovered on 10 February 1907, by astronomer August Kopff at Heidelberg Observatory in southwest Germany, and named after the Trojan prince Hector, from Greek mythology. It has one small 12-kilometer sized satellite, Skamandrios, discovered in 2006.

Description

Hektor is a D-type asteroid, dark and reddish in colour. It lies in Jupiter's leading Lagrangian point, L₄, called the Greek camp after one of the two sides in the legendary Trojan War. Hektor is named after the Trojan hero Hektor and is thus one of two trojan asteroids that is "misplaced" in the wrong camp (the other one being 617 Patroclus in the Trojan camp).

Contact binary plus moon

Hektor is one of the most elongated bodies of its size in the Solar System, being approximately 403 km in its longest dimension, but averaging only around 201 km in its other dimensions, with a total volume equivalent to an approx 250 km diameter sphere, and an estimated mass of 7.9×10¹⁸ kg (thus density of 1.0g/cm³). It is thought that Hektor might be a contact binary (two asteroids joined by gravitational attraction) like 216 Kleopatra, composed of two more rounded lobes of 220 and 183 km mean diameters. Hubble Space Telescope observations of Hektor in 1993 did not show an obvious bilobate shape because of a limited angular resolution. On 17 July 2006, the Keck 10-meter-II-telescope and its laser guide star adaptive optics (AO) system indicated a bilobate shape for Hektor, which was reinforced by later studies that, together with multiple historical lightcurves, suggest a rotation period of 6.9205 hours.

Additionally, a 12-km-diameter moon of Hektor, named Skamandrios, S/2006 (624) 1, was detected orbiting with a semi-major axis of 623.5 km and an orbital period of 2.9651 days (71.162 hours). It was confirmed with Keck observations in November 2011. No mass estimate was provided, but the equivalent volume suggests an approximate mass of 8.74×10¹⁴ kg if the two bodies are of the same density.

Hektor is, so far, the only known binary trojan asteroid in the L₄ point and the first known trojan with a satellite companion. 617 Patroclus, another large trojan asteroid located in the L₅, is composed of two almost equal-sized components.

Studies

624 Hektor was in a 2003 study of asteroids using the Hubble FGS. Asteroids studied include (63) Ausonia, (15) Eunomia, (43) Ariadne, (44) Nysa, and (624) Hektor. It has since been revisited several times, particularly as a test of the upgraded resolution of the Keck Observatory's LGS Adaptive Optics system which allowed Earth-based observation of binary asteroids for the first time. The asteroid has also been imaged by the NEOWISE and AKARI all-sky studies, which reported highly divergent size estimates of 147.4 and 231.0 kilometers  respectively, although this mostly arises from large differences in estimated albedo (approximately 0.107 for NEOWISE, and a much lower 0.034 for AKARI) rather than its absolute magnitude being measured only briefly at opposing extremes of a widely varying cycle such as thought to account for the uncertainty over the size of 1173 Anchises (624 Hektor's own abs. mag. recorded as a relatively similar 7.20 and 7.49 by the two studies). It is, unusually, not included in the published IRAS results, and is therefore the largest Jupiter trojan to be omitted from that study.

Euphrosyne

Euphrosyne (minor planet designation: 31 Euphrosyne) is the 12th largest and the 5th most massive asteroid in the asteroid belt, discovered by James Ferguson on September 1, 1854. It was the first asteroid found from North America. It is named after Euphrosyne, one of the Charites in Greek mythology.

It is a fairly dark body near the belt's outer edge. Consequently, Euphrosyne is never visible with binoculars, having a maximum magnitude at the best possible opposition of around +10.2, as in November 2011, which is actually fainter than any of the thirty asteroids previously discovered.

It is a very little-studied body despite being one of the largest asteroids. It is a C-type asteroid with a primitive surface. Its orbit, however, is quite unusual and bears a considerable resemblance to that of 2 Pallas in its high inclination and eccentricity. Whereas Pallas and Eris—the only larger bodies with comparably tilted orbits—have nodes near perihelion and aphelion, Euphrosyne's perihelion lies at the northernmost point of its orbit. During a rare perihelic opposition Euphrosyne is very high in the sky from northern latitudes, but invisible from such countries as New Zealand and Chile.

The mass estimate of Euphrosyne in Baer (2011) makes it apparently the 5th-most-massive asteroid, coming after only the "big four". It also has the highest estimated density, indicating that it is a solid body like the other largest asteroids. However, all large asteroids with comparable densities (16 Psyche and 532 Herculina) have very large uncertainties, so both the mass and density are likely to be lower than the median estimate.

Its rotation period is typical for large asteroids. Nothing is known of its axial tilt.

Euphrosyne has been studied by radar.

This object is the namesake of a family of 323–2066 asteroids that share similar spectral properties and orbital elements; hence they may have arisen from the same collisional event. All members have a relatively high orbital inclination.

Cybele

Cybele (/ˈsɪbəli/ SIB-ə-lee minor planet designation: 65 Cybele) is one of the largest asteroids in the Solar System and is located in the outer asteroid belt. It gives its name to the Cybele group of asteroids that orbit outward from the Sun from the 2:1 orbital resonance with Jupiter. The X-type asteroid has a relatively short rotation period of 6.0814 hours. It was discovered by Wilhelm Tempel in 1861, and named after Cybele, the earth goddess.

Discovery and naming

Cybele was discovered on 8 March 1861, by German astronomer Wilhelm Tempel from the Marseilles Observatory in southeastern France. A minor controversy arose from its naming process. Tempel had awarded the honour of naming the asteroid to Carl August von Steinheil in recognition of his achievements in telescope production. Von Steinheil elected to name it "Maximiliana" after the reigning monarch Maximilian II of Bavaria. At the time, asteroids were conventionally given classical names, and a number of astronomers protested this contemporary appellation. The name Cybele was chosen instead, referring to the Phrygian goddess of the earth. (The previously discovered 45 Eugenia, 54 Alexandra, and 64 Angelina had nevertheless also been given non-classical names; 64 Angelina had also been discovered by Tempel, but its name stood despite similar protests.)

Physical characteristics

The first Cybelian stellar occultation was observed on 17 October 1979, in the Soviet Union. The asteroid appeared to have an irregular shape, with the longest chord being measured as 245 km, closely matching results determined by the IRAS satellite in 1983 (see below). During the same 1979 occultation, a hint of a possible 11 km wide minor-planet moon at 917 km distance was detected, but has since never been corroborated. As of 2017, neither the Asteroid Lightcurve Data Base nor Johnston's archiveconsider Cybele to be a binary asteroid.

Diameter estimates

Mean-diameter estimates for Cybele range between 218.56 and 300.54 kilometers. According to observations by the Infrared Astronomical Satellite IRAS in 1983, the asteroid has a diameter of 237.26 km. The NEOWISE mission of NASA's Wide-field Infrared Survey Explorer gave a diameter of 218.56 and 276.58 km. The largest estimates of 300.54 km is from the Japanese Akari satellite. In 2004, Müller estimated Cybele using thermophysical modelling (TPM) to have dimensions of 302 × 290 × 232 km, which corresponds to a mean-diameter of 273.0 km.

Spectrum

Examination of the asteroid's infrared spectrum shows an absorption feature that is similar to the one present in the spectrum of 24 Themis. This can be explained by the presence of water ice. The asteroid may be covered in a layer of fine silicate dust mixed with small amounts of water-ice and organic solids.

Recent occultations

On August 24, 2008, Cybele occulted 2UCAC 24389317, a 12.7-magnitude star in the constellation Ophiuchus which showed a long axis of at least 294 km. On 11 October 2009, Cybele occulted a 13.4-magnitude star in the constellation Aquarius.

Geographos

1620 Geographos (/dʒiːoʊˈɡræfɒs/), provisional designation 1951 RA, is a highly elongated, stony asteroid, near-Earth object and potentially hazardous asteroid of the Apollo group, with a mean-diameter of approximately 2.5 km (1.6 mi).

It was discovered on 14 September 1951, by astronomers Albert George Wilson and Rudolph Minkowski at the Palomar Observatoryin California, United States. The asteroid was named in honor of the National Geographic Society.

Orbit and classification

Geographos orbits the Sun at a distance of 0.8–1.7 AU once every 1 years and 5 months (508 days). Its orbit has an eccentricity of 0.34 and an inclination of 13° with respect to the ecliptic. Its orbit is well-determined for the next several hundred years. Due to its high eccentricity, Geographos is also a Mars-crosser asteroid.

The body's observation arc begins at Palomar, two weeks prior to its official discovery observation.

Close approaches

As a potentially hazardous asteroid, Geographos has a minimum orbital intersection distance (MOID) with Earth of less than 0.05 AU and a diameter of greater than 150 meters. The Earth-MOID is currently 0.0301 AU (4,500,000 km), which translates into 11.7 lunar distances. In 1994, Geographos made its closest approach to Earth in two centuries at 5.0 Gm – which will not be bettered until 2586.

Failed Clementine mission

Geographos was to be explored by the U.S.'s Clementine mission which was launched in January 1994. However, a malfunctioning thruster ended the mission before it could approach the asteroid.

Physical characteristics

Spectral type

In the Tholen and SMASS classification, Geographos is an S-type asteroid. This means that it is highly reflective and composed of nickel-iron mixed with iron- and magnesium-silicates.

Rotation period

Since the 1970s, several rotational lightcurve of Geographos have been obtained from photometric observations. Lightcurve analysis gave a rotation period (retrograde sense of rotation) between 5.222 and 5.224 hours with a very high brightness variation between 1.02 and 2.03 magnitude (U=3/3/3/2/3/3/3).

The Yarkovsky effect is causing a decrease in the orbital semimajor axis of 27.4±5.7 m yr⁻¹, while the YORP effect is increasing the asteroid's rotation at the rate of (1.5±0.2)×10⁻³ rad yr⁻².

Spin axis

Several lightcurve were also modeled from the abundant photometric observations. In 1994 and 1995, Polish astronomers obtained a concurring period 5.223328 hours and found a spin axis of (54.0°, −52.0°) in ecliptic coordinates (λ, β) (Q=3/3). Radiometric observations gave a period of 5.223327 hours and a pole of (55.0°, −46.0°). Two other international studies obtained a period of 5.223326 hours and a pole at (56.0°, −47.0°) and (55.0°, −45.0°), respectively (Q=3/3).

Shape and structure

The light curve shows a high amplitude, indicative of its elongated shape, measuring 5.0 × 2.0 × 2.1 kilometers, which corresponds to a mean-diameter of 2.5 km.

The interior of the asteroid probably has a rubble-pile structure. The asteroid's high thermal inertia indicates the surface is most likely a mix of fine grains and large rocks and boulders. During the asteroid's close approach to Earth in 1994, a radar study of it was conducted by the Deep Space Network at the Goldstone Observatory, California. The resultant images show Geographos to be the most elongated object in the Solar System.

Diameter and albedo

According to the observations with the Goldstone Observatory and the space-based surveys carried out by the Infrared Astronomical Satellite IRAS and the NEOWISE mission of the Wide-field Infrared Survey Explorer, Geographos measures between 1.77 and 2.56 kilometers in diameter and its surface has an albedo between 0.26 and 0.3258.

The Collaborative Asteroid Lightcurve Link adopts an albedo of 0.26 and a diameter of 2.5 kilometers based on an absolute magnitude of 15.09.

Naming

This minor planet was named after the National Geographic Society, in recognition of its contribution to astronomy by supporting the National Geographic Society – Palomar Observatory Sky Survey (NGS-POSS), which produced a photographic atlas of the entire northern sky in the 1950s. NGS-POSS was headed by the second discoverer, Rudolph Minkowski. The Greek word geographos means geographer (from geo– 'Earth' + graphos 'drawer/writer').

The official naming citation was published by the Minor Planet Center before November 1977 (M.P.C. 1468).

Notes

 Hamanowa (2011): lightcurve plot of (1620) Geographos, by H. & H. Hamanowa at the Hamanowa Astronomical Observatory (D91). Obs. date: 2008-02-10; rotation period 5.22204 hours (0.217585 days) with a brightness amplitude of 1.39 mag. Quality Code of 3. Summary figures at the LCDB

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.