Editing Rigel (section)

== Stellar system ==

<div style="float:right; width:240px; border:solid black 1px; text-align: center; margin: 8px; padding: 4px; font-size: 90%;">
{{chart/start}}
{{chart|border=1| |, |S1| |S1='''Rigel''' }}
{{chart|border=0| |! |PA|PA=Separation = 9.5″{{break}}Period = 24,000{{spaces}}y }}
{{chart|border=1|-|( | |, |- |S2|S2='''Ba''' }}
{{chart|border=0| |! |, |( |PB|PB=Separation = {{val|0.58|u=mas}}{{break}}Period = {{Val|9.860|u=days}} }}
{{chart|border=1| |! |! |` |- |S3|S3='''Bb''' }}
{{chart|border=0| |` |( |PC|PC=Separation = {{Val|0.1|u=arcsecond}}{{break}}Period = 63{{spaces}}y }}
{{chart|border=1| | |` |S4| |S4='''C''' }}
{{chart/end}}
[[Star systems#Designations and nomenclature|Hierarchical scheme]] for Rigel's components<ref name=msc/>
<!--This hierarchy structure separates stars into subsystems which are assumed to be gravitationally attached. As some stars in the Rigel system have little evidence of orbital motion, which is questioned by some astronomers, we cannot say it is a "hierarchy of orbits".-->
</div>

The [[multiple star system|star system]] of which Rigel is a part has at least four components. Rigel (sometimes called Rigel A to distinguish from the other components) has a [[Visual double star|visual companion]], which is likely a close triple-star system. A fainter star at a wider separation might be a fifth 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,<ref name="Herschel1782"/> usually abbreviated to H{{spaces}}II{{spaces}}33, or as H{{spaces}}2{{spaces}}33 in the Washington Double Star Catalogue.<ref name="WDS"/> [[Friedrich Georg Wilhelm von Struve]] first measured the relative position of the companion in 1822, cataloguing the visual pair as Σ 668.<ref name=FGWStruve/><ref name="Webb1917"/> The secondary star is often referred to as Rigel B or β Orionis B. The angular separation of Rigel B from Rigel A is 9.5 arc seconds to its south along [[position angle]] 204°.<ref name="WDS"/><ref name=bakich/> Although not particularly faint at [[visual magnitude]] 6.7, the overall difference in brightness from Rigel A (about 6.6 magnitudes or 440 times fainter) makes it a challenging target for telescope apertures smaller than {{convert|15|cm|in|abbr=on|sigfig=1}}.<ref name=sanford/>

At Rigel's estimated distance, Rigel B's [[projected separation]] from Rigel A is over 2,200{{spaces}}[[astronomical unit]]s (AU). Since its discovery, there has been no sign of orbital motion, although both stars share a similar [[proper motion|common proper motion]].<ref name="Jedicke1992" /><ref name=sb9/> The pair would have an estimated orbital period of 24,000{{spaces}}years.<ref name=msc/> Gaia Data Release 3{{nbsp}}(DR3) contains a probably unreliable parallax for Rigel B, placing it at about {{convert|309|pc|ly|abbr=off|order=flip}}, further away than the Hipparcos distance for Rigel, but similar to the Taurus-Orion R1 association. There is no parallax for Rigel in Gaia DR3. The Gaia DR3 proper motions for Rigel B and the Hipparcos proper motions for Rigel are both small, although not quite the same.<ref name="Gaia3"/>

In 1871, [[Sherburne Wesley Burnham]] suspected Rigel B to be a binary system, and in 1878, he resolved it into two components.<ref name="burnham1900"/> This visual companion is designated as component C (Rigel C), with a measured separation from component B that varies from less than {{val|0.1|u="}} to around {{val|0.3|u="}}.<ref name=WDS/><ref name="burnham1900"/> In 2009, [[speckle interferometric|speckle interferometry]] showed the two almost identical components separated by {{val|0.124|u="}},<ref name=mason/> with visual magnitudes of 7.5 and 7.6, respectively.<ref name=WDS/> Their estimated orbital period is 63{{spaces}}years.<ref name=msc/> Burnham listed the Rigel multiple system as β{{spaces}}555 in his [[Burnham Double Star Catalogue|double star catalog]]<ref name="burnham1900"/> or BU{{spaces}}555 in modern use.<ref name=WDS/>

Component B is a double-lined [[spectroscopic binary]] system, which shows two sets of [[spectral line]]s combined within its single [[stellar spectrum]]. Periodic changes observed in relative positions of these lines indicate an orbital period of 9.86{{spaces}}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, is likely a physical triple-star system,<ref name=sb9/> although Rigel C cannot be detected in the spectrum, which is inconsistent with its observed brightness.<ref name=sanford/>

In 1878, Burnham found another possibly associated star of approximately 13th magnitude. He listed it as component D of β{{spaces}}555,<ref name="burnham1900"/> although it is unclear whether it is physically related or a coincidental alignment. Its 2017 separation from Rigel was {{val|44.5|ul="}}, almost due north at a position angle of 1°.<ref name=WDS/> Gaia DR2 finds it to be a 12th magnitude sunlike star<!--Does "orange dwarf" count as "sunlike"?--> at approximately the same distance as Rigel.<ref name=dr2d/> Likely a [[K-type main-sequence star]], this star would have an orbital period of around 250,000 years, if it is part of the Rigel system.<ref name="kalerrigel"/>

A spectroscopic companion to Rigel was reported on the basis of radial velocity variations, and its orbit was even calculated, but subsequent work suggests the star does not exist and that observed pulsations are intrinsic to Rigel itself.<ref name=sb9/>