Editing Rigel (section)

== Observation ==
[[File:RigelAandB.jpg|left|thumb|Rigel A and Rigel B as they appear in a small telescope]]
Rigel is an [[Variable star|intrinsic variable]] star with an [[apparent magnitude]] ranging from 0.05 to 0.18.<ref name=guinan/> It is typically the seventh-brightest star in the [[celestial sphere]], excluding the Sun, although occasionally fainter than Betelgeuse.<ref name=burnham/> It is fainter than [[Capella]], which may also vary slightly in brightness.<ref name=Samus2017/> Rigel appears slightly blue-white and has a [[B-V color]] index of −0.06.<ref name="csiro"/> It contrasts strongly with reddish Betelgeuse.<ref name=DK/>

[[Culmination|Culminating]] every year at midnight on 12 December, and at 9:00{{spaces}}pm on 24 January, Rigel is visible on winter evenings in the [[Northern Hemisphere]] and on summer evenings in the [[Southern Hemisphere]].<ref name="schaaf"/> In the Southern Hemisphere, Rigel is the first bright star of Orion visible as the constellation rises.<ref name=ellyard/> Correspondingly, it is also the first star of Orion to set in most of the Northern Hemisphere. The star is a vertex of the "[[Winter Hexagon]]", an [[asterism (astronomy)|asterism]] that includes [[Aldebaran]], Capella, [[Pollux (star)|Pollux]], [[Procyon]], and [[Sirius]]. Rigel is a prominent [[List of stars for navigation#Equatorial stars|equatorial navigation star]], being easily located and readily visible in all the world's oceans (the exception is the area north of the [[82nd parallel north]]).<ref name=kerigan/>

=== Spectroscopy ===

Rigel's [[Stellar classification#Modern classification|spectral type]] is a defining point of the classification sequence for supergiants.<ref name=morgan1978/><ref name=morgan1950/> The overall spectrum is typical for a [[Late-type star|late]] B class star, with strong [[absorption line]]s of the hydrogen [[Balmer series]] as well as neutral helium lines and some of heavier elements such as oxygen, calcium, and magnesium.<ref name=abetti/> 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.<ref name=mkk/> Variations in the spectrum have resulted in the assignment of different classes to Rigel, such as B8 Ia, B8 Iab, and B8 Iae.<ref name=schultz/><ref name=bally/>

As early as 1888, the heliocentric [[radial velocity]] of Rigel, as estimated from the [[Doppler shift]]s of its spectral lines, was seen to vary. This was confirmed and interpreted at the time as being due to a spectroscopic companion with a period of about 22 days.<ref name=plaskett/> The radial velocity has since been measured to vary by about {{val|10|ul=km/s}} around a mean of {{val|21.5|u=km/s}}.<ref name=morrison/>

In 1933, the [[Hα]] line in Rigel's spectrum was seen to be unusually weak and shifted {{val|0.1|ul=nm}} towards shorter wavelengths, while there was a narrow [[emission line|emission spike]] about {{val|1.5|u=nm}} to the long wavelength side of the main absorption line.<ref name=struve/> 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 a dense wind close to the star and absorption from circumstellar material expanding away from the star.<ref name=struve/>

The unusual Hα line profile is observed to vary unpredictably. It is a normal absorption line around a third of the time. 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.<ref name=morrison/><!--cites five previous sentences--> 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 [[coronal loop|looping structures]] arising from the [[photosphere]] and driven by magnetic fields.<ref name=israelian/>

=== Variability ===
[[File:RigelLightCurve.png|thumb|right|A [[light curve]] for Rigel, adapted from Moravveji ''et al.'' (2012)<ref name=apj2012_747_108/>]]
Rigel has been known to vary in brightness since at least 1930. The small amplitude of Rigel's brightness variation requires [[Photoelectric sensor|photoelectric]] or [[CCD photometry]] to be reliably detected. This brightness variation has 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 [[color index]] varies slightly, but this is not significantly correlated with its brightness variations.<ref name=guinan1985/>

From analysis of ''[[Hipparcos]]'' satellite photometry, Rigel is identified as belonging to the [[Alpha Cygni variable|Alpha Cygni]] class of variable stars,<ref name="waelkens 1998"/> defined as "non-radially pulsating supergiants of the Bep–AepIa spectral types".<ref name=Samus2017/> In those 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<ref name=AAVSOvartyps/> or have [[quasi-period]]s.<ref name=vangenderen/> Rigel was added to the General Catalogue of Variable Stars in the 74th name-list of variable stars on the basis of the Hipparcos photometry,<ref name=IBVS/> which showed variations with a photographic amplitude of 0.039 magnitudes and a possible period of 2.075 days.<ref name=lefevre/> Rigel was observed with the Canadian [[Microvariability and Oscillations of STars telescope|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.<ref name=apj2012_747_108/>
{{clear left}}

=== Mass loss ===

From observations of the variable Hα spectral line, Rigel's mass-loss rate due to stellar wind is estimated be {{val|1.5|0.4|e=−7}} solar masses per year ({{solar mass|link=n}}/yr)—about ten million times more than the mass-loss rate from the [[Sun]].<ref name=chesneau2010/> More detailed optical and [[K band (infrared)|K{{nbsp}}band]] infrared spectroscopic observations, together with [[VLTI]] interferometry, were taken from 2006 to 2010. Analysis of the Hα and [[Hγ]] line 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 {{val|9.4|0.9|e=−7|u={{solar mass}}/yr}} in 2006-7 and {{val|7.6|1.1|e=−7|u={{solar mass}}/yr}} in 2009–10. Calculations using the Hα line give lower results, around {{val|1.5|e=−7|u={{solar mass}}/yr}}. The terminal wind velocity is {{val|300|u=km/s}}.<ref name=chesneau2014/> It is estimated that Rigel has lost about three solar masses ({{solar mass}}) since beginning life as a star of {{val|24|3|u={{solar mass}}}} seven to nine million years ago.<ref name=aaa445_3_1099/>