Star of the Month

Rigel, Orion’s true al­pha if not for a cler­i­cal er­ror…

Sky at Night Magazine - - CONTENTS -

There are two fa­mous su­per­giant stars in the main pat­tern of Orion. In the north­east cor­ner lurks orange-hued Betel­geuse (Al­pha (_) Ori­o­nis), a red-su­per­giant star com­ing to the end of its life. In the south­west cor­ner is the blue-su­per­giant Rigel (Beta (`) Ori­o­nis).

Rigel rep­re­sents Orion’s foot – the name orig­i­nat­ing from the Ara­bic ‘rijl al-Jauza’ mean­ing ‘foot of the Cen­tral One’ (Orion was known to an­cient Ara­bic as­tronomers as the Cen­tral One). Al­though des­ig­nated beta, at mag. +0.1 Rigel is brighter than Betel­geuse. One pos­si­ble ex­pla­na­tion is that Betel­geuse is vari­able and may have been close to peak bright­ness when the la­belling was de­cided.

The sev­enth bright­est star in the sky, Rigel lies 863 lightyears from the Sun. It’s a mas­sive ob­ject es­ti­mated to be 79 times larger than the Sun and around 120,000 times as lu­mi­nous. It is classed as an Al­pha Cygni-type vari­able ex­hibit­ing a small mag­ni­tude vari­a­tion be­tween +0.05 and +0.18. At around 10 mil­lion years old it is a rel­a­tively young star. Rigel has a mag. +6.7 com­pan­ion 9 arc­sec­onds away. Al­though not par­tic­u­larly faint, Rigel’s bril­liance makes its com­pan­ion hard to see. Rigel B is lo­cated 2,500 AU from Rigel and al­though it shares the same proper motion, its or­bital pe­riod is un­known, prob­a­bly around 20,000 years. The com­pan­ion is ac­tu­ally a close bi­nary in its own right, its two com­po­nents in a mu­tual 400-year or­bit sep­a­rated by 100 AU. A fourth mem­ber of the sys­tem sits 44 arc­sec­onds from Rigel, shin­ing away at a rather dim 15th mag­ni­tude.

A pop­u­lar as­so­ci­ated tar­get for astropho­tog­ra­phy is the blue re­flec­tion neb­ula known as the Witch Head Neb­ula, IC 2118. It gets this name be­cause it looks like the pro­file of a witch’s face. The neb­ula sits 2.5° north­west of Rigel and is vis­ually ex­tremely faint. Amaz­ingly, al­though it’s phys­i­cally lo­cated 40 lightyears from Rigel, it is Rigel’s light that il­lu­mi­nates it.

Mas­sive, bright, young and vari­able, Rigel is one of the most os­ten­ta­tious stars in the sky

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