10 Mu Cephei
The garnet explosion
The red supergiant Mu Cephei is one of the largest stars known to humanity. With a luminosity around 100,000-times greater than our Sun, and with a radius pegged at around 1,260 solar radii, its rich reddish hue led the 18th-century astronomer William Herschel to describe its “very fine deep garnet colour”, which he likened to the red giant Mira. As a result Mu Cephei very quickly earned the moniker of ‘Herschel’s Garnet Star’.
A 10-million-year-old variable star, records of its changeable magnitude – which varies between +3.4 and +5.1 – have been maintained continuously for more than 130 years, although its distance has been tentatively determined in the range of 6,000 light years. Mu Cephei may be surrounded by a vast ‘shell’, perhaps as much as 3,000 years old, which extends outward to a distance of about one-third of the star’s radius. This dusty shell is known to be somewhat asymmetrical in shape, perhaps torus-like, and Mu Cephei is thought to be losing several solar masses worth of material each year.
Despite its relative youth, Mu Cephei has already exhausted its hydrogen supply and is in the process of fusing helium into carbon. As is the case with many large stars, the conversion of elements to iron will precipitate the final cataclysmic collapse of the core, a spectacular supernova leaving a feeble remnant of what once was. It seems plausible that Mu Cephei will die in a Type II supernova – rapidly collapsing and violently exploding within a million years – although the form its remains will subsequently take is unknown.