Strongly colored stars have always fascinated astronomers. The long history of red-star observations begins in the early 19th century, with famous observers such as Angelo Secchi and Thomas Espin. And those who think that stars are not highly colored need only look at Mu Cephei.
Often called “Herschel’s Garnet Star” (honoring both this cool, red star and William Herschel, who discovered infrared radiation), the color of this 4th-magnitude star, easily found in the constellation Cepheus, will leave you dazzled.
Classed as an M2 Ia supergiant, the “Ia” implying the brightest kind, Mu Cephei is among the most luminous and largest stars in the Galaxy. The Garnet Star is seen to radiate 350,000 times more energy than the Sun, which, in turn, implies a radius of 7 Astronomical Units – 40-percent larger than the orbit of Jupiter. Mu Cephei is so large that its actual apparent disk is readily discernable with professional telescopes!
To find the Garnet Star, place Alpha Cephei at the northwestern edge of a 5° field and Mu will be diametrically opposite. The deep orange-red color of this red giant is nicely brought out in 10×50 binoculars. It has a variability of a bit under a magnitude, but is usually around 4th magnitude.
Finder map – field width 60°, stars to magnitude +5.5.