The Reddest Stars
July 06, 2014
Photographer: Greg Parker
Summary Author: Greg Parker
The above photo shows the star Mu Cephei (the Garnet star) in the upper frame and CE Tauri (the Ruby star, 119 Tauri) in the lower frame. These are the two reddest, naked eye stars in the sky. Mu Cephei is a red supergiant (surface temperature of approximately 3,690 K) in the constellation of Cephus. It's often referred to as Herschel's Garnet Star. Mu Cephei (surface temperature of approximately 3,500 K) has a B-V index of +2.35 making it redder than 119 Tauri (in the constellation of Taurus). Its apparent magnitude is +4.08, making it brighter than 119 Tauri as well. Note that the Sun's surface temperaturer is 5,778 k.
119 Tauri is an M-type, red supergiant. Its mean apparent magnitude is +4.32. 119 Tauri is one of the largest known stars -- a diameter of perhaps 600 times that of the Sun. The Ruby Star has a B-V index of +2/07. Note that the more positive the B-V index is, the redder the star.
At this stage, you might be thinking that Betelgeuse, Arcturus and Aldebaran must all be brighter and redder than the above two stars. They're all brighter, but none are redder. The B-V index for Arcturus is only 1.34, but its magnitude is a massive -0.05 making it the fourth brightest star in the night sky. For Aldebaran the B-V index is a more impressive 1.78 but with a lower magnitude of 0.87. Finally, Betelgeuse comes in with a B-V index of 1.85 and a magnitude of 0.45. So, Mu Cephei and C.E.Tauri really are the brightest/reddest stars in the entire night sky.
Image details: The image of 119 Tauri was taken using the Hyperstar III on a C11 and Mu Cephei was captured with the mini-WASP array at the New Forest Observatory. Both images were processed by Noel Carboni in Florida, U.S.