Mu Cephi

 

Mu Cephi, Cepheus. PIRATE robotic telescope BVR filters. Tenerife. Data credit: Open Observatories, Open University, telescope.org.

" Mu Cephei (Herschel’s Garnet Star) is a bright red supergiant with spectral type M2 Ia. Its line profile spectrum is heavily dominated by low-temperature molecular absorption bands and neutral metal lines, featuring a red-dominated continuum profile.

Key Spectral Characteristics

• ​Molecular Bands (Titanium Oxide - TiO): The most prominent features are deep, sawtooth-shaped TiO absorption bands (especially near 4954 Å, 5167 Å, 5448 Å, 5847 Å, 6158 Å, and 7054 Å). These create sharp flux drops toward shorter wavelengths with gradual recoveries toward the red.
• ​Neutral Metal Lines: Numerous absorption lines of neutral and weakly ionized heavy elements, including Fe I, Ti I, V I, Na I (D doublet at 5890/5896 Å), and Ca I.
• ​Hydrogen Balmer Lines: The H alpha line (6563 Å) typically appears as a complex, asymmetric absorption profile that can show weak variable emission or circumstellar wing distortion due to ongoing mass loss and chromospheric pulsations.
• ​Infrared Continuum Peak: Energy output shifts strongly into the red and near-infrared consistent with its low effective temperature approx 3,750 K.
• ​Infrared Dust Features: Mid-infrared line profiles show silicate emission features arising from a cool, expanding circumstellar envelope of dust." - Gary Gemini Coms Manager at the Jodrell Plank Observatory.