S Persei — a red supergiant near the Double Cluster in Perseus

Characteristics

S Per displays properties typical of red supergiants: a spectral type dominated by molecular absorption at optical wavelengths, a very large radius compared with the Sun, and a low effective temperature. Surrounding the star is a circumstellar envelope where molecules and dust form. Observations have detected strong maser emission associated with this envelope, which is a diagnostic of outflow kinematics and density.

• Variable brightness: shows irregular or semiregular variability common among red supergiants.
• Circumstellar masers: maser lines originate in the gas expelled from the star and are used to map motions.
• Mass loss: strong stellar winds drive material into the surrounding medium.

S Per is located in the sky close to the famous Double Cluster; it lies north of the open cluster NGC 869 and in the same general field as the pair of young clusters often referred to as the Double Cluster. Catalog entries and detailed measurements for the star are maintained in astronomical databases and survey catalogues (astronomical databases).

Its variable-star designation follows the conventional naming sequence used for stars with detectable brightness changes: the single-letter name "S" marks it among the early-discovered variables in the constellation. While not unique among red supergiants, S Per remains a frequent target because its maser emission can be traced with very long baseline interferometry to study the structure and velocity field of the outflowing material.

Scientific interest in S Per focuses on mass-loss physics, circumstellar chemistry, and the role of red supergiants in enriching the interstellar medium. By comparing S Per with other red supergiants, researchers refine models of how massive stars evolve, lose mass, and influence their surroundings prior to core collapse.