S Orionis: a Mira variable red giant in the constellation Orion

Physical characteristics

As a Mira-type star, S Orionis is an extremely cool, luminous red giant with an extended atmosphere. Its visual brightness changes by several magnitudes over a pulsation cycle that lasts on the order of hundreds of days. These pulsations produce changes in radius, surface temperature and spectral appearance; the star appears redder and larger at some phases and hotter and smaller at others.

Evolution and origin

S Orionis is in a late stage of stellar evolution known as the asymptotic giant branch (AGB). Stars at this stage have exhausted core hydrogen and helium burning and undergo thermal pulses and strong mass loss. Over time such stars shed their outer layers, contributing gas and dust back to the interstellar medium and eventually evolving toward the planetary nebula phase and a white dwarf remnant.

Observational importance

Observers study S Orionis and other Mira variables to learn about stellar pulsation, atmospheric dynamics and mass-loss processes. Because its size and brightness change substantially, S Orionis provides a laboratory for testing models of radial pulsation and for probing molecular and dust formation in extended stellar atmospheres.

Notable facts and practical information

• Classification: Mira (long-period variable), late-type M spectral characteristics during much of its cycle.
• Size: an extremely large radius compared with main-sequence stars—commonly stated as several hundred times the Sun's diameter.
• Visibility: reaches peak brightness occasionally bright enough for small telescopes and careful visual monitoring by amateur astronomers; its predictable cycle makes it a frequent target for variable-star observers.

S Orionis exemplifies the dramatic changes that occur in dying intermediate-mass stars and remains of interest for both professional research and amateur monitoring programs studying stellar variability and late-stage stellar evolution.