Why Do Massive Stars Turn Red and What Happens Next?

The most massive stars, up to about 30 times the Sun's mass, undergo a spectacular change: after the main‑sequence stage they swell into red supergiants (RSG). In a typical stellar population, nearly 80 % of these giants pass through this phase.

RSGs are enormous plasma balloons, with radii that can exceed a thousand solar radii. Their surface is cool (around 3,500 K) and gravity is weak, fostering intense internal convection. This convective churn pushes energy outward, making the star pulsate and generate powerful stellar winds. Some RSGs lose mass steadily, while others eject large gas shells in brief outbursts, like a violent sigh.

Mass loss is crucial: it decides how much material stays in the core when the star runs out of fuel. Eventually, most RSGs explode as type‑II supernovae, one of the most common cosmic blasts. Theoretical studies of this stage help astronomers grasp stellar lifecycles, heavy‑element creation, and galaxy dynamics. Future precise observations and advanced computer simulations will reveal the hidden details of these red behemoths.

Red supergiants are dominated by a convective zone that occupies almost the entire stellar volume. Convection transports energy from the core to the surface at supersonic speeds, generating shock waves that can trigger mass ejections. Stellar evolution models include mass‑loss rates (Ṁ) ranging from 10⁻⁶ to 10⁻⁴ M☉ yr⁻¹, depending on metallicity and pulsation. Metallicity affects dust formation in the photosphere, enhancing radiative wind efficiency. Moreover, the star’s initial rotation can induce chemical mixing, altering core composition and the RSG phase duration. Three‑dimensional simulations reveal that convective instabilities can create gigantic convective cells, akin to planetary‑size “granules,” modulating observed luminosity. When an iron core forms, degeneracy pressure can no longer support the star, triggering core collapse and the ensuing type‑II supernova explosion. Observations of recent supernovae, such as SN 2023ixf, have confirmed a dense circumstellar medium (CSM), consistent with an intense mass‑loss episode shortly before explosion, supporting theoretical models of RSGs as progenitors of SN II‑P.