From the Physical Origin of Red Supergiants to Their Supernova Shock Breakout

Released: 26th Febraury 2026, Academia Sinica Institute of Astronomy & Astrophysics (ASIAA), Taiwan

Red supergiants (RSGs) are the dominant progenitors of Type II supernovae, yet the physical origin of their enormously extended envelopes—and the consequences for early supernova emission—have remained long-standing problems. In two closely connected studies, the cosmic explosion group at the Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), establishes a self-consistent theoretical framework that links massive-star evolution, metallicity, and supernova shock breakout observables. Both papers are published in the latest issue of the Astrophysical Journal.

In the first paper, Po-Sheng Ou and Ke-Jung Chen explore the “critical metallicity” for red supergiant formation using ~10,000 stellar evolution models. They show that metallicity shapes stellar structure through its effects on nuclear burning and opacity, thereby determining the stellar radius at the end of the main sequence. This radius ultimately determines the star’s supergiant fate and gives rise to a critical metallicity of about 0.1 solar. Above this threshold, stars naturally evolve into RSGs, while below it they remain compact, explaining the observed bifurcation between blue and red evolutionary branches and the metallicity dependence of supergiant populations. The lead author, Po-Sheng Ou, commented, “This study explains the physical origin of the critical metallicity required for stars to become red supergiants, providing new insight into the evolution of low-metallicity stars in the early universe.”

Building on this evolutionary foundation, the second paper, an international team led by Wun-Yi Chen and Ke-Jung Chen, explores supernova shock breakout from RSG progenitors using two-dimensional, multigroup radiation–hydrodynamic simulations for the first time. The results show that extended RSG envelopes give rise to longer-lasting and fainter shock breakout signals, shaped by radiation precursors and multidimensional fluid instabilities. These effects reconcile observed breakout durations without invoking extreme pre-supernova mass loss and highlight the limitations of previous one-dimensional models. The lead author, Wun-Yi Chen, stated, "This study presents the first two-dimensional multigroup radiation-hydrodynamic models of red supergiant shock breakout, revealing that radiation precursors and circumstellar density significantly shape the breakout light curves and color evolution."

Altogether, these two studies demonstrate how stellar interior physics imprints itself on early supernova emission, providing a predictive framework for interpreting shock breakout detections from RSG explosions in modern time-domain surveys.

Schematic diagram illustrating how a star’s size at the end of its main-sequence phase—the terminal-age main sequence (TAMS)—determines whether it becomes a red or blue supergiant. Stars that are already relatively large at the TAMS can undergo substantial envelope expansion and evolve into red supergiants. In contrast, more compact stars remain blue supergiants and eventually contract rather than expanding further. Image Credit: ASIAA/Po-Sheng Ou

Massive star models reveal a critical metallicity of Z∼0.001 (about one-tenth of the Sun’s metallicity) for red supergiant formation. Only stars above this threshold can expand into red supergiants, reaching ~1,000 solar radii during the core helium-burning phase. Stars below this metallicity instead remain as blue supergiants and do not undergo further expansion. Image Credit: ASIAA/Po-Sheng Ou

Gas and radiation energy densities shortly before shock breakout in a red super giant of 20 solar masses. A strong radiation precursor develops ahead of the shock, while Rayleigh–Taylor instabilities emerge near the contact discontinuity at ∼4 × 10¹³ cm. Cyan and red vectors represent the velocity and radiation flux, respectively. The pink dashed line indicates the location of the photosphere. Image Credit: ASIAA/Wun-Yi Chen

Bolometric light curves for all models, aligned at peak luminosity (t=0). Dense Circum-stellar medium (CSM) around red super giants can trigger radiation heating several hours prior to shock breakout and extend the rise time to maximum luminosity. While explosion energy sets the peak brightness, CSM density primarily governs the breakout timescale. Image Credit: ASIAA/Wun-Yi Chen

More Information:

This research presented in two papers “Critical Metallicity of Cool Supergiant Formation. II. Physical Origin" by Ou et al. & "Multi-wavelength Signatures of Supernova Shock Breakout from Red Supergiants in Two Dimensions" by Chen et al. in the Astrophysical Journal published in February, 2026.
Cosmic Explosion Group

Media Contact:

Dr. Ken Chen Email: kjchen@asiaa.sinica.edu.tw Tel: +886 2 2366 5457