Why Do Stars Turn Red?

Po-Sheng Ou^1,2*, Ke-Jung Chen¹

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
²Department of Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Po-Sheng Ou, email:psou@asiaa.sinica.edu.tw

When a star exhausts the hydrogen fuel in its core, the core contracts, and the envelope may expand by order of magnitude and cool down to ~3,000-4,000 K, turning red. Consequently, stars with ≦ 8 solar masses become red giants, while those with 8-35 solar masses evolve into red supergiants. However, the physics driving this extreme expansion is unclear. This letter presents a comprehensive explanation, grounded in hydrostatic equilibrium, for the expansion of stars with initial masses of 3-35 solar masses toward red giants or supergiants. From the terminal-age main sequence to the blue giant or supergiant stage, the envelope and the hydrogen burning shell exhibit reciprocal motion to maintain hydrostatic equilibrium: the envelope expands as the hydrogen burning shell moves inward. When the star's surface temperature cools to ~8,000-10,000 K, it enters the instability zone, where the surface pressure increases with the growing radius. The star then continues to expand until it stabilizes at ~4,000-5,000 K, becoming a red giant or supergiant. We demonstrate that the envelope instability can explain several elusive features in the Hertzsprung–Russell diagram, such as the Hertzsprung gap between the main sequence and the red giant branch, as well as the dichotomy of the supergiant populations in red and blue.

Keywords: Stellar evolution, Red giant, Red supergiant