Astronomy 242: Foundations of Astrophysics II

36. Stellar Evolution Main Sequence Evolution

As the hydrogen in the core is slowly used up, a star’s ‘thermostat’ gradually raises the core’s temperature. With a higher temperature, more energy flows out, and luminosity increases.

For example, the Sun had only ~70% of its present luminosity when it formed. Becomes a Red Giant inert helium Broken Thermostat core contracts photosphere A Main-Sequence Star...

photosphere

hydrogen- ‘burning’ core

hydrogen- ‘burning’ shell H begins fusing to He in a shell around the contracting He core — but this can’t stop the contraction, so the core gets hotter and the star gets even more luminous. After the Main Sequence

Once hydrogen starts burning in a shell, a star becomes larger, redder, and much more luminous — at least for a while. . . Triple-Alpha Process

When the core temperature hits 108 K, helium nuclei can overcome their repulsion and fuse to make carbon.

Compared to hydrogen fusion, this is not very efficient. Helium ‘Burning’ Star

helium fusing in core

hydrogen fusing in shell

The central thermostat once again stabilizes the star’s energy production, and the total luminosity decreases. Horizontal-Branch Stars

Models predict that stars HR diagrams of old star get smaller, bluer, and less clusters reveal stars with luminous after He ignition. the predicted properties. Alpha Capture Reactions

High core temperatures enable helium to fuse with other nuclei, producing 16O, 20Ne, 24Mg, . . . Asymptotic Giant Branch

Double-shell Supergiant 1982ApJ...260..821I

C

shell: 4H→He shell: 3He→C After the helium in the This situation is unstable. core is gone, burning Thermal pulses eject 4 continues in two shells. mass (up to 10 M⊙/yr).

Fate of the Sun Evolution of High-Mass Star (M ≥ 8 M⊙)

Main-Sequence Star Red Supergiant Helium-burning Star

core: inert He photosphere

photosphere

core: 4H→He

shell: 4H→He

Initial evolution follows same shell: 4H→He core: 3He→C sequence as a low-mass star. . . . Continued

Double-shell Supergiant Carbon Ignition

core: C burning

C

shell: 4H→He shell: 3He→C shell: 4H→He shell: 3He→C After the helium in the Until the core contracts core is gone, burning enough to ignite carbon. continues in two shells. Late Stages of Evolution H, He By the end of its life, a high-mass star has multiple shells of nuclear burning, making a wide range of elements. He

Reactants Temperature Products

12C + 12C 6×108 K 24Mg, 23Mg + n, 23Na + p, 20Ne + 4He, 16O + 2 4He

20Ne + 4He 1.2×109 K 24Mg C, O, Mg

16O + 16O 1.5×109 K 32S, 31S + n, 31P + p, 28Si + 4He, 24Mg + 2 4He

32S, 28Si, He ~3×109 K 56Fe, 56Co, 56Ni

S, Si

However, the iron-group elements represent Fe the end of this process.