Giant Stars irfu cea fr Energy Reserves When

Giant Stars irfu. cea. fr

Energy Reserves • When stars reach the main sequence they are in equilibrium between gravity and radiation. Hydrogen-rich zones – Hydrogen to helium for 90% of lifetime – Too cool for helium fusion • Helium builds up in the core. Helium-rich core

Red Giant • The helium in the core stops fusion except right along a hydrogen shell. Expanded hydrogen-rich zones • The helium core compresses to degenerate matter. – Density 1000 kg/cm 3 • The hydrogen expands and cools to orange and red. Hydrogen-burning shell Helium core

The Future Sun • The Sun will spend about 10 billion years on the main sequence. – Then expansion to red giant – Cool to class K or M Sun • The Sun will expand to engulf at least Mercury and Venus.

Stellar Evolution 10 M 3 M 0. 5 M 0. 02 M • Other mass stars will also run out of fuel and become giants. – Low mass stars expand a little – High mass stars grow more after less time

Helium Fusion • Three helium nuclei can fuse into one carbon nucleus. – Temperature over 108 K (100 million) – High density to collide within 10 -8 s photon helium-4 beryllium-8 carbon-12

Helium Flash • In a red giant, helium starts fusing and more energy is produced. – Mass over 2 M goes to slow helium burning – Mass from 0. 5 M to 2 M , less stable equilibrium – Lasts a few hours • The rapid helium burning is called a helium flash. – Star slightly contracts after explosion – Horizontal branch of HR diagram

Cluster Evolution • The M 3 Globular cluster shows stars going giant. horizontal branch Asymptotic giant branch red giant branch main sequence – Main sequence – Red giant branch – Horizontal branch • The largest giants are on an asymptotic giant branch. – Degenerate carbon core