4. Helium Fusion

The Horizontal Branch

``main sequence'' for core helium burning

- Helium fused to make Carbon

- Hydrogen still burning in shell around core

- beginning of the ``onion skin'' interior structure

Eventually Helium too is depleted in core

... and when the fuel's spent?

5. Evolution off the Horizontal Branch

Relentless pull of gravity

Contraction of the Carbon core -- ``ash''

Burning of Helium in a shell around core

Burning of Hydrogen in a shell around Helium shell

Very intense -

star gets brighter,

But not hotter!

it must get bigger

A red Super-Giant

(a) the different interior zones in a Main Sequence star

(b) the distribution of evolved stars in the HR diagram

(c) the thermal layers in the collapsing proto-stellar cloud

(d) the layers of ``burning'' of different elements around the core

(e) the different time-scales for ``burning'' different elements

(a) The force of gravity is balanced by heat and pressure from fusion

(b) The ratio of protons to neutrons remains constant

(c) degeneracy pressure equals regular pressure

(d) Carbon is being generated at the rate Hydrogen is depleted

(e) energy and matter are converted