Astro 103 - Lecture 11

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STELLAR GUTS


1. Steady Burning on the Main Sequence


Hydrogen Helium

in the core;

slow depletion of Hydrogen


Cool stars' interiors are totally convective.

  • replentish Hydrogen in core
  • leads to even longer life



. . . and when the fuel's spent?



2. Evolution off the Main Sequence


Relentless pull of gravity

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

Burning of Hydrogen in a shell around core

Very intense -

star gets brighter,

But not hotter!

it must get bigger

A red Giant!



3. ``Helium flash''


Where: At the tip of the red giant branch

When: Helium fusion ignites explosively

Why: Due to ``electron degeneracy''

When electrons (or any fundamental particles) are squeezed too hard together, they exert a quantum mechanical pressure due to the ``exclusion principle:''

No two particles can exist in the same state at the same place in space and time.
W. Pauli (1900-1958)

But why the ``flash?''

Because this electron pressure does not respond to changes in temperature

Once fusion starts, it continues explosively until conditions are so hot the ``degeneracy'' is removed.



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



Q11.1 The ``Onion Skin'' refers to what aspect of stellar evolution?

(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



Q11.2 What is at balance in order for the Main Sequence or Horizontal Branch to exist?

(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


Lectures Lecture page Astro103 page
Last updated: Aug 23, 2011 Matthew A. Bershady