- Short revisit of main-sequence lifetimes.
First, careful job of calculating the time for the Sun to
use up 10% of its Hydrogen fuel gives a main-sequence lifetime
of 10 billion years.
- So just what happens to ``end'' the main-sequence
lifetime of a star?
The sequences in the H-R Diagram were known long before stellar
evolution was understood and there were thoughts that stars
might evolve along the main-sequence. But, with stellar models
it is possible to let the models ``evolve'' and see what
happens to stars as they begin to use up their H fuel in the
regions where it is hot enough for fusion.
- The Sun is changing its structure. It is building up
- Does the He fuse together to form beryllium?
No, for two reasons.
- Because He nuclei have 2+
each so the temperature needs to be even higher for
- Beryllium nuclei have MORE mass than the
sum of two He nuclei so energy is not released in this
reaction it is absorbed.
- So, the Sun's central structure is now a small
He core surrounded by a hydrogen fusion shell. The
He core starts to contract under its own gravity.
This contraction releases GPE which increases the
temperature in the Hydrogen fusion shell and
the total energy production goes up.
- So the Sun's core is getting hotter, Luminosity is
increasing and the envelope begins to expand so
the outer surface temperature decreases.
- The Sun is becoming a Red Giant.
- For the Sun, we think it is a little more than 1/2 of the way through
its main-sequence lifetime and it will reach the base of
the giant branch in about 4 billion years.
- At the tip of the giant branch, the Sun will have a radius
about the size of the Earth's orbit. We will need a new home
by the time this happens.
As a red giant the Sun will have these properties:
- Let's remember the equilibria and energy sources:
- Proto-star: Gravity is winning and this object is contracting.
GPE is the energy source.
- Main-sequence Star: Here gravity is balanced by thermal pressure and
we say these stars are in Hydrostatic Equilibrium. The energy is provided
by hydrogen fusion in the core.
- Red Giant: Here, we say there is a quasi-static equilibrium with
thermal pressure barely winning the battle in the envelope and gravity
winning in the core. Red giants have two energy sources, GPE in the
contracting core and H-fusion in a shell around the core.