Transcript Red Giants

I'll give you a short answer and a longer one. The
short answer is that towards the end of a star's life, the
temperature near the core rises and this causes the
size of the star to expand. This is the fate of the Sun in
about 5 billion years. You might want to mark your
calendar!The long answer is that stars convert
hydrogen to helium to produce light (and other
radiation). As time progresses, the heavier helium
sinks to the center of the star, with a shell of hydrogen
around this helium center core. The hydrogen is
depleted so it no longer generates enough energy and
pressure to support the outer layers of the star. As the
star collapses, the pressure and temperature rise until
it is high enough for helium to fuse into carbon, i.e.
helium burning begins. To radiate the energy produced
by the helium burning, the star expands into a Red
Giant.
All through the long main sequence stage, the relentless compression of gravity is balanced by
the outward pressure from the nuclear fusion reactions in the core. Eventually the hydrogen in
the core is all converted to helium and the nuclear reactions stop. Gravity takes over and the
core shrinks. The layers outside the core collapse too, the ones closer to the center collapse
quicker than the ones near the surface. As the layers collapses, the gas compresses and heats
up.
Eventually, the layer just outside the core called the ``shell layer'' gets hot and dense enough
for fusion to start. The fusion in the layer just outside the core is called shell burning. This
fusion is very rapid because the shell layer is still compressing and increasing in temperature.
The luminosity of the star increases from its main sequence value. The gas envelope
surrounding the core puffs outward under the action of the extra outward pressure. As the star
begins to expand it becomes a subgiant and then a red giant
In the distant future, when the Sun begins to expand and evolves into a "giant" star,.
distance of the planet from the star is about 300 million kilometers (or twice the distance of the Earth from the Sun). The planet's orbital