Summary: ASTR 1120 Section 1 (3 credit hours): Spring 2006
SUMMARY OF KEY CONCEPTS: WEEK #3
Lecture #5 textbook Chapter 14 (4th
edition) or 15 (3rd
edition): `Our star'
Concept of gravitational equilibrium how does the Sun and other stars support itself stably
against the force of gravity which otherwise would cause a rapid collapse? We noted two
things: first that at each radius in the Sun there is a balance between the inward force of gravity
and an outward pressure gradient (high pressure in the center, low pressure at the surface). The
high pressure comes about because an ordinary gas' pressure increases with temperature and
density, and the center of the Sun is very hot! Second, the core of a star acts as a kind of
thermostat to prevent a runaway nuclear reaction if the rate of nuclear reactions increases the
core expands and (counter-intuitively) cools off, restoring the balance.
Photons produced in the Solar core leak out of the Sun very slowly over thousands of years or
longer because of multiple collisions along the way.
Nuclear reactions in the core also produce subatomic particles called neutrinos, which have the
property of interacting extremely weakly with ordinary matter. They can easily escape the Sun
without being scattered or absorbed. This allows us a window into the Solar core flip side is
that it's very hard to capture the neutrinos in experiments on Earth and detect them.
The Solar neutrino problem was an apparently large discrepancy between the predicted number