Physics 110: Homework #3 Solutions

3.1

The Sun is hot enough in its center to fuse hydrogen to helium through the p-p chain, which is the source of the Sun's prodigious luminosity. The p-p chain reaction only takes place within the inner 25% in radius of the Sun, because beyond this point the temperature has dropped below what is needed for nuclear fusion. The surface temperature of the Sun sets the properties of the gas (ionization and energy level populations) at the surface that define the Sun's spectral class (G2V).

3.2

Radiative diffusion, which uses photons to transport energy, is most important for carrying energy from the core out to a radius of about 70% for a 1 star. Convection carries energy in the outer part of a 1 star through the bulk motion of large cells of gas.

3.3

Solar neutrinos tell us about energy generation in the Sun's core at the present time. The current ``solar neutrino problem'' (astronomers observe fewer neutrinos than predicted by the standard solar model) may indicate that our models of the interior of the Sun are incomplete or that the properties of neutrinos are different from what we currently understand.

3.4

Infrared observations are used to probe the cores of molecular clouds and the protostars that form there. Molecular clouds in general are studied using radio observations. A protostar is said to have formed when a collapsing core has become dense enough that it blocks, or is opaque to, infrared radiation. It is largely the dust in the core that provides the opacity. This is important because it means that the protostar is now opaque to its own infrared radiation, which cannot escape so easily, so the protostar can begin to heat up.

3.5

Gravity is the dominant force that drives the collapse of gas clouds to form stars. Angular momentum, to some extent, inhibits gravitational collapse and thereby slows down the process of star formation. Angular momentum plays a critical role in defining the shape of the nebular disk around a protostar. Without gravity, stars could not form. Stars would form at a much greater rate in the absence of angular momentum and would be likely to grow more massive too.

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