DepartmentAstronomy & Astrophysics
Chapter 14: Our Star
Why does the sun shine?
Using Einstein’s famous formula E = mc2 it was able to show that the suns mass
itself contains an enormous amount of potential energy through the process of
The Stable Sun
Nuclear fusion requires high temperatures and high densities, which is found in
the core. Core must always be kept hot and dense for steady shine
Maintains these conditions through a balance of two forces, gravity pulling
inward and pressure pushing outward, aka gravitational equilibrium
How Fusion Started:
Sun was formed from a collapsing cloud if interstellar gas. The contractions released
gravitational potential energy
Most of the energy was radiated away from the surface as thermal radiation, the rest
remained inside causing the temp and pressure inside to spike.
When the core was hot, and dense enough to sustain nuclear fusion, the energy in the
suns interior was balanced with the energy lost in the form of radiation. This balance in
turn stabilized the size of the sun achieving gravitational equilibrium.
Basic Properties of the Sun
Radius just under 700,000 km and sunspots can be even larger than earth
300,000 times the mass of earth and nearly 1000 times the mass of all the planets put
Sun does not rotate at the same rate. Solar equator completes one rotation in 25 days, and
the rotation period increases with latitude to about 30 days at the poles.
The suns power output or luminosity, is 3.8 x 1026 watts.
The Suns Atmosphere:
Solar wind – stream of charged particles continually blown outward in all direction from
the Sun. It also helps shape the magnetospheres of planets
Outermost layer is called the corona and is very high in temperature, about 1 million K. It
explains why this region emits most of the suns x-rays.
Temperature drops to about 10,000 K when reaching the chromospheres. It is the middle
layer of the solar atmosphere and this is the regions that radiates the suns ultraviolet light.
The lowest layer of the atmosphere, the photosphere, the visible surface of the sun.
Appears to be a well defined surface from earth but, it consists of gas much less dense
than and earth’s atmosphere. Region where sunspots would be located
The Sun’s Interior:
The convection zone, the area in the sun where energy generated in the core travel
upward transported by the rising gas and falling if cool gas
The radiation zone, where energy move outward in the form of photons of light. Temp
rises to about 10 million K. Large amounts of x-rays are present in this area
The core, the source of the suns energy: nuclear fusion transforms hydrogen into helium.
Temperature is about 15 million K, 100 times denser than water, and the pressure is 200
billion times that on earth’s surface.
14.2 – The Cosmic Crucible
Nuclear fission – Process of splitting a nucleus into two smaller nuclei
Nuclear fusion – process of combining nuclei to make a nucleus with a greater number of
protons or neutrons.
How does nuclear fusion occur in the Sun?
Occurs because of the 15 million K plasma core
Strong force is the only force that can bind protons and neutrons together. It can
overcome electromagnetic repulsion between two positively charged nuclei.
Key to nuclear fusion is to push the two positively charged nuclei so close together that
the strong force outmuscles electromagnetic forces.
High temperature is important because nuclei must collide at very high speeds if they are
to come close enough together to fuse
High pressure of the overlying layers is important because the hot plasma of the core
would explode without it
The Proton – Proton Chain:
Overall hydrogen fusion reactions transform four individual protons into a helium
nucleus containing two protons and two neutrons.
Three steps are involved:
1. Two protons fuse to make a deuterium nucleus (1 proton and 1 neutron). This step
occurs twice in the overall reaction. A positron and a neutrino are created in the