The Sun – A star is born.
The sun is about 5 billion years old. It is a relatively small star, as stars go. However it is by far the
largest body in our solar system. Its size (1,400,000 km) is roughly a hundred times larger than the
earth. Its mass (2.0 x 1030 kg) is over 300,000 times that of Earth. Its average density is 1500 kg/m 3,
compared to Earth’s 5500 kg/m3.
The Sun is burning (fusing) hydrogen to produce helium. As with any process consuming fuel, it will
burn out. That is the bad news. The good news is that the Sun still has about another 5 billion years
of fuel. Once that fuel is gone it will be a very bad day here on Earth. More on that later……
Composition: Over 98% of the Sun’s mass is composed of only 2 elements. Hydrogen (75%) and
helium (over 23%). The other elements (carbon, oxygen, neon, iron) make up the remainder. The
helium (and other elements present) come from the process of nuclear fusion.
The Sun puts out a tremendous amount of energy (much less than other stars). Here on Earth we
receive about 1400 Watts/m2 (solar constant). In order to calculate the total energy output of the
Sun, we have to find the area of a sphere with the Sun at its center and the edge of the sphere
extending to the surface of the earth.
Calculations: 150,000,000 km to the Sun.
Convert to meters = 150,000,000,000 m
To find the area of a sphere we use 4π r2. = 2.83 x 1023 m2
Multiply the area of the sphere times the solar constant (1400 W/m2)
And we get 4 x 1026 Watts
Regardless of where we go in the solar system (or universe) the Total Solar Output of our Sun
will remain constant. Even though the solar constant will increase as we move closer to the
Sun and diminish as we move away, the size of the area of the sphere will change also,
decreasing as we move toward the Sun and increasing in size as we move away. The Total
Solar Output will not change.
Structure of the Sun
Core: Radius of 200,000 km, temperature of 15,000,000 K. This temperature and the
corresponding pressure supports nuclear fusion. http://www.buzzle.com/articles/nuclear-fusion-in-
the-sun.html
Radiation Zone: 300,000 km thick, temperature is about 12,000,000 K. Very dense region of
the Sun, so dense in fact tht it will not allow particles to move. Energy is transferred only through
thermal radiation, very, very slow.
Convection Zone: 200,000 km thick, temperature of 2,000,000 K. This area, although dense, is
not so dense that particles cannot move. Just like convection currents here on Earth, hotter particles
rise and cooler particles fall. This is the manner in which the surface of the Sun receives it energy.
Atmosphere http://imagine.gsfc.nasa.gov/docs/science/know_l1/sun.html
Photosphere: 500 km thick, temperature of 5800 K. This is the part of the Sun we see.
Chromosphere: 2500 km thick, temperature from 4500 – 10,000 K.
Corona: Stretches millions of km away from the Sun, temperature is several million
degrees.
Balancing act within the Sun
There are two forces acting on all stars, including our Sun. Most of the time those forces are at
equilibrium. The outward force of the super heated particles are compressed by the enormous
gravitational forces of the Sun. When they are NOT at equilibrium, it is a very, very bad day for those
close to the star.
In another 5 billion years the Sun will run out of hydrogen, because of its small size there is not
enough pressure to fuse helium. Gravity remains the same, but outward force of superheated
particles is less, the Sun will contract because the gravitational force is greater than the outward
force. In a split second the collapsing matter will create enough pressure to fuse helium (helium flash)
and the Sun will expand enormously, so much so that the surface of the Sun will reach the orbit of
Earth, bad day all around. This helium fusion will go on, but not for long and eventually the Sun will
burn up all its helium. As a result the Sun will collapse again, but this time it will not carry on nuclear
fusion anymore. The only element left is carbon (and some heavier ones) and the Sun does not have
enough mass to ever fuse carbon. It will slowly cool and become nothing but a burnt sphere of
carbon.