Coal or Wood burning:
- Note: is equivalent to 1 ton of coal burned per hour for every
square foot of the Sun's surface
- By the mid-1800's it was recognized that the Earth and Sun
were at least millions of years old. This was a puzzle - the most
common source of energy on Earth could not be used to explain the
energy production and longevity of the the Sun.
Another Energy Source possibility: Gravitational Potential Energy
Anytime you have a collection of mass (for example a gas of atoms
and molecules) it has an associated Gravitational Potential Energy - GPE.
For a big ball of gas, the GPE goes like:
where R is the radius of the ball of gas, M is the total mass and
G is the ``gravitational constant''.
Similarly, if because the Earth exerts a gravitational pull on
objects at the surface and in space, objects all have an
- So, if a gas cloud shrinks in size without changing mass, it must
Release GPE. Energy is always conserved, so as a cloud shrinks
it heats up and emits radiation.
- At the surface of the Earth, an object at a large radius
from the center of the Earth has more GPE than the same object at a
smaller radius (here the radius is the radius of the Earth plus
the height of the object above the surface of the Earth
- Perhaps the Sun is slowly shrinking in size and radiating
away the lost GPE.
- OK, how long would the Sun last at its current luminosity if it
was GPE powered? (Note that it would contract at about 40m per year)
This converts to 16 million years - still too short! The Earth is around 4 billion
years old (plus the Sun would have been much bigger in the past).
- Another possibility considered was the GPE of comets and meteors falling into
the Sun. To produce would require about 1 Earth mass per year to
be accreted (not that much). This can be ruled out because the resulting
change in the Earth's orbit has not been measured.
- The answer had to await some advances in physics and the discovery of
Fri Feb 6 23:24:56 PST 1998