|MadSci Network: Astronomy|
The Sun is using losing 4 billion kilograms of mass a second; as four
hydrogen nuclei are converted to a single helium one, this loss in mass
provides the energy for the Sun to shine. (Mass and energy are equivalent,
as shown by Einstein's famous formula
though it sounds,
this mass loss is actually insignificant compared to the Sun's total mass. The
Sun's total mass is 2 x 1030 (or 2 nonillion) kilograms. Another way
of looking at the Sun's mass loss is to consider how long it would take for it
to "evaporate" at its current rate of mass loss; it would take 14 trillion
years. (Remember that the Universe is only about 12 billion years old.) So
although the Sun's gravitational pull is reducing, the effect isn't
To understand how this mass loss affects the Sun, we need to consider the Sun's structure; hydrogen burning requires a huge temperature, only achieved right at the Sun's core---so most of the Sun isn't directly involved. There is a delicate balance between gravity, attempting to force the Sun to collapse, and the pressure from the radiation which supports the outer layers, and it is this balance that acts as a thermostat to control the process.
Fuel is therefore burnt at a constant rate, and the Sun shines with a constant luminosity (the Sun is actually variable, but with no overall pattern and for different, poorly understood reasons).
Before any of the effects you mention come to play a large part, the Sun will run out of hydrogen in its core and switch to burning helium instead--- causing many changes including the swelling of the star to a red giant with a radius approximately that of the Earth's orbit. Don't worry, though; we have about 5 billion years worth of fuel remaining!
[In fact, the Sun is actually getting hotter. As 4 hydrogen atoms turns into 1 helium atom, the total number of particles in the Sun's core decreases. In order for the pressure within the Sun to stay constant, and continue to support itself against gravity, the temperature of the Sun must increase. This increase in temperature results in the Sun becoming brighter as well. In turn, this makes the Earth warmer because the intensity of sunlight is increasing. It is expected that within 1 billion years or so (well before the Sun runs out of hydrogen fuel) the temperature on Earth will increase to the point that a runaway greenhouse will take place. The result is that the Earth will end up looking much like Venus today. Moderator]
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