The Sun has existed for around five billion years. At the beginning of that time, the Sun had some specific amount of angular momentum of rotation. Our traditional understanding of Physics says that angular momentum must be conserved. A commonly seen example is when an ice skater is slowly spinning with arms extended, draws her arms in and suddenly spins very rapidly.
In the case of the Sun, a peculiar condition exists. In the process of creating its energy, some of its mass disappears! When this mass disappears, what happens with the angular momentum of rotation that had been carried by that mass? Does it disappear with the mass that disappears? That would imply that the Sun would continue to rotate at the same rate. Or does it remain, distributed among the remaining atoms of the Sun? That would imply that the Sun should begin to spin faster!
The Sun creates a LOT of energy. Every second, about 3.86 * 1026 joules (watt-seconds) of energy is radiated outward from the Sun's surface. That energy was all created from the conversion of mass to energy in accordance with Einstein's E=mc2. Knowing that the speed of light is 3.0 * 108 meters per second, we can easily determine that 4.3 * 109 kg of mass disappears every second from the Sun. Isn't that something? 4.3 BILLION kilograms of the Sun disappear every second! (It will still last for many billions of years to come!)
For comparison, this is roughly 1.4 * 1014 tons per year. The Earth has a mass of about 6.0 * 1021 metric tonnes, which indicates that the Sun loses a mass equal to the entire Earth around every 40 million years! In the Sun's probable 5 billion years of creating energy by fusion, the Sun has therefore had the equivalent to more than a hundred Earths "disappear"!
Continuing this logic, the Sun has existed for around 1.6 * 1017 seconds, so, if its radiation has been relatively constant during that time, a total of about 7 * 1026 kg has already disappeared in this way. The present mass of the Sun is 1.99 * 1030, so only about one part in 2500 has already disappeared.
When the mass disappears, it disappears near the very center of the Sun, where it is hottest. In other words, the atoms of the Sun that disappear in the conversion to energy, have minimal rotational angular momentum. This suggests that this effect would have a tiny increase in the rotation rate of the Sun, about one part in two million. Pretty much irrelevant!
The other effect that may be affecting the rotation rate of the Sun has to do with a lot of turbulence that exists on and in the Sun. We can see the churning motions of the surface. We assume that the heat driven effects continue throughout the Sun, where the rising heat from the center (where the energy is created) is moving upward and outward to get to the outer surface where it can be radiated into space. Convection cells are certainly one of the main processes inside the Sun in accomplishing this heat transfer.
That churning motion must necessarily create frictional heat losses. Some of the kinetic energy of rotational motion must be converted into frictional heat, which means there would be less remaining kinetic energy of rotation. This would imply a gradual slowing of the rotation rate. In the process, the amount of rotational angular momentum would also decrease, but that would go into the turbulence eddies.
For more information on this effect, please see my similar article on the rotation of Jupiter and Saturn.
This effect in the Sun appears to be much more substantial than the 'disappearance effect' mentioned above. That suggests that the Sun may have significantly slowed down in its rotation during the past five billion years. Trying to quantify this effect is difficult, because there are so many variables inside the Sun that are unknown.
( http://mb-soft.com/public/othersci.html )
C Johnson, Physicist, Physics Degree from Univ of Chicago