If the sun consumes "x" amount of mass per second during nuclear fusion, and it is believed to be "y" years old, how big (diameter) was it believed to be when it was first "fired up"?

I cannot give you a quantitative estimate of the sun's size, but it was smaller than it is today. A star is born when nuclear reactions begin in the core of the collapsing protostar. Once the star starts nuclear fusion, its size remains nearly constant throughout its life in the main sequence. As you may know, the star is supported by hydrostatic equilibrium, and the temperture of the core is almost constant through the main sequence.

But, there are slight variations in the size and luminosity of the star during the main sequence. As the star ages, the temperature of the core gets slightly hotter (the core shrinks slightly), causing the nuclear reactions to proceed at a faster rate, causing the energy output to go up. Consequently, to maintain hydrostatic equilibrium, the star's envelope expands a little bit and the luminosity goes up slightly. Further, the temperature of the surface of the star goes down slightly (even though the core is hotter).

Thus, even though one thinks that the sun must have been bigger in the past as it had more mass, that is not the case, and it is actually bigger now than it was in the past.

This page was last updated June 28, 2015.