Question

Why mass do not remain Constant at Sun?

Answer 1

The centre of our solar system, Sun, can be considered as a giant nuclear reactor. It produces that tremendous amount of energy by Nuclear Fusion of Hydrogen and Helium.

2H1 + 2H1 —-> 4He2 + Energy

If only masses are considered, right side of equation falls short of mass than the left side.

This difference of mass makes up the energy, by Einstein's most famous equation

Energy E = m. c^2

Due to fusion, Sun loses about 4,000 kg of mass per second. This rate can fairly be assumed constant from its birth. So till now Sun has lost about 7.5*10^20 kg of mass.

So yes, the sun is losing mass.