A mass balance.
The effect of gravity is measured.
You might discover that solids have mass and they have volume.
That depends on the circumstances (what information is given). Quite often, you can use the definition of density:density = mass / volume
The atomic number.
Density = Mass/ Volume
The age of the material.
The two main factors in determining the average atomic mass of an element are:the isotopic composition of the element (the fraction of each isotope)the atomic mass of each isotope
... mass.
Look on a periodic table.
Mass is a major factor.
It is the mass of the star.
Just adapt the formula for density to find the mass of an object. After adapting it, it should be like this: Volume*Density=Mass