Suppose that we decide to weigh a couple of cats. This is true anywhere in the universe. In this course, we will use the mass of our sun as 1 sun and will talk about larger stars as having, for example, a mass of 1.25 suns. A smaller star might be only . Density compares the masses of different substances.
Scientists take a cubic centimeter as their measure.
This is roughly the size of a sugar cube. Here you see some cubes with their densities. You would need 3 cubic centimeters of water to get the same mass as 1 cubic centimeter of rock. You would need 8 cubic centimeters of water to get the same mass as 1 cubic centimeter of iron. This is why the core of the earth is made of iron, and why the continents, made mostly of rock, are on the crust.