The correct answer is option 2: Solar mass.
When measuring the masses of stars and galaxies, scientists typically use the unit of a solar mass, which represents the mass of our Sun. This is because the mass of stars and galaxies are often much larger than the mass of the Earth or the Moon, making these units less practical for comparison.
The term "solar mass" refers to the amount of mass contained in our Sun, which is approximately 2 x 10^30 kilograms. It provides a standard unit of measurement that allows scientists to easily compare the masses of different stars and galaxies. Using solar mass as a unit also helps in understanding the structure and dynamics of celestial objects on a larger scale.