Final answer:
The mass of the Milky Way Galaxy is determined by studying its rotation, using the rotation speed and Kepler's third law to calculate the galaxy's mass. The mass includes luminous components like stars and interstellar matter, as well as dark matter, which is substantially more massive.
Step-by-step explanation:
The distribution of the mass of the Milky Way Galaxy is determined by studying the rotation of the galaxy. This method includes observing how fast objects in the outer regions of the galaxy are orbiting the center and using that data with Kepler's third law to deduce the galactic mass. Through measurements of their rotation speeds, indicated by wavelength shifts due to the Doppler effect, astronomers can estimate the mass. For instance, the mass of interstellar matter in the Milky Way is approximately 15% that of the stars, and when the mass of the luminous matter is based on about 1011 stars of average mass 1.5 times that of our Sun, we find the dark matter to be 10 times as massive as the luminous matter.
The mass-to-light ratios help to estimate the masses of different kinds of galaxies like spiral or elliptical. Galaxies with active star formation typically have lower mass-to-light ratios ranging from 1 to 10, whereas elliptical galaxies, which have mostly old stars, display different mass-to-light ratios.