Calculating Galactic Mass
In this lesson, we will outline three different ways to measure the masses of galaxies (or any large interstellar object for that matter).
One way that we detect massive objects is by the projection or distortion of distant galaxies. When the light from a distant object passes near a massive object, its light is affected by gravity. Depending on the source of the gravity (whether it is round or square, etc.), the light will be affected in different ways. Based on the images that we view and our knowledge of the actual image, astronomers are able to reconstruct the mass distributions in galaxies and galaxy clusters.
Another characteristic that astronomers can detect is the velocity at which objects are moving. This fact proves crucial to the second method of determining the mass of galaxies. Through the observation of various stellar velocities, astronomers can use the seemingly random velocities and make order of them. Using Kepler’s and Newton’s laws astronomers are able to calculate the minimum mass necessary to maintain the current orbits of stars around the center of their galaxies.
A third characteristic of galaxies that astronomers can detect is the temperature of gases that are in the galaxy. By observing the x-ray emissions from intergalactic dust, astronomers are able to determine the temperature of the gas based on the emission spectra. Based on the temperature, astronomers are able to use the principles of hydrostatic equilibrium to determine the amount of mass necessary to keep this gas within gravity’s clutches.