How do you calculate the mass of a galaxy?

Since the average galaxy has between 100 billion and 300 billion stars, cataloguing the relative sizes of all the stars within that galaxy is extremely difficult and error-prone. As a result, any attempt to calculate the relative sizes of all the stars within a galaxy is at best an educated guess based on scientific knowledge.

The Keck Observatory in Hawaii is home to a team of scientists who have devoted their lives to cataloguing stars within our own galaxy. They have found that the stars closest to the Milky Way's black hole are particularly elusive in terms of both size and number.

Now, looking at Andromeda, the largest galaxy nearest to us, the task is easier because we can see it completely and distinguish between stars more readily. The difficulty stems from its distance from us; while it is relatively close at 2.5 million light years, the difficulty increases with galaxy distance.

In conclusion, they rely on guesswork and will do so until our detection technology significantly advances beyond what it is now.

Measurements of the Doppler shift of spectral lines emitted by these objects are commonly used to determine the mass distribution within a galaxy; additional information about the mass distribution of a galaxy can be obtained from gravitational lensing effects resulting from the galaxy's mass. A galaxy's mass is determined by observing the rotational velocity of stars and gas clouds within the galaxy and applying Kepler's laws of motion or Newton's laws of gravitation.