How Do Astronomers Measure the Mass That the Galaxy Contains

question 31

Multiple Choice

How do astronomers measure the mass that the Galaxy contains inside the orbit of the Sun?

• A) they count the number of stars one by one and multiply by the average mass of a star
• B) they add up all the observations at 21-cm wavelengths (because the mass of hydrogen gas is far greater than the mass in all the stars)
• C) they measure the distance to the center of the Galaxy and the period of the Sun's orbit and then use Kepler's Third Law
• D) they measure the masses of many other spiral galaxies nearby and then take an average
• E) you can't fool me; there is no way to get the mass of something as big as our Galaxy

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