What is the primary way in which we determine the mass distribution of a spiral galaxy?
A)We calculate its mass-to-light ratio.
B)We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
C)We count the number of stars we can see at different distances from the galaxy's center.
D)We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

Question

Quizplus · Accepted Answer

The primary way to determine the mass distribution of a spiral galaxy is by constructing its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center. This allows us to directly measure the mass of the galaxy at different radii and from this, we can determine the total mass distribution of the galaxy. The other options, such as calculating the mass-to-light ratio or counting stars at different distances, can provide some information but they are not as reliable or accurate as measuring the rotation curve. Kepler's third law can also be used in some cases, but it is not the primary method.

What Is the Primary Way in Which We Determine the Mass