In order to turn a star's proper motion into its space velocity, we must also know:
A)its mass and spectral type.
B)its temperature and age.
C)its distance and radial velocity.
D)its distance and mass.
E)its spectral type and period of rotation.

Question

Quizplus · Accepted Answer

Proper motion provides us with the component of a star's motion across the sky, but in order to determine its space velocity, we also need to know its radial velocity (the component of its motion towards or away from us) and distance from Earth. Choice A, B, D, and E are not directly related to determining a star's space velocity. Proper motion only measures the motion of a star transverse to our line of sight, and is thus only part of its space velocity. To determine the full space velocity, we must also know its distance (so we can calculate the star's motion in three dimensions) and its radial velocity (the component of the star's velocity that is directed towards or away from us). Knowing the star's mass, spectral type, temperature, age, or period of rotation is helpful in determining other characteristics of the star, but does not contribute to calculating its space velocity from its proper motion.

In Order to Turn a Star's Proper Motion into Its