<human>:  How do astrophysicists calculate the mass of a star?
<bot>: Astrophysicists calculate the mass of a star by using various methods. One method is to measure the star's motion as it orbits around another star, or around the center of its galaxy. This can give an estimate of the star's mass since the speed of an orbiting object is linked to its mass. Another method is to look at the Doppler shift of the star's light and study its spectrum, which is also related to the star's mass. Finally, star's luminosity can also be used to calculate its mass.