Activity #6d - Weighing a Galaxy

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Worksheet Answers

1. The calculation for each of the planets should result in a value for M of 2.0 x 10³⁰ kg, which is the mass of the Sun. It is the same for each because the central mass for the solar system is concentrated in the Sun.

2. Distance (kpc)	Velocity (km/s)	Mass (kg)
   3.0	95.0	2.1 x 1040
   10.0	110.0	5.6 x 1040
   15.0	110.0	8.4 x 1040

   The mass increases as the distance from the center of the galaxy increases. This is because stars move under the gravitational influence of all the matter within their orbit. So stars at greater distances move under the influence of more mass than stars closer to the center.

   The best estimate for the mass of the galaxy is the one which includes the most amount of mass. From this calculation, the largest value is 8.4 x 10⁴⁰ kg. The actual mass of the galaxy is likely to be more than this. From Part 1, we know the mass of the Sun is 2.0 x 10³⁰ kg. So this galaxy is at least 4.2 x 10¹⁰ times more massive than the sun.