Chapter 16, Problem 2LL

The Tully-Fischer method relies on being able to relate the mass of a galaxy to its rotation velocity. Stars in the outer-most regions of the Milky Way galaxy, located at a distance of 50 kpc from the galactic centre, are observed to orbit at a speed vrot determine the mass in the Milky Way that lies interior to 50 kpc. Express your answer in units of the Solar mass. 250 km s-1. Using Kepler's 3rd Law,

A given star orbits the center of its galaxy at an average speed of v_star, at a distance of r_star​ from the center.  The galaxy has 2 spiral arms, and the arms themselves orbit slower than the star -- at the same radius, they orbit at a speed of v_arm (in the same direction as the star).  The galaxy's age is t_gal. In the history of this galaxy, how many times did this star cross through a spiral arm? Values: v_star​ = 200 km/s, r_star​ = 9 kpc, v_arms​ = 46 km/s, t_gal​ = 4 Gyr

Among the globular clusters orbiting a distant galaxy, one is moving at 534 km/s and is located 14 kpc from the center of the galaxy. Assuming the globular cluster is located outside most of the mass of the galaxy, what is the mass of the galaxy? Convert your answer to solar masses. (Hint: Use the formula for circular velocity, Vc =    GM r ; make sure you convert relevant quantities to units of meters, kilograms, and seconds. Note: 1 pc = 3.1 ✕ 1016 m.)