Use the distance to the Andromeda Galaxy (2.4 × 10 19 km) and the speed of light to determine the lookback time to the galaxy. Now take the difference between the velocities you measured for the left ...Use the distance to the Andromeda Galaxy (2.4 × 10 19 km) and the speed of light to determine the lookback time to the galaxy. Now take the difference between the velocities you measured for the left and right hand sides of the galaxy, and divide by two to find the average rotational velocity of the Andromeda galaxy. Use the distance to the galaxy from part I and the systemic velocity from part II to calculate how long it will take for M31 to collide with the Milky Way.