In order to get a feeling for the importance of dark matter in our galaxy, Mary sets the mass of the dark matter to Mdark = 3.8 1041 kg and the simulation to constant radius mode (the radius is automatically set to 7.7 1020 m in this case). She notes the speed of the star in this configuration. Using the value for Mord = 1.8 1041 kg, she calculates the speed of the star if the dark matter were not present assuming that the radius of the orbit were to remain constant. She calculates the speed of the star under these conditions and checks her result by running the simulation with the dark matter mass set to zero