Galaxies I

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     M31, M32, and M33 are members of the Local Group, an assemblage of more than 54 galaxies in the neighborhood of the Milky Way, the galaxy which contains our solar system. Like the Milky Way, M31 and M33 are spiral galaxies, whereas M32 is a dwarf elliptical galaxy. Of the three, M31, also known as the Andromeda Galaxy, is the largest, with a mass that has been estimated in recent studies to be equal to or greater than that of the Milky Way. Comprised primarily of older faint stars, M32 is a substantially smaller galaxy and a satellite of Andromeda. M33, known as the Triangulum Galaxy, is more distant and less massive than Andromeda and is believed to have collided with that galaxy in the past.
     The attributes of these four galaxies may reflect their past interactions and are likely to shape future encounters. For example, astronomers are not currently sure how M32's compact ellipsoid shape took form, but they suspect that M32 may have had a spiral shape earlier that was transformed by a tidal field from Andromeda into its current elliptical shape. Meanwhile, Triangulum and Andromeda are connected by a stream of hydrogen and stars, which is evidence that the two galaxies have interacted in the past between 2 and 8 billion years ago. Finally, among the trio M31, M33 and the Milky, every pair is potentially on a collision course compelled by gravity. Triangulum might be ripped apart and absorbed by M31, it might collide with the Milky Way before the latter has any violent interaction with Andromeda, or it might participate in the collision between the Milky Way and the Andromeda Galaxy, which is expected to occur in about 4 billion years.

According to the passage, the difference in shape between M32 and the other galaxies discussed can be explained with reference to which of the following factors?