Why do stars in the disk of a spiral galaxy orbit in roughly the same direction, in the same plane in space?
A) It is pure coincidence; the stars just happen to be going in the same direction.
B) The gas that formed the stars was shaped as a giant, rotating disk.
C) Collisions between the stars caused their motions to organize into a general, spinning disk direction.
D) After the stars formed, gravity pulled the stars down into a plane in space, all orbiting a supermassive black hole.

Question

Quizplus · Accepted Answer

The gas that formed the stars was shaped as a giant, rotating disk. This is due to conservation of angular momentum, where the gas cloud initially had some net rotation, and as it collapsed under its own gravity, it became flattened into a disk shape. The stars that later formed out of this gas cloud inherited this net rotation and orbital direction, resulting in most stars orbiting in the same direction in a common, thin disk.

Why Do Stars in the Disk of a Spiral Galaxy