The Orion Nebula is a star birth region of the Milky Way galaxy. From our perspective, this star birth region lies on the next further out spiral arm of our galaxy. The nebula consists mostly of hydrogen gas that is slowly collapsing under the influence of gravity. The collapsing balls of gas eventually become massive enough that their own crushing density becomes sufficiently great to cause the hydrogen to fuse into helium in a massive nuclear fusion reaction that can run for billions of years. In this way, a star is born.