Theories of the evolution of our galaxy - and any other  - is under continual refinement. Much of what we understand about galactic formation is based on what we know about Dark Matter.

In a way, galactic formation is similar to stellar formation, only on a much larger scale. With the early Universe mixed with Dark Matter and gases containing molecular hydrogen, collisions eventually occurred. Collisions led to conservation of energy eventually resulting in collapse.

There are two methods of collapse:

• Stars form early out of fragmentation, resulting in shrinking and forming of elliptical galaxies and globular clusters
• Cloud shrinks faster resulting in rotation of the cloud, then star formation occurs

So for elliptical galaxies, stars formed first. Spiral galaxies like our Milky Way formed their stars later - after the cloud began rotation.

But what about the spiral structure?

The leading theory is that the spiral structure of our galaxy is a consequence of the distribution of OB Associations and hot stars. In addition, Density Waves are believed to travel through the disk stimulating star birth in their wake.

Standard rotation does not explain the spiral structure since the end result is what is called the "winding problem."

(Image credit: Brooks/Cole Thomson Learning)

The spiral arms will overtake each other. Here is an image that gives an example of the density wave-OB Association (and hot star) theory of spiral structure.

This is a composite ultraviolet and infrared image of M81. The infrared (shown in red) shows the distribution of "normal" stars. The blue indicates the hot OB type stars.

Compare the above image with the true color image below:

It is clear that the hot OB type stars are tracing the spiral structure of the galaxy.

If we could have a top-down view of our own galaxy, there is no doubt the view would be the same.

Much of what we learn about galactic formation and evolution come from computer simulations. The study of Dark Matter and galactic formation as well as the evolution of our Universe all go hand-in-hand - so stay tuned!

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