Structure of The Milky Way Galaxy

This is an artist's rendition of our Milky Way Galaxy, our galactic home.

Image Credit: https://www.nasa.gov/mission_pages/GLAST/science/milky_way_galaxy.html

Beautiful, isn't it?

Let's take a look at the structure of our glorious galaxy.

The Galaxy From the Inside Out

Much like how the Earth is not the center of our solar system, our solar system is NOT the center of our galaxy. So what is at the center of our galaxy?
Spinning like a glorious ballerina of darkness at the center of the galaxy is a supermassive black hole. While we cannot observe our black hole directly, we have observed a compact radio wave source that most likely is powered by the accretion disk of materials encircling our black hole. This, and the black hole are known as Sagittarius A* (pronounced Sagittarius A Star). Our black hole has a mass about 4.5 billion times larger than the sun. The x-ray closeup in the image below, taken at the Chandra Observatory, shows the accretion disk of our galaxy's black hole.

Image Credit: https://www.nasa.gov/mission_pages/chandra/multimedia/black-hole-SagittariusA.html

Here's a link to an interesting article about a particularly large flare up in Sagittarius A*'s activity last year: http://www.space.com/28193-monster-black-hole-largest-flare-ever.html

The black hole is also surrounded by a bar in the center of our galactic neighborhood. This bar is full of stars that orbit quickly about the center of the galaxy. For a visual interpretation, take a look at the first picture in this post. The galactic bar is the section seen in the center with a more yellowish hue.
This bar is contained within the bulge at the center of our galaxy, visible as the yellowish blob in the center of the picture below.

Image Credit: http://www.space.com/18199-milky-way-core-star-photos.html

As we move further out, we can see the spiral arms of our galaxy. The spiral arms are comprised of stars and the interstellar medium: clouds of Hydrogen, other gasses, and dust that allow for star formation to occur. The arms are density waves, meaning that they are longstanding structures that orbit as a structure about the center of the galaxy, much like the way that a record rotates as it is played. This means that the farthest reaches of the arms are moving at faster speeds than the center of the arms. Stars, however, orbit at similar rates of speed throughout all sections of the spiral arms. This means that stars that are formed in the arms flow out towards the ends of the arms and may wind up between arms or in a new arm entirely before their stellar deaths.
Our solar system is contained within a spur between two arms called the Orion Spur and our sun orbits the center of the galaxy at a distance of approximately 26,000 light years. The entirety of the disk of our galaxy is approximately 100,000 light years in diameter.

Image Credit: https://www.nasa.gov/jpl/charting-the-milky-way-from-the-inside-out

Given our position within the galaxy, observations from Earth show the Milky Way as a thick band of stars and dust that forms a circle about the sky. This is because we view the galaxy edge-on, looking towards the spiral arms that are around us. 

Image Credit: https://www.nasa.gov/topics/earth/features/2012-alignment.html

Due to this positioning, we can't actually take an image of our entire galaxy, although the work of talented artists gives us a good idea of what it most likely looks like.

Beyond the spiral arms of the galaxy exists a less dense, spherical cloud of stars and other materials we also consider as part of the galaxy. We call this the galactic halo. The halo extends to a radius of approximately 300,000 lightyears from the center of the galaxy. The prevalence of globular clusters in the halo, along with the absence of new star formation leads many astronomers to believe that the halo is an older structure than the disk of our galaxy.

Image Credit: http://www.cefns.nau.edu/geology/naml/Meteorite/Book-GlossaryG.html

 Much like how the study of our solar system allowed for the creation of the solar nebula theory which explains both our sun's formation and the formation of other stars, this study of our galaxy provides insights to the formation of our galaxy as well as other spiral galaxies like our own.