The Milky Way Galaxy

Determining the structure of our galaxy is not an easy task because we are stuck inside the Galaxy and can only look in all different directions. Our situation is like you having to determine the layout of your town from just looking out of your front door (or back door) and not being able to move even across the street. The fact that we see a narrow band of stars tells us that our galaxy is shaped like a thin disk. If we lived in a more spherical galaxy, we would see the stars distributed more uniformly around the sky. There is a hint of a bulge in the direction of the Sagittarius constellation. Careful star counts and determining their distances shows hints of a spiral pattern in the disk. The interstellar dust limits our view a small section of the Galaxy. However, clear evidence of the spiral structure in the disk comes from the 21 cm radiation discussed in the previous chapter. Our galaxy, the Milky Way, is disk-shaped with spiral arms. It has an elliptical bulge in the centre and a spherical halo that is denser closer to the Galaxy centre. It is about 100,000 light years across and our solar system is about two thirds of the way out from the centre.

The image to the right shows the probable structure of the Milky Way viewed from a point beyond our galaxy. Move the mouse pointer over part of the image to reveal the name of that area. The dark unmapped section lies directly opposite from our location - we cannot study this area due to the fact that the galactic centre is in the way.
This diagram is adapted from 'Universe: Origins and Evolution' by T.P. Snow & K.R. Brownsberger