Question

In My Science class we are talking about the Milky Way, If we can't go outside the galaxy how do we know its a spiral galaxy?

It is indeed hard for astronomers to determine the detailed structure of our own galaxy. The main problem is the opaque dust clouds that block our view. About 50 years ago, however, astronomers began to study the distribution, distance, and rotational speed of clouds of hydrogen gas, using radio at a wavelength of 21 cm. The dust is transparent to these radio waves. The clouds of hydrogen form a spiral structure. Since then there have been many other studies, for example of the dense clouds where stars are forming; these also define the spiral structure. There remain many gaps in our knowledge, but the fact that the Milky Way is a spiral galaxy is well established. Very recently, radio astronomers have measured star-forming regions on nearly the opposite side of our galaxy and revealed that it is larger than previously thought, about the same size as the Andromeda spiral galaxy. See the article “Milky Way” in Wikipedia for more information.

David Morrison
NAI Senior Scientist
February 17, 2009