Section 4.1.3: What shape is the Milky Way?

At this point, you may be wondering what shape our own Milky Way galaxy is. It’s not obvious at first because we’re inside the Milky Way and can’t see the galaxy as a whole. However, the fact that we see a concentration of stars in a band across the night sky indicates that most of the Milky Way’s stars lie in a flat disk. This has been confirmed by both professional and amateur astronomers, who have pieced together photographs taken of the entire night sky to create an image of the Milky Way, an example of which is shown below:

A image of the Milky Way, made up of multiple shots on large-format film (By Digital Sky LLC [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons)

From the above image you cannot tell if the Milky Way has any spiral arms and if it does, how many there are. To find that out, astronomers have to measure the velocity at which different parts of the galaxy are rotating. These observations have revealed that the Milky Way is a barred spiral galaxy, although astronomers still can’t agree on whether it has two or four arms! An artist’s impression of the Milky Way, based on observations from the Spitzer space telescope, is shown below:

An Artist's impression of the Milky Way based on Spitzer observations. Credit: NASA/JPL-Caltech