Milky Way Cosmology

Herschel counted stars and produced the map of the Milky Way shown below. The Sun is approximately in the center (slightly to the left) and the dark lanes through Sagittarius give the "jaw" on the right.

During the 18th and 19th centuries the Milky Way was thought to be the entire Universe, so Herschel's map put the Solar System close to the center of the Universe.

But observations with radio and infrared radiation that could pass through the obscuring dust clouds in Sagittarius and elsewhere show that the Sun is quite far out in the Milky Way. The picture above is an infrared image taken by the DIRBE instrument on the COBE satellite. The picture shows half the sky, so stars which are as far out as the Sun appear at the left or right edge of the picture. (The Sun itself is of course "behind the camera" and not in the picture.) So we now know that the Milky Way is many times larger than Herschel thought it was, and that the Sun is quite far from the center.

Furthermore, many extragalactic nebulae such as NGC 891 look just like the DIRBE picture of the Milky Way. Thus the Solar System is well out from the center of a typical spiral galaxy that is like many other galaxies.

For further reading, see "The Alchemy of the Heavens: Searching for Meaning in the Milky Way", by Ken Croswell.