|The first model of our galaxy made by William Herschel in the 18th century looked like the one shown here (the Sun's position is denoted by the yellow star). Herschel did not know about interstellar dust.|
About the same time, American Astronomer Shapley, used distance estimates
to Globular Clusters to get a different scale and structure to our Galaxy,
(shown right). He argued that these massive clusters must be orbiting the center of our
Galaxy, which was NOT at the position of the Sun (which was correct!).
In the 1930s, dust was discovered to exist through out the galaxy. This dims stars which earlier astronomers mistakeningly took as a greater distance. Now, correcting the dimming from dust, astronomers began to put together a fairly accurate scale and structure to our galaxy.
The Scale and Structure of the Milky Way
We know the scale and structure of our galaxy much better now.
The Sun is WAY out away from the center.
The main components are:
Disk: the flattened region which contains stars of all ages and dust & gas. The spiral arms lie within the disk.
(Nuclear) Bulge:a flattened sphere of stars, centered on the center of the galaxy. It contains mostly old stars.
Halo: The spheriodal region emcompassing the entire galaxy. It contains only very very old stars and star clusters, no gas or dust.
(Galactic) Nucleus: at the very center of the Galaxy. The location of a million solar mass black hole.