Diameter of the Milky Way

The Milky Way Galaxy, a spiral galaxy, homes our solar system together with lest 200 billion other stars. The Milky Way is also referred to the portion of the Milky Way Galaxy that can be seen by the naked eye from the earth. It appears to look like a milk band of light stretching across the sky.
The structure if the galaxy is like a disk with a bulge in the center with clouds of dust and gases which surrounds it and prevents us from seeing into the center. The diameter has been measured to be about 100,000 light-years. One light year is the distance at which light travels in a year (9.46 trillion kilometers).
The diameter of the galaxy was determined by the use of variable stars (also known as Cepheid variables), or stars whose luminosity changes with time. Harlow Shapley was the astronomer that discovered the relationship between its brightness and distance. It was found that the change in its brightness was predictable because its change from maximum brightness to dimness occurred in a cycle. The advantage to this finding allowed Shapley to find its absolute brightness by measuring the period. Shapley thus obtained the distance when he compared the brightness to a Cepheid variable distance scale because the distance to other stars were known. This pave the way for other astronomers to determine not only the distance of our galaxy but others too.
Karian Fung -- 1998