The Physics
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Diameter of the Milky Way

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Coble, Charles R., Elaine G. Murray & Dale R. Rice. Earth Science. New Jersey: Prentice-Hall, 1987: 46. "The Milky Way Galaxy to be about 100,000 light-years in diameter" 100,000 light year
"Milky Way." Microsoft Encarta 96 Encyclopedia CD-Rom. Funk & Wagnalls, 1996. "The diameter of the disk is about 100,000 light-years" 100,000 light year
Goodwin, Simon. Hubble's Universe: A Portrait of Our Cosmos.New York: Penguin, 1996: 9. "Disk-shaped island about 100,000 light years across" 100,000 light year
Arny, Thomas and Nicholas A. Pananides. Introductory Astronomy. 2nd ed. Massachusetts: Addison-Wesley, 1979: 291. "The Milky Way is about 100,000 light years across" 100,000 light year
Cohen, Nathan, and Donald Goldsmith. Misteries of the Milky Way. Chicago: Contemporary Books, 1991: 291. "a hundred thousand light-years from rim to rim" 100,000 light year

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