Observational evidence for the Milky Way being made up of distant stars first came when [[Galileo Galilei]] pointed his [[telescope]] towards the Milky Way, observing a large amount of faint stars.<ref>http://www-gap.dcs.st-and.ac.uk/~history/Biographies/Galileo.html</ref> [[William Herschel]] in 1785 was the first to attempt to map out the shape of the Milky Way. He created a diagram based on counting the stars in the different regions of the sky, giving a rough shape of the galaxy with the Solar System near the center. It wasn't until [[Harlow Shapley]] in the end of the 19th century was the Milky Way was discovered to be a flat disk with the Solar System far from the center.<ref>http://www.scientificamerican.com/article.cfm?id=how-did-scientists-determ</ref>
Although the [[Andromeda galaxy]] was recorded as long ago as the 10th century, and both it and the [[Magellanic Clouds]] were known perhaps since ancient times, historically they were not seen as other galaxies, but as nebulae. In 1917, Heber Curtis observed a [[nova (star)|nova]] in what was then called the "''Great Andromeda Nebula''", that along with previous novae found based on subsequent photographic evidence, discovered they were ten magnitudes fainter then novae events in our own galaxy. based on this new evidence, he became a proponent of the "'''island universe hypothesis'''" first proposed by [[Immanuel Kant]] in 1755, that the spiral nebulae were actually other galaxies.<ref>http://adsabs.harvard.edu/abs/1988PASP..100....6C</ref> This was later proven by [[Edward Hubble]] in the 1920's1920s, when he was able to determine the distance to the other galaxies though using [[Cepheid variables]] located within them as standard candles to determine their distance, showing the "nebulae" were too distant to be part of the Milky Way and hence other galaxies.<ref>http://adsabs.harvard.edu/cgi-bin/bib_query?1929ApJ....69..103H</ref>
==Environment of the Milky Way==
The disk of the galaxy is the location of the spiral arms and holds various [[metallicity|Population I stars]], including our own sun, ranging from intermediate age stars to very young, hot stars, as well as open clusters, diffuse nebulae, and the interstellar dust and gas responsible for future star formation. Virtually all of the largest and brightest stars (class B and O stars) are found here.
The discovery and mapping of the spiral arms began in the 1950's 1950s using radio based observations of the abundant hydrogen gas in the galaxy. This lead led to the theory that there are four distinct spiral arms known as '''Norma''', '''Scutum-Centaurus''', '''Sagittarius''', and '''Perseus'''. Our own solar system located in a partial arm known as the '''Orion Spur''' (sometimes called the Orion Arm) between the arms of Sagittarius, and Perseus.<ref>http://www.space.com/scienceastronomy/080603-aas-spiral-arms.html</ref>
However starting the 1990s when it was discovered using infrared observations that the Milky Way was a '''barred spiral''', the original four armed design was challenged. Later in 2005, the bar in the center of the galaxy was found to extend farther out then previously believed, further bringing into question the prevailing four spiral arm theory. In 2008, new infrared imagery from the [[Spitzer Space Telescope]] made the case that the Milky Way has just two major spiral arms, which is known to be common for barred spiral galaxies.<ref>http://adsabs.harvard.edu/abs/2008ASPC..387..375B</ref> These two arms are the '''Scutum-Centaurus''', and '''Perseus''' arms, which hold the greatest density of stars and join up with the ends of the central bar of the galaxy. The Sagittarius and Norma arms are now seen as minor arms of the galaxy, filled with gas and groups of young stars.<ref name="david"/>