The Milky Way
 

The Greek philosopher Democritus (450-370 BC) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars [19] Aristotle (384-322 BC). however believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions The Neoplatonist philosopher Olympiodorus the Younger (c - 570 AD) was critical of this view, arguing that if the Milky Way is sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax which it does not. In his view, the Milky Way is celestial.

 

The Arabian astronomer Alhazen (965-1037) made the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere The Persian astronomer al-Biruni (973-1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars. The Andalusian astronomer Ibn Bajjah ("Avempace", d. 1138) proposed that the Milky Way is made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near [20] in the 14th century the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars"

^{The shape of the Milky Way as estimated from star counts by William Herschel in 1785, the solar system was assumed to be near the center.}

 Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. In 1750 the English astronomer Thomas Wright, in his An Original Theory or New Hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band in the sky from our perspective inside the disk. In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way

 

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center Using a refined approach, Kapteyn 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center A different method by Harlow Shapley based on the cataloging of globular clusters led to a radically different picture a flat disk with a diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged

 

Distinction from other nebulae

 

A few galaxies outside the Milky Way are visible in the night sky to the unaided eye In the 10th century the Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a small cloud In 964. Al-Sufi identified the Large Magellanic Cloud in his Book of Fixed Stars, it was not seen by Europeans until Magellan's voyage in the 16th century. The Andromeda Galaxy was independently noted by Simon Marius in 1612.

 

In 1750. Thomas Wright speculated (correctly) that the Milky Way is a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways In 1755, Immanuel Kant used the term "island Universe" to describe these distant nebulae.

 

Toward the end of the 18th century. Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.

 

In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition Slipher discovered that spiral nebulae have high red shifts. indicating that they are moving away from the Milky Way at a rate exceeding the Milky Way's escape velocity Thus, they are not gravitationally bound to the Milky Way and are unlikely to be a part of the galaxy

 

In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy. Messier object M31, was then known). Searching the photographic record, he found 11 more novae Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes hypothesis, which holds that spiral nebulae are independent galaxies

 

In 1920 the so-called Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way spiral nebulae, and the dimensions of the Universe To support his claim that the Great Andromeda Nebula is an external galaxy. Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.

 

In 1922. the Estonian astronomer Ernst Opik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object Using the new 100-inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae they were far too distant to be part of the Milky Way In 1936 Hubble produced a classification of galactic morphology that is used to this day.