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The Milky Way
Galactic Center of Milky
Way and a meteorThe Greek philosopher Democritus (450–370 B.C.) proposed that the bright band on the night sky known
as the Milky Way might consist of distant stars. Aristotle (384–322 B.C.), however, believed the Milky Way to be caused
by "the ignition of the fiery exhalation of some stars which 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 which is continuous with the heavenly
motions." The Neoplatonist philosopher Olympiodorus the Younger (c. 495-570 A.D.) criticized this view, arguing that
if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should
have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic
world.
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 was very remote from the Earth and did not belong to the atmosphere."
The Persian astronomer Abu Rayhan 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 was 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. The Syrian-born Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way
galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars".
Actual proof of the Milky Way consisting of many stars came in 1610 when 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 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 on 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 shape
of the Milky Way as deduced from star counts by William Herschel in 1785; the solar system was assumed to be near the center.The
first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel
in 1785 by carefully 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 in 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 cataloguing of globular clusters led to a radically different picture: a flat disk with 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 galaxy, the Milky Way, emerged.
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