GALAXIES

The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. However Aristotle, 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 was critical of this view, saying that if the Milky Way is sublunary it should appear different at different times and places on Earth, and that it should have parallax.

Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at measuring and observing the Milky Way's parallax, and he "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-Bīrūnī proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." The Andalusian astronomer Ibn Bâjjah proposed that the Milky Way is made up of many stars that almost touch one another and appear to be a continuous image. 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."

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 big number of faint stars. In 1750 the English astronomer Thomas Wright, 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. 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 in 1920 arrived at the picture of a small 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 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. After Robert Julius Trumpler quantified this effect in 1930 by studying open clusters the Milky Way, emerged.

Galaxies have their own magnetic fields. They are strong enough to be dynamically important: they drive mass inflow into the centers of galaxies, they modify the formation of spiral arms and also they can affect the rotation of gas in the outer regions of galaxies. Magnetic fields provide the transport of angular momentum required for the collapse of gas clouds and that is formation of new stars. The typical average equipartition strength for spiral galaxies is about 10 μG or 1 nT. For comparison, the Earth's magnetic field has an average strength of about 0.3 G. Radio-faint galaxies like M 31 and M 33, our Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found in starburst galaxies. For example in M 82 and the Antennae and in nuclear starburst regions, for example in the centers of NGC 1097 and of other barred galaxies.