Milky Way
The Milky Way is by far the largest object in the night sky, which it occupies an important fraction. It is observable preferably through a night without moon and especially without light pollution. The Milky Way appears as a long white trail that runs through the night sky 360 degrees. This is due to the very large number of stars that make up the Milky Way, their lights superimposed to give this whitish appearance. It is by observing with a telescope, Galileo was able to resolve individual stars. All the stars we can see with the naked eye are part of the Milky Way.
The solar system is part of the Milky Way. Our Galaxy is dominated by his record, a huge "plate". Being inside the disk we see it from the side, like a long trail of stars, the Milky Way. It contains from 150 to 250 billion stars, gas in different phases (hot and molecular) and dust that absorbs visible light, which explains the dark streaks in the most central parts of the Galaxy.
The structure of our Galaxy
Our Galaxy is a type of galaxy "spiral". It is dominated by a disc with the stars revolve around the center, which consists of a bulb and a bar. Between 70% to 80% of stars are in the disc which has a diameter of 90,000 light years to a thickness of just 2,500 light years. The disc consists of several spiral arms in which the majority of stars concentrated. It is believed that the arms are formed naturally by gravitational instabilities induced during rotation of the disk. The two spiral arms dominate the galaxy are those of Scutum-Centaurus and Perseus, the Sun lying in a secondary arm called the Orion Arm.
Our position in the Galaxy
The Sun is one of the many stars of the galaxy disk. It is almost exactly in the plane of the disc, with the Galactic Centre distance of 28,000 light years. The Sun orbits the Galactic center with a speed of 230 km / s.
To know exactly our position in the Galaxy, it is necessary to determine the distances of stars. The distance to nearby stars is determined by the effect called parallax. They use their changes in position on the sky when viewed from Earth at two different positions around the sun. The GAIA satellite to be launched by ESA in 2013 can observe the position of almost a billion stars! The distance to the most distant stars is determined by the specific properties of certain variable stars, Cepheids, whose variability period depends on their intrinsic luminosities.
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