Galaxy is a word that derives from the Greek term kyklos galaxies which means "milky circle". A galaxy is a gigantic system formed by millions, billions or trillions of stars and other celestial bodies, which remain linked together due to gravitational interactions.
The galaxies are represented in space in an isolated way and it is possible to contemplate with the naked eye, from Earth, only three of them: Andromeda, Large Magellanic Cloud and Small Magellanic Cloud.
The galaxy where we are located is called the Milky Way and is composed of numerous stars, one of them the Sun, the center of our Solar System.
It is estimated that in the Universe there are billions of galaxies that can be morphologically characterized by irregular, elliptical, spiral or bar spirals. The Milky Way is an example of a spiral galaxy because it has a core and large spiral arms made up of stars and clouds of cosmic dust.
In order to achieve a better characterization of the Milky Way, in 2013 the European Space Agency will start the Gaia Mission, a ambitious project lasting 5 years, which aims to carry out the three-dimensional mapping of millions of stars in the Milky Way and galaxies neighbors.
