We all have a direct experience of earth, which is a planet possessing a satellite -the moon- and evolving with several other planets around a star called the sun. The Ancient Greeks knew this and arrived at this conclusion without sophisticated instruments as a telescope. This concept was forgotten during 1500 years until Galileo, following Copernicus, made this discovery again, because he could observe the firmament with the recently available telescope. He challenged the current paradigm that the earth, the Microcosm, was the center of the Universe and that the Macrocosm, heavens, covered it.
Our sun is a dim common star that belongs to a group called a galaxy. On clear nights, the Milky Way beautifully sweeps across the sky (fig. 1.1).
Figure 1.1. The Milky Way must look very similar to Andromede (Messier31) here represented.
It is a very classical galaxy having the form of a flat ellipsoid, with a diameter of 9 x 1017 kilometers (that is 900,000,000,000,000,000 km). Such a distance requires quite an effort of imagination to visualize. It is composed of about 100 billion stars. A billion is a thousand million, 1,000,000,000 i.e. 109. In 16th century France, Bi-Million meant a million million. The British kept this meaning but the French and Americans changed it for a thousand million. In French, a million million is now a milliard, while milliard is a thousand millions in Great Britain.
Our galaxy has two small neighbors called the clouds of Magellan, and the next more important galaxy is the galaxy of Andromede, which was, until 1923, recognized as being the farthest lying star still discernible with the naked eye. Now it is recognized that it is not a star but a pack of hundred billion stars lying at about 21 x 1018 kilometers away from the Milky Way The few recorded planets are not counted. In 1998, 12 planets have been recorded. In 2000, a total of 30 planets are known to orbit distant suns. In 2003, 101 planets were on the record and more are constantly discovered.
Figure 1.2. The closest galaxies to the milky way, lying within a distance of 21 x 1018 km , are not distributed uniformly in space.
On a small scale, galaxies are not disposed uniformly in space (fig. 1.2). About twenty galaxies form the local group, of which the two most important members are Andromede and the Milky Way. Our closest neighbors are the clouds of Magellan. The evaluation of cosmic distances relies on a common yardstick. This yardstick is the distance at which lie the Clouds of Magellan. A direct technique for determining this distance is the parallax, which measures how much the stars move back and forth in the sky as the Earth orbits the Sun. It is inapplicable to the Clouds of Magellan, whose distance is somewhere between 40 kilo parsecs (130,000 light-years) and 60 kilo parsecs. Hence an uncertainty of 40% for all subsequently calculated cosmic distances.
Our 24 neighbor galaxies are comprised within an area of 3 x 1019 kilometers and form the local group. It is a matter of observation that the galaxies lying outside this local group are also assembled in groups.
If we localize the other groups of galaxies at further distances, we find, with the exception of small areas located north and south that cannot be probed because of the presence of the Milky Way, that there are about fifty groups of galaxies present. Sometimes these groups form themselves into clusters such as the cluster of the Virgin (Virgo). These clusters are apparently themselves assembled into clusters of clusters, and our local group seems to be located at the far-left end of such a Super-Galaxy (Fig.1.3). Down to 3 x 1022 km, there are about 108 galaxies i.e. a hundred million galaxies.
Figure 1.3. A probing down to 4.5 x 1020 kilometers reveals that galaxies are distributed in about 50 groups, here represented by spheres. Our local group is in the center, located at the far-end of a Super-galaxy. Galaxial dust of the Milky Way hinders any probing North or South.