The Milky Way, viewed from the outside, has a disc-shaped structure with a bulging central core and spiral arms. Positioned spherically around the galactic disc are very old "globular clusters", which appear spherical and consist of up to several hundred thousand stars (in the side view they are represented as spheres). The entire galatic system consists of more than 100 billion stars which constitute around 85 to 90 percent of the total volume as well as clouds of interstellar gas mixed with some dust, out of which stars are formed and to which, at the end of their life cycle, the part of their matter that does not become a "stellar remnant" is returned. Because dust obscures the light of the stars behind, the dust component of these clouds can be recognised as dark stripes in the side view of the Milky Way and as dark regions on the star chart. The Milky Way also rotates on its own axis. The Sun is about 26,000 light years away from its centre and one revolution takes approximately 250 million years.
For stars, astronomical units (i.e. average distance between Earth and Sun [about 150 Mio. km]) are too small to measure distances, which is why we introduce Lightyears (Ly). This is the distance the light travels within a year with its vacuum velocity of 300 000 km per second, equivalent to about 10 Trillion km. In order to get a better impression of this number we can express all distances in this unit: travelling from Earth to Moon the light needs little more than a second, towards the Sun (1 AU) about 8 minutes, from Sun to Neptune about 4 hours and to the very next neighbouring star Alpha Centauri 4.3 years (a distance of 4.3 Ly)!
The brightest stars of the night sky have distances of ca. 100 Ly while the ?closeby? star forming region of Orion nebula (which can be observe with the naked eye) is about 500 Ly away. All of these objects must be regarded as our closest comical neighbourhood in our Milky Way, near the spiral arm where the Sun is located. All visible stars belong to the Milky Way, our Galaxy. The centre of the Milky Way is about 26 000 Ly away from the Sun while the entire Milky Way has a diameter of about 100 000 Ly. The Andromeda galaxy (which still can be observed with the naked eye) has a distance of about 2 Mio. Ly. Like the Milky Way it belongs to the ?Local Group? - this is the name of our galaxy cluster.
Most of the galaxies are found in such galaxy clusters, most of which consist of much more members (up to thousands of galaxies) than the Local Group (about 40 members). Galaxy clusters are not yet the largest hierarchical structure in the universe. These are called superclusters. We belong to the Virgo supercluster. Only recently it was discovered that superclusters have a string-like structure.
However, this only counts for the matter in the universe which can be observed because it is radiating in some range of the electromagnetic spectrum, for example in the visible light or in the X ray range. Today we know that this adds up to only 15% of the matter in the universe. The rest is ?Dark Matter? which can be noticed only by its gravitational action..
A. Schulz; Ü: Joanne Moar, M. Dahl