Most of the stars are concentrated in the luminous band of the Milky Way. Sagittarius about 30.000 light years away. We think of Galaxies as collections of stars, but irregular Galaxies and spiral Galaxies like our own contain gas as well. Some of the gas is in the form of Hydrogen left over from the original formation of each Galaxy fresch from the Big-Bang. The Milky Way disk is about 100.000 light years in diameter and 2000 light years by the side .
Globular clusters are well known as the oldest conglomerations of stars in the Milky Way .
The concept of a Univers expanding according to the Einstein general theory of relativity dates from 1920. The Greek Claudius Ptolemy's book known as the Almagest centred a Univers, stationery Earth around which the Moon, Sun, Planets, and stars moved each day on circular paths . The Ancient Greeks measured star brightnesses in magnitudes 2000 years ago and were refined by modern astronomers . Each spiral galaxy rotates round an axis which is perpendicular to the galaxy's disk . The galaxy milky way is a spiral galaxy, our galaxy contains as much mass as a trillion sunsized stars .