What is a Galaxy

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"What is a Galaxy"
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A star filled sky on clear nights has provided mankind with a fascinating spectacle since time immemorial. For our ancestors the night sky was a realm of myths, legends and the supernatural; for us, however space probes and giant telescopes have made breathtaking views of the universe and scientific discussions on its secrets possible. In spite of all that we know, we are still awed by the wonders of the heavens. The planets Earth represent a mere speck of dust in the immense expanse of space.

The origins of the Universe

According to a generally accepted theory, our universe originated about 15 billions years ago. A mighty explosion or "big bang" signaled the moment the universe came into being. Space, time and matter originated from literally out of nowhere. In the 1920s, the American astronomer E.P. Hubble found the explanation for the continually expanding universe. He proved that our galaxy, the Milky Way has been drifting through the universe for billions of years, moving further and further away from its neighboring galaxies. Billions of cubic meters of new space come between the galaxies and push them further apart each day. The farther apart they are from each other, the quicker they move away from each other and the quicker the distance grows between them. But the reverse is also true; that the further back in time one goes, the closer they must have been to each other. Ultimately this means that in the beginning the universe must have been incredibly dense. All matter must have been unified in one place from which the universe eventually exploded.

Today the universe has taken on vast dimensions. The most distant recognizable objects at the moment are quasars which are about 12 billion light years away. A light year refers to the distance a ray of light can travel in one year. The speed of light is c.300, 000 km/sec. Cosmic background radiation (called 3-K-radiation) is considered to be further evidence for the Big Bang theory. 3-K-radiation is electromagnetic radiation left over from the beginning of the universe. Einstein in 1916 used his Theory of Relativity to prove that large masses such as the Sun through their gravitation can cause the curvature of light thereby creating the belief that the universe is the form of a bent shape that cannot be represented pictorially. There are two possibilities for the future of the universe according to the Theory of Relativity: the "Big Crunch" (massive collapse) after reaching its maximum extent, or the currently favored possibility that its further expansion will continue with diminishing velocity.


Galaxies are believed to has completed their formation process an estimated 5 billions years after the Big Bang. While the universe was still young and had not expanded so far, the galaxies must have been far closer together than they are today. Indeed, the farther back one looks into the past, the closer they were. Collisions and the melting down of galaxies were the result. This could happen to our galaxies, the Milky Way, which is moving towards Andromeda, our neighboring galaxy within the "Local Group" that is 1.7 million light-years away. Our Milky Way system belongs to the "Local Group" which is a cluster of galaxies with around 30 known members: 3 large spiral systems, 10 elliptical and 4 irregular systems. The nearest cluster of galaxies is the Virgo Cluster with around 3000 members. The unimaginably large number of galaxies, cluster of galaxies and vast clusters in the sponge-like structure of the universe not known. However, the variety of known forms that galaxies can take extends from spiral clouds and barred spirals to entirely irregular shapes. The shape also reflects the speed which the process of formation required. With galaxies (the Milky Way system) it result in the formation of sun and planets.


Stars in contrast to planets, are self-illuminating heavenly bodies. From any one observation point on the Earth only about 2,500 can be seen with the naked eye, as it is impossible to see the whole world at one time. Were it possible to see all the stars with a so-to-speak globally naked eye, there would be about 6,000. With a powerful telescope there are up to 10`` to be seen. Stars are distributed in different ways; they form open or galactic stellar clusters and ball-like stellar clusters. Due to their being a great distance away, stars appear only as specks of light. Yet they are created in different ways and have corresponding characteristic qualities regarding luminosity, radius, temperature, mass, spectral class, density, energy production, speed of rotation, magnetic field and chemical composition. Until the introduction of astrophysics, they were categorized by their stellar position and brightness. Then they were categorized by their spectrums. Fundamental information on the state and development of stars is given by the "Herzsprung-Russell-Diagram," named after its developers. It is graph on which the surface temperatures (spectral class) of stars are plotted against their luminosities. The "main line" stars, including the Sun are differentiated from the other stars, which have greater luminosity by the relationship between surface temperature and spectral class. When a star grows old, its move from the main sequence into the area of the giant and supergiant. At the end of its life, it shrinks to become a "white dwarf".


The confusing multiplicity of stars in the night sky has always stimulated man to look for orientation in the stars. Thus stars in the sky were arranged into figures which became to constellations that were used for astrology and religious purposes, as well as for navigation and to compile calendars. The appearance of the star Sirius at down warned the Egyptians that the flooding of the Nile was drawing closer. The constellations that we use today date back not only to the Greeks, but also the Egyptians and Babylonian cultures of Antiquity and Pre-Antiquity. Today there are 88 internationally recognized historical constellations, 31 in the northern, 45 in the southern celestial sphere and 12 on both sides of the Celestial Equator.

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