Galaxies
Origin
Galaxies are huge congregations of stars that hold together by force of gravity. Studies of distant spaces with optical and radio telescopes indicate that there may be about 100 billion galaxies in the visible universe.
Galaxies seem to be scattered in space. Galaxies tend to be grouped together into Clusters, and some Clusters appear to be grouped into Super Clusters.
When the expanding material of the universe broke up in the first instance, billions of islands of gaseous matter were formed in space. These gaseous islands or Proto-Galaxies rotated, each with its own speed of rotation. Those with very low rotational speed assumed nearly spherical shapes. Others assumed elliptical forms, with varying degrees of elongation, depending on their rotational speed.
Most of these gaseous islands had such high rotational speed that their bodies were flattened out into the shape of discs, from whose edges spiral arms streamed.
As the gaseous islands were settling down, local condensations, Protostars developed at many points within the galaxy. These condensations began to contract under their own weight into dense gas spheres. As a result of this contraction, the temperature of the gas spheres rose steadily and their heated surfaces began to emit heat waves and then the shorter wavelengths of visible light.
As the central atmosphere of these contracting proto-stars reached the ignition point (about 10 million degrees centigrade), contraction stopped, thermonuclear reactions began and millions of bright burning globules of gas emerged - the stars.
When the stars appeared, the originally cool and dark proto-galaxies were transformed into the bright stellar galaxies.
Structure & Composition of Galaxies
A structural analysis of the known galaxies brings out three major forms:
- (i) Spiral
- (ii) Elliptical
- (iii) Irregular
Spiral galaxies have a central nucleus with great spiral arms trailing round it, The Milky Way and the Andromeda Galaxy belong to this group. The spirals comprise around eighty percent of the galaxies so far known.
Elliptical galaxies show purely elliptical shapes without any spiral arms. Irregular galaxies are youthful galaxies, while spiral galaxies represent middle age and elliptical galaxies are of old age.
Cluster of Galaxies
Most of the observable galaxies seem to be scattered in space more or less at random but there are numerous cases of galaxies clustering into groups, which may contain as many as several hundred individual galaxies.
Our own galaxy, the "Milky Way", belongs to a cluster of some 24 galaxies called the Local Group. This group covers an area of about 3 million light years in diameter.
The two largest galaxies in the group are the Milky Way and the Andromeda Galaxy, both of them are spiral.
Local Group
The "Local Group" is a term applied to indicate our galaxy and its nearby galaxies. The Group now numbers around two dozen.
The latest known member of the group is a "Dwarf Galaxy" discovered by the Siding Spring Observatory in Australia. It is in Carina and consists of a loose swarm of very faint stars.