Galaxy:
A galaxy is a system of many thousands of million of stars together with interstellar gas and dust.
OR
Galaxy is a fundamental unit of universe.it is composed of hundreds of thousands stars together with intercellular gas & dust.
Origin of Galaxies:
The galaxies resulted from the accumulation of gas on the proto-galaxies that were formed by density fluctuations and gravityinstabilities in the expanding primordial fireball. It is thought that they originally consisted only of those formed in the big bang (mainly hydrogen and Helium).
Number of Galaxies:
The number of galaxies in the universe is estimated to be between 100 billion and a trillion (1011 to 1012). Each galaxy contains about 100 billion stars.
Classification:
The galaxies were classified by Hubble in 1924. According to his thesis, there are three types of galaxies, which are as under.
01)
Elliptical galaxies.
02)
Spiral galaxies.
03)
Irregular galaxies.
Elliptical Galaxies:
Elliptical galaxies (E) appear as luminous elliptical disc with a smooth distribution of light. Their surface brightness decreases out ward from the center. Elliptical galaxies are further divided into EO-7, according their aspect in the sky. For example, EO types are seen face on as circular discs, while E-7 types are seenalmost edge on.
Spiral Galaxies:
Spiral galaxies are either normal(S), in which spiral-shared arms emerge directly form the center. Spiral galaxies are further divided into types a, b and c, depending on the extant of theunwinding of the arms and on the relative clarity of the arms and the nucleus (Centre).
Irregular Galaxies:
Irregular galaxies do not have a definite structure. However, two types are distinguished i.e. Irr I and Irr II.
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