SOLAR SYSTEM

• THE UNIVERSE:

The Branch of physical science, which deals with the nature of the Universe, its overall structure and origin, is called cosmology. Until the 17the century, the Universe was thought to be state, infinite and unchanging: this is called the Steady-State Theory. However, Edwin Hubble (1889-1953), an American astronomer showed in 1920s that the space between the galaxies is increasing and the Universe is therefore expanding like a balloon that is being inflated. The known Universe is finite and its edge lies at a distance of about 1024 km.


• BIG BANG THEORY:

According to the Big Bang theory, the Universe came into existence about 15 to 20 billions years ago when a cataclysmic explosion took place. The Echo of this big bang can still be heard in the form of microwave radio signal from space. This is also called as the 3K microwave background radiation first detected by Arno Penzias and Robert Wilson for which they were awarded the Nobel Prize. Before the birth of our Universe, there was no time, no matter and no space. All the energy and matter of the future cosmos were concentrated in a single point of energy. All the forces were unified in this single point and were in perfect harmony. By someway this harmony was disturbed and this triggered a catastrophic explosion, filling our space with every particle of matter hurled away from every other particle. Thus, this is how our universe exploded into life; and matter, space and time came into being.



• GALAXY:

Introduction:
Galaxy is a fundamental unit of the Universe. It consist of several hundred thousands of stars together which interstellar gas and dust.

Origin of Galaxies:
The galaxies resulted from the accumulation of gas on the proto-galaxies that were formed by density fluctuations and gravity instabilities in the expanding primordial fireball. It is thought that they originally consisted only of those elements 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:
Hubble classified the galaxies in 1924. According to his thesis, there are three types of galaxies, which are as under:

1-Elliptical galaxies: -
Elliptical galaxy (E) appear as luminous elliptical discs with a smooth distribution of light. Their surface brightness decreases outward from the center. Elliptical galaxies are further divided into EO-7, according to their aspect in the sky. For instance, EO types are seen faces on as circular discs, which E-7 types, are seen almost edge on.


2-Spiral Galaxies: -
Spiral Galaxies (S), in which spiral-shaped arms emerge directly from the center. Spiral galaxies are further divided into types a, a and c, depending on the extent of the ‘unwinding’ of the arms and on the relative clarity of the arms and the nucleus (center).


3-Irregular Galaxies: –
Irregular galaxies do not have a definite structure. However, two types are distinguished i.e. IrrI & IrrII.