The totality of the heavenly bodies from grand galaxies down to
the sm cosmic dust filling the endless space describes the Universe w dimension
is beyond our comprehension.
There are t divergent theories on the mode of origin of the
universe. These are the B Bang and the Steady State theories. According to 'Big
Bang' theory universe has a finite beginning, through a tremendous explosive
expansion superdense and very condensed matter. The "steady state"
theory states the universe expands through a process of continuous creation of
matt which (the process) remains unchanged through eternity.
Galaxy: A galaxy is a rotating stellar system (star system)
consisting of a swarm of stars which are held together by mutual gravitational
attraction. In other words, a galaxy is an enormously large cluster of stars.
Our galaxy to which the sun along with its planet families belong is an
aggregate of aboii one hundred billion stars. Ours is called "Milky
way" galaxy which looks like a discuss (a disc-shaped metal used in discus
throw) when viewed through powerful optical telescope.
It is a pancake shaped whirling body which is gigantic in size.
A part of the "Milky way" is clearly seen with naked eye as a faint
luminous broad band across the sky during a clear autumn night. It takes about
100,000 years for the light to go from one end of our galaxy to the other end.
There are billions of galaxies in the universe. The nearest galaxy comparable
to our Milky Way in size is the Andromeda galaxy.
Stars and Planets: A star is a self-luminous and incandescent
globe of hot gases. Stars produce their own light and are seen twinkling in the
sky. The stars vary in size, temperature and constituent materials. Many stars
occur in pairs (twins). They may even occur in triplets or quadruplets.
Most of the stars have their own systems of planets and
satellites revolving round them. In such cases, a star constitutes a star
system. Our sun is a star having its own planetary and satellite system called
the "solar system".
Planets and satellites are non-luminous heavenly bodies which
shine only because of reflection of star (sun) light from their surface. The
planets whirl round their stars in definite orbits.
Planets
Satellite