Competitive Exams: Astronomy and Space Science


Giant clusters of stars; our solar-system is part of galaxy called ‘Milky-Way.’ Evolution of Space:

Geocentric Theory: It was founded by Ptolemy in 140 A. D. It states that earth is at the centre of the universe.

Helio-centric Theory: It was propounded by Copernicus in 1453 A. D. It states that all celestial body revolves around the sun.

William Herschel: It showed that the sun is a member of galaxy, the Milky Way and placed at one corner of Milky Way.

Edwin Hubble (1925): He proved presence of other galaxies. He also proved that galaxies are receding constantly from each other. With the help of analyzing red shifty Doppler Effect, he also proved the rate at which galaxies are receding.

Big Bang Theory

It was put forth by Abbe George Lemaitre. It states that the receding movement of galaxies is the result of implosion. Big bang occurred at least 12 billion years ago. Evidences: Discovery of back-ground radiation proved by COBE, cosmic background experiment, discovering uneven ripple of microwave radiation, which suggests sudden explosion.


It is extremely concentrated matter. The pull of gravity is so powerful that nothing, not even light, can emerge from it. So, black-hole can't be seen. Black-hole formation indicates ultimate death of a star. The collapsing star core (due to finishing up of fuel hydrogen), imparts much energy to star exterior and explosion with very high luminosity takes place. After the explosion the highly dense residue of a comparatively smaller star. Mass less than equal to 1.4 of solar mass is called as Chandrashekhar.

Pulsating Theory

It goes beyond Big-Bang theory. It says that receding nature of galaxy will not continue for ever, and gravity would ultimately stop expansion and thereafter galaxy will collapse inward and ultimately forming pre-mordial substance once again.

Event Horizon: Rim of the black-hole from which nothing could escape. Across event-horizon matter and energy pass in only one direction. Event-horizon indicates beginning of black-holes.