- Study of universe is called cosmology.
- Galaxy = stars + planets + gases + dust.
- Our nearest galaxy is Andromeda.
- Milky Way contains 300 billion stars.
- In AD 140, Ptolemy gave theory that earth is at the centre of the universe and sun and the other heavenly bodies revolve around.
- In 1543, Copernicus stated that sun is at the centre of the universe and not earth.
- Kepler supported Copernicus but said that sun is centre of the solar system not universe.
- In 1924, Edwine Hubble first demonstrated existence of galaxies beyond Milky Way.
- Structurally galaxies are found in three forms.
- Spiral: – Have a central nucleus with great spiral arm. E.g. Milky Way and Andromeda.
- Irregular: – With no shape.
- Elliptical: – Without spiral arm.
- Evolution of universe-
- Big bang theory:- By Georges Le Maitre.( explosion occurred 13.8 billion years ago)
- Steady state theory: – By Bondi, Gold and Fred Hoyle. ( it has no beginning and no end)
- The pulsating theory: – At present universe is expanding.
- Components of solar system-
- Sun, eight planets and their respective satellites.
- Interstellar debris such as asteroids, meteroids, comets.
- Electrically charged gases called plasma.
- Inter planetary dust particles.
- And small solar system bodies (SSSB).( other than planets, dwarf planets and satellites)
- Various theories were given to explain origin of solar system-
| HYPOTHESIS | PROPOUNDER |
| Gaseous | Kant |
| Nebular | Laplace |
| Planetesimal | Chamberline and Moulton |
| Tidal | Sir James jeans and Harold jeffreys |
| Binary star | H n Russell |
| Supernova | F hoyte |
| Inter stellar dust | Otto Schmidt |
| Electromagnetic | H Alfven |
| Protoplanet | G kuiper |
| Nebular cloud | Dr Von weizsacker |