► The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies and the contents of intergalactic space.
► The study of universe is known as Cosmology.
Cosmology = cosmos (universe) + logos (science)
► The universe has no limit.
► A galaxy is a vast system of billions of stars, which also contains a large number of gas clouds (mainly of hydrogen gas) and dust, isolated in space from simiiar systems.
► There are about 100 billion galaxies (1011 galaxies) in the universe, and each galaxy has, on an average, 100 billion stars (1011 stars). So, the total number of
stars in the universe is 1022 stars.
► The Milky Way Galaxy is the home of the Earth and our Solar System. It is spiral in shape.
► Milky Way Galaxy was formed 5 billion years after the Big Bang.
► Latest known galaxy is the Dwarf Galaxy.
► According to the modem thought, universe can be classified into two parts namely-(a) Atmosphere and (b) Space.
► Origin of the universe is explained by the Big Bang Theory, formulated and proposed by the Belgian astronomer and cosmologist Georges Lemaitre.
► Andromeda is our nearest galaxy.
► Clumps of dust and gas in a nebula come together due to gravity and form stars.
► Stars are made of hot burning gases.
► They emit light of their own and are very large and very hot.
► Light takes about 4.3 years to reach us from the next nearest star proxima centauri.
The Big Bang Theory
► All the matter in the universe was originally a concentrated lump called primeral atom.
► Big Bang was an explosion that occurred 15 billion years ago, leading to the formation of galaxies of stars and other heavenly bodies.
► Since then, all the galaxies have been flying away from one another causing expansion of the universe.