· 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.
GALAXY
· 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 similar systems.
· There are about 10 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 modern 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.