The Universe

Existing matter and energy are together known as universe. The study of universe is known as Cosmology. Universe includes all the universal bodies and the complete solar family which comprises the sun, moon, planets, comets, planetoids etc. 

Origin of the universe is explained by the Big Bang Theory, which was formulated and proposed by the Belgian astronomer and cosmologist Georges Lemaitre, after his becoming acquainted with the findings of the American astronomers Edwin P. Hubble and Harlow Shapley on the expanding universe.

Main points of the Big Bang Theory:

·         All the matter in the universe was originally one concentrated lump called primeval 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.

Galaxy

A large group of stars, dust and light gases, bound together by their own gravity is called a galaxy. It contain  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 100 billion galaxies in the universe, and each galaxy has, on an average, 100 billion stars.

·         The Milky Way Galaxy is the home of the Earth and our Solar System. It is spiral in shape.

·         Latest known galaxy is the Dwarf Galaxy.

·         We live on the outer edge of a spiral type of galaxy called the Milky Way, which is about 100,000 light years in diameter and is rotating slowly.

Main points of the Earth’s Galaxy: The Milky Way

·         The Milky Way is a large spiral-shaped galaxy.

·         It is called the Milky Way because it appears as a soft glowing light of billions of stars. These stars are so far that they can be seen only in constellation; not separately.

·         Galileo discovered that this band of light was produced by countless individual stars which a naked eye cannot see.

·         It takes about 250 million years to complete one revolution.

Main points of the Andromeda: Earth’s closest Galactic neighbour

·         Andromeda is a spiral galaxy and also our closest neighbour.

·         It is the farthest object that can be seen with the naked eye.

·         Like stars, galaxies are grouped into clusters. Some clusters contain thousands of galaxies.

·         About 30 galaxies, along with the Milky Way and the Andromeda are grouped together in one cluster called the Local Group.

Nebulae

·         Nebulae are huge interstellar clouds of gas and dust that appear as faint, misty patches of light scattered all over the sky.

·         They appear either as bright luminous clouds or as dark patches against a brighter background.

·         A nebula depends for its luminosity upon the presence of stars that have either arisen from it or are contained in it.

STARS

·         Clumps of dust and gas in a nebula come together due to their own gravity and form stars.Stars are made of hot burning gases. They emit light of their own. They are very large and very hot.

·         Stars seem to be fixed with respect to each other. In fact they are in rapid motion but they are at such great distance that relative changes in position become noticeable only over the centuries.

The star Alpha Centaury (also called Proxima Centauri) is the closest star to the Earth after the Sun. It is about 4.3 light years away