Stars are dense ball of plasma which are held together by its own gravity.The sun is the nearest star which is the source of energy for us.They shine due to nuclear fusion going inside core and radiate it to the universe.The numerous number are visible in clear night sky.There are stars which are faint brighter cooler ,but sun is a hot star.

Classification of stars

The classification of stars is based on its spectrial characterstics like ionisation energy and its temperature.We classify them using letters of English alphabets O ,B ,A,F,G,K,M. These are placed in decresing order of their temperature.Hence O is the hottest and M is the coolest one.The clours also vary as per the temperature O are blue,B are white,G are Yellow and M are red in colour.The simple aid to memory is Oh Be A Fine Girl,Kiss Me.

Life cycle of a Star

The huge luminous globe of dust and gas collapse under its own gravity to from a protostar.They from the main sequence os a stars.When the stars expands they grow ans as a result Hydrogen and Helium runs out of the core of the star.Hence it explodes and collapse.and depending upon the original mass of the stars they becomes dwarf ,neutron star or a black hole.

Stages in life cycle are-

Nebula-They are the birth places of a stars and are very bright because of the gas which provides energy.Towards the end of the stage the star like sun converts to red giant.

Red Giant-This a bright cool star as Hydrogen is out of stock.They very large red giants are called Super Giants.

Red Dwarf-They are cool and very small faint stars.Proxima Centauri is one of the common example.

White Dwarf-It is the last stage in lifecycle of a star and is very small in size and hot.

Supernova-This is the death of a star.They are the main source of Helium.

Neutron Stars-They are produced when supernova exlplodes.Pulsars are common example.

Black Holes-The strong graviational pull ingulf the surrounding matter .It forms a massive star at the end

The largest group of stars are called Clusters ,the organised group of stars are called Galaxies and Spherical group of million of stars are called Globular Cluster.

Citations--

http://en.wikipedia.org/wiki/Star_(classification)

Britinica Encylopedia

Our Universe