Like people, stars are born. They grow and eventually die. Huge and cold clouds of gas and dust known as “nebulae” start to shrink under their own gravity to mark the formation of stars. A star is formed when the cloud breaks into clumps and each gets so hot and dense so that a nuclear reaction can take place.
After their birth, stars are at the center of a flat disc of gas and dust. These materials are then blown away by star’s radiation.
All stars belong to galaxies. Galaxies were born few hundred million years after the universe was created. 13 billion years ago, galaxies were small and closer together but as they smashed into each other they grew in size and changed shape. Collisions were common.
Most stars take million years to die. Stars end their life in different ways depending on their mass. When a star like sun has burned all of its fuel, it expands and becomes a “red giant”. After puffing off its outer layers, the star collapses and shrinks to form a very dense “white dwarf”. One teaspoon of material from a white dwarf weighs up to 100 tonnes. Over billions of years, the white dwarf cools and becomes invisible.
However, stars heavier than 8 times the mass of the sun end their life very suddenly. They first swell into red super giants. Then, they blow themselves apart in a huge explosion known as “supernova explosion”. The explosion lasts for a week or so, then it quickly fades and all that is left is a neutron star or a black hole surrounded by an expanding cloud of very hot gas. The stardust made from the explosion makes other stars and planets. 