As they grow older, stars become unstable and expand, and, in billions of years, will eventually end their existence in explosions. They expand into giant stars which eventually explode and throw off clouds of gases from their outer layers. When stars explode, clouds of helium and other gases from the original stars are formed which, over millions of years, mix with dust and gases, to make the beginnings of new stars. These materials start to spin faster as gravity pulls them together into balls.
Stars are hot and bright because they are huge nuclear reactors.
Stars produce great amounts of energy by changing hydrogen into helium through a series of nuclear reactions. This energy source in the center of the stars makes them get hot faster and become brighter. Nuclear fusion reactions begin when the temperature at the core or center of the stars reaches about 15,000,000 degrees Celsius. The energy produced by nuclear fusion heats the gases that surround the centers of the stars, causing them to shine brightly. The largest stars have the shortest lives, as they burn hotter and faster than medium-sized and smaller stars like the Sun , the closest star to the Earth.