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Stars begin as immense clouds of mostly hydrogen in gravitational collapse. As cloud density increases, atoms get close enough that nuclear fusion of hydrogen into helium begins. This releases vast amounts of energy, turning a diffuse, cold gas into a dense white-hot nuclear furnace.

Over time the hydrogen is consumed and nuclear reactions of helium and trace amounts of other elements in the initial cloud become important, producing heavier elements. Once the hydrogen in the core is exhausted, stars the size of our Sun (and a bit larger) expand to become red giants before degenerating and recycling their elements to form the next generation of stars. Stars much smaller than the Sun become white dwarfs and more massive stars can catastrophically explode as supernovae.

Supernovae are extremely luminous and their burst of radiation often briefly outshines an entire galaxy before fading from view over several weeks or months. During this short interval, a supernova blows its elements into space, radiating as much or more energy than the Sun emits over its entire life span.

Our Sun is one of about one hundred billion stars in the Galaxy and there are billions of galaxies in the Universe. The outer region of the Sun is composed of 75% hydrogen and 24% helium with less than 2% heavier elements, principally oxygen, carbon, neon, and iron. Though small compared to hydrogen and helium, these heavier elements came from the dust of generations of earlier stars recycled to the interstellar medium where the Sun formed.

". . . stars have a life cycle much like animals. They get born, they grow, they go through a definite internal development, and finally they die, to give back the material of which they are made . . ."

Hans Bethe, Nobel Prize 1967