As Carl Sagan so aptly put it, "the total number of stars in the universe is larger than all the grains of sand of all the beaches on the planet Earth." Stars, like humans, pass through a life cycle. They are born and then die. The life cycle varies depending on the volume of material contained in the star. A red dwarf is the smallest star, while a super giant is the largest. Our sun, a medium-sized star, is a main sequence star.
Protostars, or young stellar objects, are the precursors of new stars. They form from nebulae, which are interstellar clouds of dust and hydrogen. The dust and hydrogen attract each other, and the nebulae begins to condense. Gravity begins to make the cloud even more dense and compact, and the mass begins to generate heat. Eventually the hydrogen ignites, and fusion begins. Fusion converts the hydrogen gas to helium. A star is born.
Red Dwarf Stars
Red dwarf stars are the smallest and most common stars, and they live the longest. They can burn for several hundred billion years. They are 7.5 percent of the mass of the sun. Any smaller and they would not be dense enough to undergo fusion. Because of their size, they do not burn as fast or as hot as more massive stars, and they do not advance quickly through the life cycle.
The sun, a main sequence star, is following a typical star life cycle. As the hydrogen gas burns, the central core continues to collapse upon itself and grow denser and heavier. This is one of the interesting characteristics of a star; it actually grows heavier as it consumes itself. While the core is consumed, it emits energy. The energy pushes outward, counteracting the inward pull of the core, so an outer shell of hydrogen gas and dust forms. The core begins to burn hotter and brighter. Eventually all of the hydrogen gas in the inner core is consumed. The sun took about 100,000 years to go from a protostar to a new star, and roughly 50 million years to reach the mature stage. As an adult it will last about 10 to 15 billion years on its way to becoming a white dwarf.
Red Giant Life Cycle
When all of the hydrogen in the inner core is consumed, the outer shell expands, and the star becomes a red giant. The inner core slowly cools. The star begins to dim, but it emits a red light as the hydrogen in the outer shell burns. The inner core continues to condense, and the outer shell continues to expand.
White Dwarf Life Cycle
At this point, the life cycle could diverge into several directions depending on the mass of the star. In a star the size of our sun, the outer shell continues to expand until it consumes all of the hydrogen. The inner core becomes surrounded by a cloud of helium and dust that dissipates. Only a non-burning core of carbon is left. The star is a white dwarf. The star will continue to collapse upon itself and cool until nothing visible remains. The star is dead.
Super Giant Star
Stars at least 1.4 times more massive than our sun, super giant stars, do not have as peaceful a death. With super giants, the conflict between the inner core's inward contraction and the outer shell's desire to expand becomes so great, the star explodes. Depending on the extent of the explosion, these stars form novae, super novae, neutron stars or black holes. These stars burn the fastest, and may only last a few million to a few billion years.
A New Beginning
The gas and dust clouds formed as a result of a star's death provide the materials for new nebulae. These in turn provide materials for the birth of new stars. The cycle is complete.