The life cycle of a star can be described through a series of stages that depend primarily on its mass. Here's a simplified overview using the terms you've provided:
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Nebula: A star's life begins in a nebula, which is a vast cloud of gas and dust in space. Under the influence of gravity, parts of the nebula begin to contract and clump together, forming a protostar.
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Main Sequence: Once the protostar's core becomes hot and dense enough to initiate nuclear fusion, hydrogen atoms begin to fuse into helium, releasing energy. The star enters the main sequence phase, where it will spend the majority of its life (often billions of years). The Sun is currently in this stage.
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Red Giant: After exhausting the hydrogen in its core, the star’s core contracts under gravity, causing the outer layers to expand and cool, transforming it into a red giant. This stage occurs when the star starts to fuse helium into heavier elements in the core.
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Supergiant: For more massive stars, once they exhaust their helium, they can undergo further fusion processes, creating heavier elements, and can expand into a red supergiant. This phase occurs before the eventual collapse of the core.
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Supernova: Eventually, a massive star will reach a point where it can no longer sustain fusion. The core collapses dramatically, leading to a supernova—an explosive event that ejects the outer layers of the star into space and can outshine an entire galaxy for a brief period.
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White Dwarf: For stars that were less massive (like our Sun), after shedding their outer layers, they leave behind a hot core that becomes a white dwarf. This is a dense, Earth-sized remnant that will gradually cool and fade over time.
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Black Hole: In the case of massive stars, after the supernova, if the core remnants are sufficiently massive (beyond about 2-3 solar masses), they will collapse under their own gravity to form a black hole, a region of spacetime with gravity so strong that not even light can escape.
In summary, the life cycle of a star involves the processes of formation from a nebula, development through the main sequence and subsequent phases (red giant or supergiant), leading to dramatic end stages that can result in the formation of white dwarfs or black holes, depending on the star's mass.