A star starts its existence as a massive cloud of gas. Stars originate within dense regions of interstellar gas and dust called molecular clouds. In these environments, the temperatures are low enough for gases to form molecules, with atoms binding together. When stars are born, they settle onto the main sequence of the H-R Diagram, which is represented by a straight line on the chart.
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Stage 2: The T-Tauri Phase
A young star starts generating powerful winds that disperse the surrounding gas and molecules. This clearing of the surrounding material allows the emerging star to become visible for the first time.
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Stage 3: Main Sequence Stars
Eventually, the young star attains hydrostatic equilibrium, where the compressive force of gravity is balanced by the outward pressure, stabilizing its shape. At this point, the star becomes a main sequence star. It will remain in this phase for 90 percent of its lifespan, fusing hydrogen into helium in its core. Once the star exhausts its hydrogen core, it departs from the main sequence and transitions into the red giant phase.
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Stage 4: Expansion into Red Giant
After the hydrogen in a star's core is fully converted to helium, the core collapses, leading to the star's expansion. During this process, the star first becomes a sub-giant and then a red giant. Red giants have cooler surfaces compared to main sequence stars, giving them a red appearance instead of yellow. On the H-R Diagram, the oldest main sequence stars move off the main sequence, expanding in size while losing the higher temperatures they once had.
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Stage 5: Fusion of Heavier Elements
He
He
As the star expands, it starts fusing helium in its core, and the energy from this reaction prevents the core from collapsing. After helium fusion ends, the core contracts, and the star begins fusing carbon. This cycle continues until iron appears in the core. Since iron fusion absorbs energy, its presence leads to core collapse. If the star is sufficiently massive, this implosion results in a supernova.
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Stage 6: Death/Compact Object
The end of a star's life results in a brilliant stellar explosion known as a supernova, one of the brightest events in the universe. The specific manner in which a star dies is influenced, in part, by its mass.