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--- courses:ast100:3.3 [2026/02/24 09:34] – [2. Life of Stars] asad
+++ courses:ast100:3.3 [2026/02/24 09:37] (current) – [2. Life of Stars] asad
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 Conversely, a massive star follows the rapid and dramatic blue track across the top of the H-R diagram. Starting its stable life as a highly luminous and immensely hot blue giant on the upper main sequence, it exhausts its nuclear fuel much faster. As it evolves, it expands outward, moving horizontally across the spectral classes to become a yellow giant and eventually a massive, cooler red supergiant. Unlike its smaller counterparts, the giant star's life culminates in a catastrophic supernova explosion at the top right of the diagram, violently dispersing heavy elements into the cosmos.
+==== - Small stars ====
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+For small stars, like our Sun, this main-sequence phase is just one chapter in a cyclical cosmic journey. After billions of years, when the core's hydrogen fuel is depleted, the star leaves the main sequence and expands dramatically into a red giant, swelling to roughly 30 times its original size. It briefly contracts into a yellow giant before expanding once more into a second red giant stage. In its final evolutionary act, the dying star sheds its outer layers to form an expansive planetary nebula, stretching approximately one light-year across. The exposed core remains as a hot, dense central star that ultimately shrinks and cools into a white dwarf—a stellar remnant only about 1/100th the size of the Sun. Meanwhile, the expanding shell of ejected gas sweeps outward, carrying newly forged heavy elements back into the interstellar medium to enrich the next generation of interstellar clouds.
+==== - Big stars ====
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+For massive stars, this main-sequence phase marks the beginning of a rapid and incredibly violent lifecycle. After quickly burning through their core fuel, these large stars expand into pulsating yellow giants before swelling enormously into red supergiants. Their lives culminate in a cataclysmic supernova explosion, blowing apart the star and creating an expanding remnant that stretches approximately 10 light-years across. This violent detonation ejects gas enriched with very heavy elements back into the interstellar medium; the high speed of this ejected material can even compress nearby interstellar clouds, triggering new waves of star formation. Left behind in the wake of the explosion is an incredibly dense core remnant—either a neutron star or a black hole—measuring a mere 10 kilometers across.