Astronomy - Formation and Evolution of Stars - Quick Guides | Astronomy

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Study GuideAstronomy–FormaƟon and EvoluƟon of Stars1. EvoluƟon of Stars1.1 Inside a Main Sequence StarA typical main sequence star, like our Sun, produces energy at its center. Thecoreis the hottest anddensest region, where hydrogen is converted into helium through nuclear fusion.In stars like the Sun, energy production does not happen at just one tiny point. Instead, it occursthroughout a large central region, extending to about25% of the star’s radius.Over time, the star’s composition slowly changes. Hydrogen is gradually used up and turned intohelium. In the4.6 billion yearssince the Sun formed, abouthalf of the hydrogen in its corehasalready been converted into helium.These slow changes in chemical composition cause slow changes in the star’s structure and surfaceproperties. Astronomers call this long-term change in a star’s lifestellar evolution.

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Study Guide1.2 Life on the Main SequenceZero-Age Main Sequence (ZAMS)When nuclear fusion first begins in the core, the star enters thezero-age main sequence (ZAMS)stage. At this point:•The star has the same chemical composition throughout.•Its surface temperature is slightly warmer.•Its luminosity is slightly lower than older main sequence stars.As hydrogen fusion continues:•Hydrogen in the core decreases.•Helium builds up.•The core slowly contracts to maintain pressure.•Core temperature rises.These changes cause the star to:

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Study Guide•Becomemore luminous•Slightlycool at the surface•Driftupward and to the righton the HR diagramTerminal-Age Main Sequence (TAMS)When most hydrogen in the core is used up, the star reaches theterminal-age main sequence(TAMS). This marks theend of the main sequence phase.1.3 How Long Do Stars Live?A star’s lifetime depends on:•How much fuel it has(mass)•How fast it uses that fuel(luminosity)

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Study GuideMassive stars burn fuel very quickly, so they:•Are brighter•Live much shorter livesLow-mass stars burn fuel slowly and can livebillions or even trillions of years.Mathematically:•Luminosity∝mass³·⁵•Lifetime∝mass⁻²·⁵This explains why large stars evolve faster than small ones.1.4 The Red Giant Stage

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Study GuideWhen core hydrogen is exhausted, fusion stops in the core. Energy is now supplied bygravitationalcontraction, which heats the core further.At this stage:•Ahelium coreforms.•Hydrogen fusion continues in ashell around the core.•The outer layers expand and cool.The star becomes asub-giant, then ared giant.On the HR diagram, it moves:•To theright(cooler surface)•Thenupward(higher luminosity)As surface temperature drops below3,000 K, the outer layers become convective. This causes arapid increase in brightness as stored energy reaches the surface. This path is called theHayashitrack, also known as thered giant branch.1.5 Helium Burning and the Triple-Alpha ProcessAs the core contracts further, temperature rises to about200 million K. At this point:•Helium nuclei can combine to form carbon.•This requires three helium nuclei colliding almost at once.This reaction is called thetriple-alpha process.Because the core is supported byelectron degeneracy pressure, the start of helium fusion happenssuddenly in an explosive event called thehelium flash.After the flash:•The core expands•Degeneracy pressure is removed•Helium fusion becomes stable•The star shrinks slightly and becomes hotter

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