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Astrophysics I

5.2 Post-main sequence evolution and giant stars

2 min readLast Updated on July 25, 2024

Stars evolve dramatically after depleting their core hydrogen. They swell into giants, becoming cooler but brighter. This transformation marks a crucial phase in stellar life, setting the stage for advanced fusion processes.

Post-main sequence evolution varies based on a star's mass. Low-mass stars become red giants, while high-mass stars form supergiants. These changes involve complex nucleosynthesis, producing heavier elements and shaping the star's ultimate fate.

Post-Main Sequence Evolution

Changes in hydrogen-depleted stars

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  • Core contraction triggers gravitational collapse raising core temperature
  • Shell hydrogen burning initiates fusion in shell surrounding inert helium core boosting energy output
  • Envelope expansion causes outer layers to swell and cool dramatically increasing stellar radius (up to 100x)
  • Luminosity increase results from heightened energy production making star brighter
  • Movement off main sequence shifts star towards red giant branch on H-R diagram

Formation of giant stars

  • Red giants form from main sequence stars (0.5M<M<8M)(0.5M_☉ < M < 8M_☉) developing expanded cool envelope and hot dense core
  • Red supergiants evolve from high-mass stars (M>8M)(M > 8M_☉) reaching enormous radii (up to 1000x Sun)
  • Common traits include low surface temperatures (3000-4500 K), high luminosities, convective outer layers
  • Red giants possess helium core with hydrogen-burning shell while red supergiants have carbon-oxygen core with multiple fusion shells

Stellar Nucleosynthesis and Evolution

Fusion in post-main sequence cores

  • Helium fusion (triple-alpha process) combines three helium nuclei into carbon at 10810^8 K
  • Carbon fusion merges carbon nuclei forming heavier elements (oxygen, neon) at 5×1085 \times 10^8 K
  • Neon, oxygen, and silicon burning occur in high-mass stars producing progressively heavier elements
  • s-process (slow neutron capture) generates elements heavier than iron in AGB stars
  • Degenerate helium flash ignites helium fusion suddenly in low-mass stars

Factors in stellar evolution paths

  • Initial mass primarily determines post-main sequence evolution and possible fusion processes
  • Metallicity impacts opacity and energy transport affecting H-R diagram position
  • Rotation rate influences elemental mixing within star altering lifetime and evolution
  • Mass loss through stellar winds or binary interactions shapes future evolution and final fate
  • Convection vs. radiative energy transport efficiency affects internal energy transfer
  • Core mass at main sequence end influences onset and nature of post-main sequence fusion

Term 1 of 16

Antares
See definition

Antares is a red supergiant star located in the constellation Scorpius, known for its distinct reddish hue and prominence as one of the brightest stars in the night sky. As a post-main sequence star, Antares has evolved beyond the main hydrogen-burning phase of its life cycle, undergoing significant changes in size, luminosity, and temperature as it prepares for its eventual fate as a supernova. This star serves as an important example of the characteristics and behaviors of giant stars during their late evolutionary stages.

Key Terms to Review (16)

Term 1 of 16

Antares
See definition

Antares is a red supergiant star located in the constellation Scorpius, known for its distinct reddish hue and prominence as one of the brightest stars in the night sky. As a post-main sequence star, Antares has evolved beyond the main hydrogen-burning phase of its life cycle, undergoing significant changes in size, luminosity, and temperature as it prepares for its eventual fate as a supernova. This star serves as an important example of the characteristics and behaviors of giant stars during their late evolutionary stages.

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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

Term 1 of 16

Antares
See definition

Antares is a red supergiant star located in the constellation Scorpius, known for its distinct reddish hue and prominence as one of the brightest stars in the night sky. As a post-main sequence star, Antares has evolved beyond the main hydrogen-burning phase of its life cycle, undergoing significant changes in size, luminosity, and temperature as it prepares for its eventual fate as a supernova. This star serves as an important example of the characteristics and behaviors of giant stars during their late evolutionary stages.



© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.