Stars evolve over time, transitioning from main sequence to red giants. This process begins as hydrogen in a star's core depletes, causing it to contract and heat up. The star's outer layers then expand and cool, transforming it into a red giant or supergiant.

A star's mass greatly influences its evolution. More massive stars burn through their fuel faster, leading to shorter lifespans and more dramatic transformations. Less massive stars evolve more slowly, eventually becoming red giants before expelling their outer layers and leaving behind white dwarfs.

Stellar Evolution: From Main Sequence to Red Giants

Hydrogen depletion in stellar cores

• Main sequence stars fuse hydrogen into helium in their cores releasing energy that creates outward pressure balancing inward gravitational force
• Over time, hydrogen in the core becomes depleted due to fusion leaving the core dominated by helium which does not undergo fusion at this stage
• As hydrogen depletes, the core contracts due to reduced outward pressure causing core temperature to increase
• Increased core temperature makes the outer layers of the star expand and cool transforming the star into a red giant (Sun) or supergiant (Betelgeuse) depending on its initial mass
• This process is part of the broader concept of stellar nucleosynthesis, which describes the creation of heavier elements within stars

Mass influence on stellar evolution

• More massive stars have shorter main sequence lifetimes because higher mass leads to higher core temperatures and pressures causing fusion to occur faster and deplete hydrogen more quickly (O-type stars, ~10 million years)
  • These stars evolve into red supergiants after the main sequence since they have sufficient mass to fuse heavier elements in their cores and undergo significant mass loss through stellar winds (Wolf-Rayet stars)
• Less massive stars have longer main sequence lifetimes because lower mass leads to lower core temperatures and pressures causing fusion to occur slower and deplete hydrogen more slowly (M-type stars, ~100 billion years)
  • These stars evolve into red giants after the main sequence since they do not have sufficient mass to fuse heavier elements in their cores
  • They eventually expel their outer layers forming planetary nebulae (Ring Nebula) and leave behind white dwarf remnants (Sirius B)
• Stellar evolution tracks on the Hertzsprung-Russell diagram illustrate how stars of different masses evolve over time

Main sequence vs giant stars

• Size
  • Main sequence stars range widely in size depending on their mass (0.1-200 solar radii)
  • Red giants and supergiants are much larger than their main sequence counterparts due to expanded outer layers from increased core temperature and reduced surface temperature (100-1000 solar radii)
• Temperature
  • Main sequence stars have a wide range of surface temperatures depending on their mass with more massive stars being hotter (O-type, ~40,000 K) and less massive stars being cooler (M-type, ~3,000 K)
  • Red giants and supergiants have cooler surface temperatures than their main sequence counterparts due to expanded outer layers resulting in a larger surface area (3,000-4,000 K)
• Luminosity
  • Main sequence stars range widely in luminosity following the mass-luminosity relation: $L \propto M^{3.5}$
  • Red giants and supergiants are more luminous than their main sequence counterparts due to increased size and cooler surface temperature (100-100,000 solar luminosities)
• Spectral characteristics
  • Main sequence stars have spectral types ranging from O (hottest) to M (coolest)
  • Red giants and supergiants have spectral types of K or M indicating cooler surface temperatures
    • Their spectra show strong absorption lines of neutral metals (calcium) and molecular bands (titanium oxide)

Stellar Structure and Evolution

• Stellar structure models describe the internal layers of stars, including the core, radiative zone, and convective zone
• Post-main sequence evolution involves significant changes in stellar structure as the core contracts and outer layers expand
• Stellar interior models help predict how stars evolve over time and transition from the main sequence to giant phases
• Stellar mass loss becomes increasingly important in later stages of evolution, particularly for more massive stars