22.1 Evolution from the Main Sequence to Red Giants

Stellar Evolution: From Main Sequence to Red Giants

Hydrogen depletion in stellar cores

While a star sits on the main sequence, it fuses hydrogen into helium in its core. That fusion releases energy, creating outward radiation pressure that balances the inward pull of gravity. This balance is called hydrostatic equilibrium, and it keeps the star stable for most of its life.

Over time, though, the core's hydrogen supply runs out. The core becomes dominated by helium "ash," which can't fuse at the temperatures found in a typical main sequence core. Here's what happens next:

1. With less energy being produced, outward pressure drops and the core begins to contract under gravity.
2. That contraction heats the core and the surrounding shell of hydrogen, which does still have fuel. Hydrogen shell burning ignites around the inert helium core.
3. Shell burning actually produces more energy than core burning did, so the star's outer layers absorb that energy, expand, and cool.
4. The star swells dramatically and its surface temperature drops, shifting its color toward red.

The result? A red giant (for Sun-like stars) or a red supergiant (for high-mass stars like Betelgeuse), depending on the star's initial mass. This transformation is visible on the Hertzsprung-Russell (H-R) diagram as the star moves off the main sequence toward the upper right.

This process is part of stellar nucleosynthesis, the broader sequence by which stars forge heavier elements from lighter ones throughout their lifetimes.

Mass influence on stellar evolution

A star's mass is the single biggest factor controlling how it lives and dies. Higher mass means higher core temperatures and pressures, which drives fusion at a much faster rate.

• High-mass stars (like O-type and B-type stars) burn through their hydrogen in as little as ~10 million years. After the main sequence, they become red supergiants with enough core temperature to fuse heavier elements (helium, carbon, and beyond). They also lose significant mass through powerful stellar winds. Wolf-Rayet stars are an extreme example of this mass loss.
• Low-mass stars (like M-type red dwarfs) fuse hydrogen so slowly that their main sequence lifetimes can exceed ~100 billion years, far longer than the current age of the universe. These stars will eventually become red giants, but they lack the mass to ignite fusion of elements much heavier than helium.
• Sun-like stars fall in between. After becoming red giants, they shed their outer layers to form planetary nebulae (the Ring Nebula is a well-known example) and leave behind a dense white dwarf remnant (like Sirius B).

Evolutionary tracks on the H-R diagram show these different paths. A massive star races across the diagram quickly, while a low-mass star barely moves for billions of years before slowly migrating toward the giant branch.

Main sequence vs. giant stars

Once a star leaves the main sequence and becomes a giant, its physical properties change dramatically. Here's how they compare:

Size

• Main sequence stars range from about 0.1 to 10+ solar radii, depending on mass.
• Red giants and supergiants balloon to 100 to 1,000+ solar radii as their outer layers expand.

Surface Temperature

• Main sequence stars span a wide range: O-type stars reach ~40,000 K, while M-type stars sit around ~3,000 K.
• Red giants and supergiants cluster at cooler surface temperatures, typically 3,000–4,000 K, because their energy is spread over a much larger surface area.

Luminosity

• On the main sequence, luminosity follows the mass-luminosity relation: $L \propto M^{3.5}$. A star twice the Sun's mass is roughly 11 times more luminous.
• Red giants and supergiants are far more luminous than their main sequence predecessors, ranging from ~100 to over 100,000 solar luminosities. Even though their surfaces are cooler, their enormous size more than compensates.

Spectral Characteristics

• Main sequence stars span spectral types O through M (remembered by the sequence O, B, A, F, G, K, M).
• Red giants and supergiants are typically spectral type K or M. Their cooler atmospheres produce strong absorption lines from neutral metals like calcium and molecular bands from compounds like titanium oxide (TiO), which are signatures you won't see in hotter stars.

Stellar structure and evolution

Understanding why stars evolve requires looking at their internal structure. A main sequence star has three key regions:

• Core: where nuclear fusion occurs
• Radiative zone: where energy moves outward through radiation (photon diffusion)
• Convective zone: where energy moves outward through rising and sinking currents of hot gas

After the main sequence, this structure changes significantly. The core contracts and heats up, hydrogen shell burning begins, and the convective zone can deepen as the outer layers expand. Stellar interior models use the equations of hydrostatic equilibrium, energy transport, and nuclear reaction rates to predict these changes.

Mass loss also becomes increasingly important in later evolutionary stages. Massive stars shed material through intense stellar winds, while lower-mass giants lose their outer envelopes more gently, eventually producing planetary nebulae. The amount of mass a star retains determines what kind of remnant it leaves behind.