Giant Star Life

Overview

A star's life is determined almost exclusively by its mass. Large stars (above about three solar masses) live much shorter and violent lives than most other, average-sized stars (such as our sun). Even the processes by which they create energy are different.

This page details what occurs during the life of the latter type of star.

Energy Production During Normal Life

Stars that are slightly more massive than the sun tend to produce energy through a different process than the PP Chain. It is called the CNO Cycle.

The left-most loop, the CN Cycle, is the predominant one, occurring more than 99% of the time. The final step from ¹⁵ N -> ¹² C produces a ⁴ He. Through the loop, 4H were used, so the C, N, and O were all acting as catalysts, and were conserved.

The central cycle, ¹⁷ ON, occurs only about 0.04% of the time, but still produces the same: a ⁴ He.

The CNO cycle has a very strong temperature dependence at . Thus, in stars of the sun's mass and below, the PP Chain is more efficient. But in massive stars, the central temperature is higher, and this boosts the efficiency of the CNO cycle more than the PP Chain, and so it dominates over the PP Chain.

Compositional Changes

This process will power the star for much less time than an average star. This is because a star's luminosity is approximately proportional to its mass to the fourth power - M⁴. Due to this, it runs through its nuclear fuel much more quickly, despite its higher initial mass. The table to the right shows the Main Sequence lifetimes of a few different star masses.

However, before that happens, all of the nuclear processes that have been occurring act to change the chemical composition of the star, which causes it to alter some properties.

As the hydrogen is turned to helium, the mean molecular weight of the core rises. Given the Ideal Gas Law (right), if the mean molecular weight rises, then the pressure drops, and if the pressure drops, then the core contracts, so the density rises. This must happen for the star to stay in hydrostatic equilibrium.

As the core contracts, the virial theorem says that half the gravitational energy released by contraction must go to heating the star. The end result of all this is for the star to become more luminous as it ages. For example, the sun is currently about 140% as luminous now as it was when it began fusion about 5 billion years ago.

All these changes build up, and eventually, the core runs out of hydrogen to fuse. When this happens, the star moves off the Main Sequence and begins its journey to death.