Issue 3.18

The Astrophysics Spectator

October 18, 2006

Two new pages are added to the “Stars” path that discuss red giant stars. The first page gives a general description of red giants and points out several examples visible to the eye. The second page discusses the evolution of a red giant star.

A star generates energy through thermonuclear fusion of light elements. The first source of energy is hydrogen, and a star spends most of its lifetime burning this fuel, converting the hydrogen into helium. These stars are called main-sequence stars, because on a plot of luminosity versus photosphere temperature, these stars fall on a curved line. Most stars in our Galaxy are in this stage of their lives. Eventually, however, the hydrogen in the core of a star is exhausted, and the star evolves to burn helium and other heavier elements. These stars are the red giant stars. They are much less common than the main sequence stars, reflecting their short lifetimes: only one tenth to one quarter of that of a main-sequence star. But they are brighter than a main-sequence star of the same mass, and they are often very red.

Before a spent main-sequence star can burn helium, its core temperature must increase. The core accomplishes this by shrinking and converting gravitational potential energy into heat. Eventually the core temperature rises high enough to burn helium. But this has a perverse, counterintuitive effect on the outer parts of the star, because as the core shrinks and becomes hotter, the photosphere of a star becomes larger and cooler. The result is a large, bright, and red star.

Next Issue: The next issue of The Astrophysics Spectator is planned for release in three weeks on November 8.

Stars

Red Giant Stars. Red giant stars are red because they are physically larger than main-sequence stars of the same mass. The red giant is a star burning its last bits of nuclear fuel. Burning helium, carbon, and oxygen, these stars are very luminous but very short-lived. The most prominent bright red stars visible in the night sky are red giants. We can see several good examples of red giants and red supergiants in our night sky. (continue)

Red Giant Evolution. As a red giant burns through the helium, carbon, and oxygen at its core, it cycles through stages of expansion and reddening. These cycles begin with the exhaustion of one fuel in the core and the contraction of the core to burn the next heavier fuel. As the core shrinks, the interior becomes hotter. To prevent the temperature gradient from getting too steep, the star expands, becoming red as the larger area of the photosphere causes the photosphere temperature to drop. Once burning recommences, the star slowly becomes brighter and bluer. A red giant's life is short compared to the life of a main-sequence star. (continue)