II.

Characteristics

A typical Cepheid variable star is anywhere from 5 to 20 times more massive than the Sun, which has a mass of 1.99 x 10³⁰ kg (4.39 x 10³⁰ lbs). Typical Cepheid variable stars also shine about 10,000 times brighter than the Sun.

Cepheid variables begin changing in brightness as they near the end of their hydrogen fuel supply. Hydrogen furnishes thermonuclear fuel for stars like the Sun. In nuclear reactions deep in the star’s interior, hydrogen atoms fuse into helium atoms, liberating energy in the process (see Nuclear Energy). A typical star spends most of its lifetime in the relatively stable hydrogen-consuming stage. However, as a star’s hydrogen fuel diminishes, an imbalance develops between the inward gravitational attraction of the star’s mass for itself and the outward pressure created by its nuclear reactions. This imbalance can make the star expand and contract. These periods of expansion and contraction may last from 1 to 50 days and change a star’s size by as much as 30 percent. A typical Cepheid variable star may oscillate (expand and contract again and again) for a million years, a short period of time relative to the several-billion-year life span of most stars.

As a Cepheid variable oscillates, its temperature and brightness also change significantly. As the star expands, its light becomes brighter, and its temperature decreases from about 7,300^o C (13,000^o F) to about 5,700^o C (10,300^o F). Astronomers have identified two major classes of Cepheids, known as Type I and Type II. Type I Cepheid variable stars, also called classical Cepheids, are typically younger stars that are 2.5 to 6.5 times brighter and have shorter oscillation periods than their Type II counterparts.