Cepheid Variables

Probably the most important kind of variable star is called a Cepheid variable, so-named because the first such star discovered was delta Cephei. As we shall see, Cepheid variables are important not only because of the variablity itself, but because the variability can be used to determine distances in a reliable way.

Characteristics of Cepheids

Cepheids are yellow supergiant stars with well-defined periods that commonly lie in the range 1-100 days. Because they are supergiants, they are luminous and can be seen in other galaxies. The following figure shows the light curve for delta Cephei (and a period-luminosity relation that we will discuss further below).

Origin of the Variability

The variability of Cepheids is a direct consequence of their pulsation in size. The following figure shows the variation in brightness, temperature, spectral class, surface velocity, and radius for the prototype Cepheid variable, delta Cephei, as a function of the fraction of the pulsation period, as does this animation.

The fluctuating radial velocity of the star (determined by Doppler methods) indicates that it is pulsing in size, causing the surface to alternately move toward us and away from us. We see that the surface temperature, radius, spectral class, and magnitude are all changing periodically as a result of the systematic pulsation. (These pulsations involve the propagation of waves inside the star and are more complex than a simple uniform expansion and contraction, but we won't go into that level of detail.)

Revised Distance Scales
When Cepheid variables were first used to establish distance scales in our galaxy and to nearby galaxies early in this century, it was not yet realized that there are two kinds of Cepheid variables. Also, some RR Lyrae variables were mistakenly assumed to be Cepheid variables in some of that work. The net effect was that earlier distance scales were later revised by about factors of 2 when these mistakes were corrected.

Two Kinds of Cepheid Variables

There are two kinds of Cepheid variables: Type I Cepheids, which are Pop I stars (delta Cephei is an example), and Type-II Cepheids, which are Pop II stars. (Recall that Pop I is the younger star population found mostly in the disk of the galaxy, while Pop II is the older population found largely in the globular clusters.)

Type I Cepheids are also called classical Cepheids and type-II Cepheids are also called W Virginis stars. These two classes of Cepheids resemble each other in most characteristics, except for subtle spectral effects and that they obey different period-luminosity relations, as we discuss further below.