Cepheid Variable

I

Introduction

Cepheid Variable, type of star that pulsates regularly and varies greatly in brightness. Cepheid variables are a special type of variable star, a star that varies in brightness over time. They are named for the star Delta Cephei, the first of these pulsating stars detected in 1784 by British astronomer John Goodricke. Astronomers have discovered over 700 Cepheid variable stars in our Milky Way Galaxy, and they have discovered several thousand in the Local Group (a term for about 30 galaxies including the Milky Way, Andromeda, and galaxies known as the Magellanic Clouds).

Cepheid variable stars contract and expand like some other variable stars, but they differ in that the average brightness of a Cepheid variable star is always in direct proportion to its oscillation period (the time it takes the star to go from dim to bright to dim again). Astronomers use this period of oscillation to determine the actual amount of light emitted by Cepheid variables. Knowing the actual brightness of a star allows astronomers to calculate its distance from Earth. By observing and measuring Cepheid variables across the universe, astronomers have been able to accurately measure vast distances between stars and between galaxies.

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.

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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.

III

Importance to Astronomy

Astronomers identify a star as a Cepheid variable by its light curve—a graph showing how its light varies over time. Light emitted from a Cepheid variable star typically increases dramatically in brightness, followed by a gradual decrease. Astronomers have identified Cepheid variable stars in many parts of the sky by their light curves. Most classical Cepheid variable stars (Type I) are found in the gaseous spiral arms of our Milky Way galaxy and in the spiral arms of other galaxies. Type II Cepheid variable stars are found in globular star clusters and the centers of galaxies (see Globular Cluster).

In 1912 American astronomer Henrietta Leavitt noticed that the average brightness of a Cepheid variable star was related to its period of oscillation. From this research, other astronomers developed the period-luminosity law, which states that the brightness of Cepheid variable stars increases with its period (because larger Cepheid variable stars have longer periods, and larger stars are intrinsically brighter).

By measuring a Cepheid variable star’s period of oscillation, astronomers can calculate its intrinsic brightness (the amount of light the star emits). Then, by comparing the intrinsic brightness of the Cepheid variable with the actual amount of the star’s light that reaches Earth, astronomers can calculate the star’s distance from Earth. In this way, Cepheid variable stars are valuable for measuring distances in space. Astronomers have used Cepheid variable stars to help them estimate the size of the entire universe (See also Cosmology).

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