The Astronomy Book

134

later. These are stars that regularly
vary in brightness with a period
(cycle length) that could be anything
from one to more than 120 days.
Cepheid variables are reasonably
easy to recognize because they are
among the brightest variable stars,
and they have a characteristic light
curve, showing fairly rapid increases

in brightness followed by a slower

tailing off. Today, they are known to

be giant yellow stars that “pulsate”—

varying in diameter as well as

brightness over their cycles—and

are very rare. As a class of stars,

they also have an exceptionally

high average brightness, which

means they stand out even in other

MEASURING THE UNIVERSE

galaxies. In examining her records

of Cepheid variables in either the

LMC or SMC, Leavitt noticed

something that seemed significant.

Cepheids with longer periods

seemed to be brighter on average

than those with shorter periods.

In other words, there was a

relationship between the rate at

which Cepheids “blinked” and their

brightness. Furthermore, Leavitt

correctly inferred that, since the

Cepheids she was comparing were

all in the same distant nebula

(either the LMC or the SMC), they

were all at much the same distance

from Earth. It followed that any

difference in their brightness as

viewed from Earth (their apparent

magnitude) was directly related

to differences in their true or

intrinsic brightness (their absolute

magnitude). This meant there was

a definite relationship between the

periods of Cepheid variables and

their average intrinsic brightness

or their optical luminosity (the rate

at which they emit light energy).

Leavitt published her initial

findings in a paper that first

appeared in the Annals of the

A Cepheid variable belongs to a class of star called a pulsating

variable. These stars expand and contract over a regular cycle, at the

same time regularly varying in brightness. They are hottest and brightest

shortly after reaching their most contracted phase. The graph of the

star’s luminosity (light output) against time is called its light curve.

The period of the fluctuation in brightness of a

Cepheid variable is closely related to its intrinsic brightness.

Measuring its period

gives a value for its

intrinsic brightness.

Comparing its intrinsic

brightness to its

apparent brightness

from Earth gives a value

for its distance from Earth.

Cepheid variables can be used as “standard candles”

to measure distances in the universe.

Hottest state Coolest state

Period of one pulsation Light

curve

TIME

LUMINOSITY

A straight line can readily

be drawn among each of

the two series of points

corresponding to maxima and

minima, thus showing that

there is a simple relation

between the brightness of the

variables and their periods.

Henrietta Swan Leavitt