The Astronomy Book

135

Astronomical Observatory of
Harvard College in 1908. Then,
in 1912, after further study, which
included plotting graphs of the
periods of Cepheid variables
in the SMC against values for
their minimum and maximum
brightness, she confirmed her
discovery in more detail. It became
known as the “period−luminosity”
relationship. Formally, it stated
that the logarithm of the period
of a Cepheid variable is linearly
(i.e., directly) related to the star’s
average measured brightness.

Building on Leavitt’s work
Although it is possible that
Leavitt did not realize the full
implications right away, she had
discovered an extremely valuable
tool for measuring distances in the
universe, far beyond the limitations
of parallax measurements. Cepheid
variables were to become the first
“standard candles”—a class of
celestial objects that have a known
luminosity, allowing them to be
used as tools to measure vast
distances in space.

One of the first people to appreciate

the significance of Leavitt’s

discovery was Danish astronomer

Ejnar Hertzsprung. Due to the

period−luminosity relationship

discovered by Leavitt, Hertzsprung

realized that by measuring the

period of any Cepheid variable

it should be possible to determine

its luminosity and intrinsic

brightness. Then by comparing

its intrinsic brightness to its

apparent magnitude (measured

average brightness from Earth), it

should be possible to calculate the

distance to the Cepheid variable.

In this way, it should also be

possible to determine the distance

to any object that contained one

or more Cepheid variable star.

However, there was still a

problem to be solved: although

Leavitt had established the

important period−luminosity

relationship, initially all this

promised was a system for

measuring the distance to remote

objects relative to the distance

to the SMC. The reason for this

is that Leavitt had no accurate

THE RISE OF ASTROPHYSICS

I should be willing to pay

thirty cents an hour in view

of the quality of your work,

although our usual price,

in such cases, is twenty

five cents an hour.

Edward C. Pickering

information about the distance to

the SMC, nor indeed any accurate

data about the intrinsic brightness

of any Cepheid variable.

Calibrating the variables

To turn Leavitt’s finding into

a system that could be used to

determine absolute distances, not

just relative distances, it needed

calibrating in some way. In order

to do this, it would be necessary to

measure accurately the distances

to and intrinsic brightness of a few

Cepheid variables. Hertzsprung

therefore set about determining the

distances to a handful of Cepheids

in the Milky Way galaxy, using an

alternative complex method called

statistical parallax, which involves

calculating the average movement

of a set of stars assumed to be at

a similar distance from the sun.

Having obtained the stars’

distances, it was a straightforward

step to figure out the intrinsic

brightness of each of the nearby

Cepheids. Hertzsprung used

these values to calibrate a scale,

which allowed him to calculate

the distance to the SMC and the

intrinsic brightness of each of

Leavitt’s Cepheids in the SMC. ❯❯

Apparent magnitude

is the brightness

of a star as viewed

from Earth.

Absolute visual

magnitude is the

brightness of a star

as viewed from a set

distance and indicates

the true or intrinsic

brightness of a star.

Optical luminosity is

the rate at which a star

emits light energy from

its surface and is closely

related to absolute

visual magnitude.

Brightness and magnitudes of stars