86
See also: A new kind of star 48–49 ■ Measuring the universe 130–37 ■
Beyond the Milky Way 172–77A
ncient Greek astronomers
were the first to classify
stars by their apparent
brightness—that is, their brightness
as observed from Earth. In the
18th century, British amateur
astronomer John Goodricke grew
interested in changes in apparent
brightness after his neighbor,
astronomer Edward Pigott,
provided him with a list of stars
known to vary. In the course of his
observations, he discovered more.
In 1782, Goodricke observed
the variation in brightness of Algol,
a bright star in the constellation
Perseus. He was the first person
to propose a reason for this change
in brightness, suggesting that
Algol was in fact a pair of stars
orbiting one another, with one
brighter than the other. When
the dimmer of the two stars
passed in front of the brighter
one, the eclipse would reduce the
brightness detected by observers.
Today this is referred to as an
eclipsing binary system (it is
now known that Algol is actually
a three-star system).Goodricke also discovered that the
star Delta Cephei in the constellation
Cepheus varies in brightness with
a regular period. It is now known
that Delta Cephei is one of a class
of stars whose apparent brightness
varies because the star itself
changes. Stars such as this are
called Cepheid variables, and they
are key to calculating the distance
to other galaxies.
Goodricke presented his
findings to the Royal Society in- He died shortly after from
pneumonia, at just 21 years old. ■
THE BRIGHTNESS
OF THE STAR
AS ALTEREDW
VARIABLE STARS
IN CONTEXT
KEY ASTRONOMER
John Goodricke (176 4 –178 6)BEFORE
130 bce Hipparchus defines
a magnitude scale for the
apparent brightness of stars,
which is popularized by
Ptolemy in the Almagest.1596 David Fabricius
discovers that the star Mira
Ceti varies in brightness with
periodic regularity.AFTER
1912 Henrietta Swan Leavitt
discovers that the period of
some variable stars is related to
their absolute (true) brightness.1913 Ejnar Hertzsprung
calibrates this variation in
brightness, allowing Cepheid
variables to be used as
“standard candles” to calculate
the distance to galaxies.
1929 Edwin Hubble identifies
the link between the velocity
of a galaxy and its distance.In an eclipsing binary system,
maximum brightness occurs when
both stars are visible (A); minimum
brightness occurs when the dim star
is eclipsing the bright star (B).Bright starDim starB
A