A Third Method of Measuring the Size of the Universe 189
But it was the astronomer Henrietta Leavitt at the Harvard College Observatory,
another “computer,” who discovered this period-luminosity relationship. While cataloguing
the magnitudes of stars, she noticed this pattern in the variable stars: the longer the varia-
tion is, the brighter the star is. She first published her findings in 1908 and confirmed them
in 1912, long before Cepheid variables were used by Shapley in the Great Debate.
Shapley studied this relationship and concluded that it could be used for distance mea-
surement if one measured the apparent magnitude and light variability period. By observing
Cepheid variable stars in globular clusters in the Milky Way galaxy, he realized that the solar
system was not in the center of the galaxy.
After astronomers were able to use Cepheid variable stars to measure distances to
heavenly bodies, the positions of heavenly bodies at distances of approximately 10 million
light-years could be accurately determined to a certain extent, and the map of the universe
was significantly redrawn. Shapley and Leavitt’s work certainly led to discoveries like the fact
that there are many galaxies outside of the Milky Way and the redshift-based evidence that
the universe is expanding.
Methods of Measuring Even Greater Distances
By using Cepheid variable stars and subsequent advances in observational techniques,
astronomers could measure the distance to heavenly bodies to approximately 100 million
light-years, as long as they resigned themselves to a certain margin of error. However, even
this distance still covered only approximately 1 percent of the visible universe. Since the area
of the universe that we can physically observe extends out to a radius of approximately 15
billion light-years from Earth, enlarging the measurement range to that distance was one of
the dreams of astronomers.
Some other measurement methods that have already been devised are introduced
below.
Measurement Based on Supernovas
A Type Ia supernova (an evolved binary star system consisting of a giant or super giant
and a white dwarf) has the property that its peak absolute magnitude is practically constant.
Moreover, its brightness is approximately 100,000 times greater than that of a Cepheid vari-
able star! Since it emits as much light as a galaxy, the distance to which it can be measured
is extremely far. However, a supernova can only be observed at the instant it explodes,
meaning the moment at which the life of that star ends.
Measurement Based on Redshift
If we consider the fact that more distant heavenly bodies are receding from Earth at a faster
rate than closer bodies, then redshift due to cosmic expansion increases in proportion to
distance. Therefore, by observing the shift in the wavelengths of the spectral lines of a gal-
axy, the velocity of that galaxy (and its distance from Earth) can be known.

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